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How will rising carbon dioxide levels in the troposphere affect photosynthetic producers?

How will rising carbon dioxide levels in the troposphere affect photosynthetic producers?


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Much discussion has been had about the affects of climate change on plantlife, but how will rising carbon dioxide concentrations affect the photosynthetic process itself? Since CO₂ is a reagent in photosynthesis, would we expect higher CO₂ to mean an increased rate of photosynthesis in a real-world context? Has there been any research on this?

I am thinking more of large-scale field tests rather than lab plants.


There are several key ways in which rising atmospheric CO₂ concentrations will affect photosynthesis, and these are related to the different types of photosynthesis. In order to properly answer your question, I'll provide some background about photosynthesis itself.

Photosynthesis evolved in a high-CO₂ atmosphere, before the oxygen-enrichment of the atmosphere (which actually happened as a result of photosynthesis). Most plant species operate C3 photosynthesis. In these plants, carbon dioxide diffuses into the cell where it is fixed by Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) into a 3-carbon molecule (hence C3), which is then polymerised to make sugars. A crucial fact about RuBisCO is that it has both carboxylase (carbon-fixing) activity and oxygenase (oxygen-fixing) activity. This means that oxygen and carbon dioxide compete for the active site on the enzyme complex, leading to RuBisCO being quite inefficient and slow at fixing carbon in higher oxygen concentrations. That didn't matter in the high-CO₂ atmosphere of the early Earth, but in todays atmosphere O₂ concentrations are high enough that they severely limit the productivity of C3 plants.

However, plants haven't just been growing slowly all that time - several mechanisms for increasing photosynthetic efficiency have evolved. The most influential systems involve concentrating carbon dioxide in a particular area, excluding oxygen, and concentrating RuBisCO in that same area. This avoids the oxygen competition for the active site and allows RuBisCO to operate more efficiently. The key adaptation here is C4 photosynthesis - the system which is present in most grasses and many of the most productive plants on Earth (e.g. maize, sugarcane, Miscanthus). It has evolved at least 62 times independently. It works by having RuBisCO concentrated within 'bundle sheath' cells which are surrounded by a layer of suberin wax. This layer prevents CO₂ escaping and O₂ from getting in. CO₂ from the atmosphere is then fixed in different cells - 'mesophyll cells' - by another enzyme - Phosphoenolpyruvate carboxylase (PEPC), resulting in a four-carbon molecule (hence C4). This 4-carbon acid, (malate or oxaloacetate depending on the system) is then shuttled into the bundle sheath cells. There, the CO₂ is released again by a variety of enzymes depending on the system, creating a high CO₂ concentration in the cell where RuBisCO can then work efficiently.

In general, C4 plants are much (about 50%) more efficient than their C3 counterparts, and they are particularly well adapted to high temperatures and moist environments. So, to answer your first question: as atmospheric CO2 levels continue to rise, C3 plants will gradually be able to photosynthesise more efficiently. Interestingly though, C4 plants are predicted to also benefit from increased atmospheric CO₂. If global temperatures rise as predicted, both C3 and C4 plants will be able to operate more efficiently than they currently do, up to a maximum temperature beyond which enzymes will begin to denature faster and efficiency will drop. One consideration is that the difference in efficiency between C3 and C4 systems will decrease, which may significantly alter the makeup of plant communities around the world.

This is a vast oversimplification, but it is accurate for the predicted overall effects. Localised effects (i.e. productivity changes in a particular region or for a particular crop) will depend on habitat, physiology, etc.

Some key papers to launch you into the literature:

  • Leakey, A.D.B., Bernacchi, C.J., Dohleman, F.G., Ort, D.R. & Long, S.P. (2004) Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future [CO2] rich atmospheres? An analysis of diurnal courses of CO2 uptake under free‐air concentration enrichment (FACE). Global Change Biology. [Online] 10 (6), 951-962. Available from: doi:10.1111/j.1529-8817.2003.00767.x [Accessed: 31 January 2012].
  • Morgan, J.A., LeCain, D.R., Pendall, E., Blumenthal, D.M., Kimball, B.A., Carrillo, Y., Williams, D.G., Heisler-White, J., Dijkstra, F.A. & West, M. (2011) C4 grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grassland. Nature. [Online] 476 (7359), 202-205. Available from: doi:10.1038/nature10274 [Accessed: 31 January 2012].

I wanted to add a little more to the excellent answer above, especially since the OP asks about research into this question in a "real-world context".

There is a substantial body of evidence on exactly this question that comes from experiments at "Free Air CO2 Enrichment" (FACE) sites. FACE is an experimental method/technology in which standing ecosystems undergo CO2 enrichment without (much) disturbance to the ecosystem. It has been known that higher CO2 increases plant growth since the 1960s or so, but the motivation for FACE was to understand what the long-term and ecosystem scale effects of rising CO2 in the atmosphere would be. Many different ecosystem types (forests, crops, shrublands, etc) have been studied with this technique to date, some for fairly long periods of time. I think that many of these sites are now being shut down.

Some key findings:

  • Net primary production (Plant C assimilation - Respiration) generally increased for plant species, but increases in productivity varied a lot between ecosystem types.
  • This increase in productivity diminished over time, an effect that was largely mediated by changes in plant N availability.
  • Different plant functional types show different responses. For example, herbaceous species saw less enhancement of assimilation (due to a decrease in leaf N) compared to woody plants at some sites.
  • At the ecosystem level, primary production is often limited by factors other than available CO2 - nitrogen or water for example.

There are a couple of excellent reviews available:

  • Nowak, R. S., Ellsworth, D. S. and Smith, S. D. (2004), Functional responses of plants to elevated atmospheric CO2- do photosynthetic and productivity data from FACE experiments support early predictions?. New Phytologist, 162: 253-280. doi: 10.1111/j.1469-8137.2004.01033.x pdf

  • Norby, R. J. and Zak, D. R. (2011), Ecological Lessons from Free-Air CO2 Enrichment (FACE) Experiments. Annual Review of Ecology, Evolution, and Systematics, 42: 181-203. doi: 10.1146/annurev-ecolsys-102209-144647 link.

  • Norby, R.J., Warren, J. M., Iversen, C. M., Medlyn, B. E., and McMurtrie, R. E. (2010), CO2 enhancement of forest productivity constrained by limited nitrogen availability. PNAS, 107 (45) 19368-19373. doi:10.1073/pnas.1006463107 link

Carbon dioxide

Included among the rate-limiting steps of the dark stage of photosynthesis are the chemical reactions by which organic compounds are formed by using carbon dioxide as a carbon source. The rates of these reactions can be increased somewhat by increasing the carbon dioxide concentration. Since the middle of the 19th century, the level of carbon dioxide in the atmosphere has been rising because of the extensive combustion of fossil fuels, cement production, and land-use changes associated with deforestation. The atmospheric level of carbon dioxide climbed from about 0.028 percent in 1860 to 0.032 percent by 1958 (when improved measurements began) and to 0.041 percent by 2020. This increase in carbon dioxide directly increases plant photosynthesis up to a point, but the size of the increase depends on the species and physiological condition of the plant. Furthermore, most scientists maintain that increasing levels of atmospheric carbon dioxide affect climate, increasing global temperatures and changing rainfall patterns. Such changes will also affect photosynthesis rates.


a. plants/producers/autotrophs convert light to chemical energy by photosynthesis

b. chlorophyll/photosynthetic pigments absorb light

c. electrons are excited/raised to higher energy level

d. excited electrons pass along chain of electron carriers

e. energy from electrons used to pump protons across thylakoid membrane/into thylakoid space

f. chemiosmosis/proton gradient used to make ATP

g. ATP synthase generates ATP

h. pigments arranged in photosystems

i. electrons from Photosystem II flow via the electron chain to Photosystem I

j. electrons from Photosystem I are used to reduce NADP

k. ATP and reduced NADP used in the light independent reactions/Calvin cycle

l. carbohydrate/glucose/carbon compounds produced containing energy

Award marking points for any point made on a clearly annotated diagram.

a. producers/plants/autotrophs obtain energy from light/sun/inorganic sources

b. food contains energy / energy passed in the form of food/carbon compounds (along food chains/between trophic levels)

c. consumers obtain energy from other organisms/from previous trophic level

This mark point distinguishes consumers from producers.

d. energy released (in organisms) by (cell) respiration

Reject energy used in respiration.

f. energy/ATP used for biosynthesis/movement/active transport/other valid use of ATP


How will rising carbon dioxide levels in the troposphere affect photosynthetic producers? - Biology

All of this extra carbon needs to go somewhere. So far, land plants and the ocean have taken up about 55 percent of the extra carbon people have put into the atmosphere while about 45 percent has stayed in the atmosphere. Eventually, the land and oceans will take up most of the extra carbon dioxide, but as much as 20 percent may remain in the atmosphere for many thousands of years.

The changes in the carbon cycle impact each reservoir. Excess carbon in the atmosphere warms the planet and helps plants on land grow more. Excess carbon in the ocean makes the water more acidic, putting marine life in danger.

Atmosphere

It is significant that so much carbon dioxide stays in the atmosphere because CO2 is the most important gas for controlling Earth&rsquos temperature. Carbon dioxide, methane, and halocarbons are greenhouse gases that absorb a wide range of energy&mdashincluding infrared energy (heat) emitted by the Earth&mdashand then re-emit it. The re-emitted energy travels out in all directions, but some returns to Earth, where it heats the surface. Without greenhouse gases, Earth would be a frozen -18 degrees Celsius (0 degrees Fahrenheit). With too many greenhouse gases, Earth would be like Venus, where the greenhouse atmosphere keeps temperatures around 400 degrees Celsius (750 Fahrenheit).

Rising concentrations of carbon dioxide are warming the atmosphere. The increased temperature results in higher evaporation rates and a wetter atmosphere, which leads to a vicious cycle of further warming. (Photograph ©2011 Patrick Wilken.)

Because scientists know which wavelengths of energy each greenhouse gas absorbs, and the concentration of the gases in the atmosphere, they can calculate how much each gas contributes to warming the planet. Carbon dioxide causes about 20 percent of Earth&rsquos greenhouse effect water vapor accounts for about 50 percent and clouds account for 25 percent. The rest is caused by small particles (aerosols) and minor greenhouse gases like methane.

Water vapor concentrations in the air are controlled by Earth&rsquos temperature. Warmer temperatures evaporate more water from the oceans, expand air masses, and lead to higher humidity. Cooling causes water vapor to condense and fall out as rain, sleet, or snow.

Carbon dioxide, on the other hand, remains a gas at a wider range of atmospheric temperatures than water. Carbon dioxide molecules provide the initial greenhouse heating needed to maintain water vapor concentrations. When carbon dioxide concentrations drop, Earth cools, some water vapor falls out of the atmosphere, and the greenhouse warming caused by water vapor drops. Likewise, when carbon dioxide concentrations rise, air temperatures go up, and more water vapor evaporates into the atmosphere&mdashwhich then amplifies greenhouse heating.

So while carbon dioxide contributes less to the overall greenhouse effect than water vapor, scientists have found that carbon dioxide is the gas that sets the temperature. Carbon dioxide controls the amount of water vapor in the atmosphere and thus the size of the greenhouse effect.

Rising carbon dioxide concentrations are already causing the planet to heat up. At the same time that greenhouse gases have been increasing, average global temperatures have risen 0.8 degrees Celsius (1.4 degrees Fahrenheit) since 1880.

With the seasonal cycle removed, the atmospheric carbon dioxide concentration measured at Mauna Loa Volcano, Hawaii, shows a steady increase since 1957. At the same time global average temperatures are rising as a result of heat trapped by the additional CO2 and increased water vapor concentration. (Graphs by Robert Simmon, using CO2 data from the NOAA Earth System Research Laboratory and temperature data from the Goddard Institute for Space Studies.)

This rise in temperature isn&rsquot all the warming we will see based on current carbon dioxide concentrations. Greenhouse warming doesn&rsquot happen right away because the ocean soaks up heat. This means that Earth&rsquos temperature will increase at least another 0.6 degrees Celsius (1 degree Fahrenheit) because of carbon dioxide already in the atmosphere. The degree to which temperatures go up beyond that depends in part on how much more carbon humans release into the atmosphere in the future.

Ocean

About 30 percent of the carbon dioxide that people have put into the atmosphere has diffused into the ocean through the direct chemical exchange. Dissolving carbon dioxide in the ocean creates carbonic acid, which increases the acidity of the water. Or rather, a slightly alkaline ocean becomes a little less alkaline. Since 1750, the pH of the ocean&rsquos surface has dropped by 0.1, a 30 percent change in acidity.

Some of the excess CO2 emitted by human activity dissolves in the ocean, becoming carbonic acid. Increases in carbon dioxide are not only leading to warmer oceans, but also to more acidic oceans. (Photograph ©2010 Way Out West News.)

Ocean acidification affects marine organisms in two ways. First, carbonic acid reacts with carbonate ions in the water to form bicarbonate. However, those same carbonate ions are what shell-building animals like coral need to create calcium carbonate shells. With less carbonate available, the animals need to expend more energy to build their shells. As a result, the shells end up being thinner and more fragile.

Second, the more acidic water is, the better it dissolves calcium carbonate. In the long run, this reaction will allow the ocean to soak up excess carbon dioxide because more acidic water will dissolve more rock, release more carbonate ions, and increase the ocean&rsquos capacity to absorb carbon dioxide. In the meantime, though, more acidic water will dissolve the carbonate shells of marine organisms, making them pitted and weak.

Warmer oceans&mdasha product of the greenhouse effect&mdashcould also decrease the abundance of phytoplankton, which grow better in cool, nutrient-rich waters. This could limit the ocean&rsquos ability to take carbon from the atmosphere through the fast carbon cycle.

On the other hand, carbon dioxide is essential for plant and phytoplankton growth. An increase in carbon dioxide could increase growth by fertilizing those few species of phytoplankton and ocean plants (like sea grasses) that take carbon dioxide directly from the water. However, most species are not helped by the increased availability of carbon dioxide.

Plants on land have taken up approximately 25 percent of the carbon dioxide that humans have put into the atmosphere. The amount of carbon that plants take up varies greatly from year to year, but in general, the world&rsquos plants have increased the amount of carbon dioxide they absorb since 1960. Only some of this increase occurred as a direct result of fossil fuel emissions.

With more atmospheric carbon dioxide available to convert to plant matter in photosynthesis, plants were able to grow more. This increased growth is referred to as carbon fertilization. Models predict that plants might grow anywhere from 12 to 76 percent more if atmospheric carbon dioxide is doubled, as long as nothing else, like water shortages, limits their growth. However, scientists don&rsquot know how much carbon dioxide is increasing plant growth in the real world, because plants need more than carbon dioxide to grow.

Plants also need water, sunlight, and nutrients, especially nitrogen. If a plant doesn&rsquot have one of these things, it won&rsquot grow regardless of how abundant the other necessities are. There is a limit to how much carbon plants can take out of the atmosphere, and that limit varies from region to region. So far, it appears that carbon dioxide fertilization increases plant growth until the plant reaches a limit in the amount of water or nitrogen available.

Some of the changes in carbon absorption are the result of land use decisions. Agriculture has become much more intensive, so we can grow more food on less land. In high and mid-latitudes, abandoned farmland is reverting to forest, and these forests store much more carbon, both in wood and soil, than crops would. In many places, we prevent plant carbon from entering the atmosphere by extinguishing wildfires. This allows woody material (which stores carbon) to build up. All of these land use decisions are helping plants absorb human-released carbon in the Northern Hemisphere.

Changes in land cover&mdashforests converted to fields and fields converted to forests&mdashhave a corresponding effect on the carbon cycle. In some Northern Hemisphere countries, many farms were abandoned in the early 20th century and the land reverted to forest. As a result, carbon was drawn out of the atmosphere and stored in trees on land. (Photograph ©2007 Husein Kadribegic.)

In the tropics, however, forests are being removed, often through fire, and this releases carbon dioxide. As of 2008, deforestation accounted for about 12 percent of all human carbon dioxide emissions.

The biggest changes in the land carbon cycle are likely to come because of climate change. Carbon dioxide increases temperatures, extending the growing season and increasing humidity. Both factors have led to some additional plant growth. However, warmer temperatures also stress plants. With a longer, warmer growing season, plants need more water to survive. Scientists are already seeing evidence that plants in the Northern Hemisphere slow their growth in the summer because of warm temperatures and water shortages.

Dry, water-stressed plants are also more susceptible to fire and insects when growing seasons become longer. In the far north, where an increase in temperature has the greatest impact, the forests have already started to burn more, releasing carbon from the plants and the soil into the atmosphere. Tropical forests may also be extremely susceptible to drying. With less water, tropical trees slow their growth and take up less carbon, or die and release their stored carbon to the atmosphere.

The warming caused by rising greenhouse gases may also &ldquobake&rdquo the soil, accelerating the rate at which carbon seeps out in some places. This is of particular concern in the far north, where frozen soil&mdashpermafrost&mdashis thawing. Permafrost contains rich deposits of carbon from plant matter that has accumulated for thousands of years because the cold slows decay. When the soil warms, the organic matter decays and carbon&mdashin the form of methane and carbon dioxide&mdashseeps into the atmosphere.

Current research estimates that permafrost in the Northern Hemisphere holds 1,672 billion tons (Petagrams) of organic carbon. If just 10 percent of this permafrost were to thaw, it could release enough extra carbon dioxide to the atmosphere to raise temperatures an additional 0.7 degrees Celsius (1.3 degrees Fahrenheit) by 2100.


Managing Water Resources in the West Under Conditions of Climate Uncertainty: A Proceedings (1991)

U.S. Department of Agriculture

The atmospheric carbon dioxide concentration has risen from about 270 parts per million (ppm) before 1700 to about 355 ppm today. Climate changes, including a mean global surface temperature rise of between 2.8 and 5.2°C, have been predicted by five independent general circulation models (GCMs) for a doubling of the carbon dioxide concentration. The objectives of this paper are to examine plant responses to rising carbon dioxide levels and climatic changes and to interpret the consequences of these changes on crop water use and water resources for the United States.

BACKGROUND: PLANT RESPONSES TO ENVIRONMENTAL FACTORS

The main purpose of irrigation is to supply plants with adequate water for transpiration and for incorporating the element hydrogen in plant tissues through photosynthesis and subsequent biosynthesis of various tissues and organs. Transpirational flux requires several hundred times more water than photosynthesis.

In a series of U.S. Department of Agriculture studies beginning in 1910 in Akron, Colorado, Briggs and Shantz (1913a,b 1914) showed that the water requirement of plants is linearly related to the biomass production of plants. They established this linear relationship by growing plants in metal containers filled with soil. Throughout the period of growth, they monitored water use carefully by weighing and adding measured amounts of water to maintain a desirable soil water content as water lost by plant transpiration was replenished.

The findings of Briggs and Shantz have been confirmed repeatedly (Allison et al., 1958 Arkley, 1963 Chang, 1968 Hanks et al., 1969 Stanhill, 1960). Figure 7.1 shows the linear relationship between biomass produced and rainfall plus irrigation water used by Sart sorghum and Starr millet in Alabama, as adapted from data of Bennett et al. (1964). De Wit (1958) examined the relationships among climatic factors, yield, and water use by crops. He found the following general linear relationship to be true, especially in semiarid climates:

Y = yield component (e.g., total above-ground biomass or seed production)

T = cumulative actual transpiration

Tmax = maximum possible cumulative transpiration

m = constant dependent on yield component and species, especially on differences among photosynthetic mechanisms

Pan evaporation was used to represent Tmax, which is proportional to climatic factors, especially air vapor pressure deficit (VPD):

es = the saturation vapor pressure at a given air temperature

ea = the actual vapor pressure that exists in the air.

Combining these relationships, we see that yield is proportional to cumulative transpiration divided by vapor pressure deficit:

where k is a constant with units millibars &bull g (dry matter) &bull g -1 (water). Like m, k depends on yield component, species, and photosynthetic mechanisms.

FIGURE 7.1 Linear relationship between biomass production and water use for two forage crops in 1956 and 1957 at Thorsby, Alabama. Squares: Sart sorghum. Triangles: Starr millet.

SOURCE: Adapted from Bennett et al., 1964.

Thus, we can see that theory predicts that yield will be proportional to cumulative transpirational water use, divided by vapor pressure deficit. There are several ways of calculating the VPD it can be computed by aggregating seasonal daytime average VPD, or by using approximation methods based on daily maximum and minimum temperatures (Jensen, 1974). As pointed out by Tanner and Sinclair (1983), the maximum es can be computed from the daily maximum temperature, and ea can be estimated from the daily minimum temperature. Tanner and Sinclair estimated that the effective daytime es falls at a point two-thirds to three-quarters of the distance between the es computed at the daily maximum temperature and the ea computed at the daily minimum temperature. The effective daytime VPD values then must be averaged over the growing season of the crop. Regardless of the method used to compute a representative VPD, yield versus cumulative transpiration linear relationships vary with the aridity of the

climate&mdashspecifically with the temperature and vapor pressure regime under which the crop is grown. Figure 7.2 (modified from Stanhill, 1960) shows water used versus dry matter yield of pastures from the latitude of Denmark (which has a cool, humid atmosphere) to the latitude of Trinidad (which has a hot, dry atmosphere). Based on comparisons among existing climates, we can expect that transpirational water requirements of plants will increase if climates get warmer.

Atmospheric carbon dioxide is known to affect plant yield. Kimball (1983) reviewed 430 observations of carbon dioxide enrichment studies conducted prior to 1982 and reported an average yield increase of 33 percent, plus or minus 6 percent, for a doubling of the carbon dioxide concentration. This value has been generally confirmed by many other studies since that time. The yield increases seem to apply for both biomass accumulation and grain yield. Thus, plants may grow larger and, considering Figure 7.1, they may use more water as the global carbon dioxide concentration increases.

Transpirational water use is clearly related to ground cover (Jensen, 1974 Doorenbos and Pruitt, 1977). Daily water use soon after crops are planted on bare soil is typically only 10 to 20 percent of water use after effective ground cover is reached. Water use rises sharply as the crop's leaf area increases. Similarly, water use drops 60 to 70 percent when hay crops such as alfalfa are cut. As leaf regrowth occurs, transpiration rates recover rapidly as the ground cover of leaves is restored. Ground cover can be quantified with a leaf area index (LAI): the ratio of leaf area per unit ground area. Therefore, any carbon dioxide-induced stimulation of early growth of leaf area or increase of total leaf area growth may increase transpiration.

Increased carbon dioxide concentrations are known to cause smaller stomatal apertures and hence to decrease the leaf conductance for water vapor (Morison, 1987). This is a second mechanism whereby increased carbon dioxide concentrations may affect plant transpiration.

Another effect of rising carbon dioxide concentrations is the change in water-use efficiency (WUE). Water-use efficiency has a range of definitions. For whole-season processes, it is best defined as the ratio of dry matter (or seed yield) produced to the amount of water used by crops. For shorter-term whole canopy processes, it is best defined as the ratio of the photosynthetic carbon dioxide uptake rate per unit land area to the transpiration rate per unit land area. Figure 7.2 demonstrates the effect of climate on WUE.

FIGURE 7.2 Cumulative dry matter yield versus cumulative potential evapotranspiration (ET) of pastures under a range of climatic regimes. Open circle: Denmark. Filled circle: The Netherlands. Open triangle: England. Filled triangle: New Jersey. Open Square: Toronto, Canada. Filled square: Gilat, Israel. Open inverted triangle: Trinidad, West Indies.

SOURCE: Adapted from Stanhill, 1960.

Equation 3 quantifies the relationship between WUE and vapor pressure deficit.

In summary, the following relationships have been established by research:

Transpiration is linearly related to biomass accumulation and yield.

Transpiration is also linearly related to the aridity of the climate&mdashin other words, to the vapor pressure deficit. Thus, rising global temperatures would increase transpiration by increasing the atmospheric vapor pressure deficit.

Transpiration is affected by the degree of ground cover.

Rising carbon dioxide concentrations will increase plant growth. More rapid leaf area development and more total leaf area could translate into more transpiration.

Rising carbon dioxide concentrations will decrease leaf stomatal conductance to water vapor. This effect could reduce transpiration.

Rising carbon dioxide concentrations and rising global temperatures could change WUE.

The following sections of this chapter will examine more closely the effects of rising carbon dioxide concentrations and climate change on vegetation, providing qualitative and quantitative assessments of how these changes will affect photosynthesis, growth, and transpiration water requirements of crops.

DIRECT EFFECTS OF CARBON DIOXIDE ON PHOTOSYNTHESIS, TRANSPIRATION, AND GROWTH OF PLANTS

Atmospheric Carbon Dioxide

The carbon dioxide concentration of the earth's atmosphere has varied throughout geologic time. Ice core data from Antarctica and Greenland have been obtained and, from entrapped air bubbles, used to show carbon dioxide and methane concentrations of the atmosphere throughout the past 160,000 years (Barnola et al., 1987 Lorius et al., 1990). Changes in the deuterium content within ice crystals have been used to establish temperature changes over this same time period (Jouzel et al., 1987). In general, carbon dioxide concentrations were as low as 180 to 200 parts per million (ppm) 13,000 to 30,000 years ago and 140,000 to 160,000 years ago during the coldest parts of the last two ice ages (Barnola et al., 1987). Carbon dioxide concentrations rose to about 270 ppm during the last interglacial period (116,000 to 140,000 years ago) and during the current interglacial period (beginning about 13,000 years ago). Ice core data since about 1700 A.D. and direct atmospheric sampling data since 1958 show that the carbon dioxide concentration increased to 315 ppm by 1958 and to about 355 ppm by 1990 (Keeling et al., 1989). The rate of increase of atmospheric carbon dioxide is about 0.5 percent per year, which means that the change is accelerating.

These changes in atmospheric carbon dioxide have important implications for plants and the global carbon cycle as well as for climate. Atmospheric carbon dioxide is the raw material for terrestrial green plant photosynthesis, and thus it represents the first molecular link in the food chain of the whole earth. In later sections, we will examine the importance of carbon dioxide for photosynthesis and plant growth, as well as the importance of potential climate change on water resources for the future.

Plant Photosynthetic Mechanisms

Three types of photosynthetic mechanisms of terrestrial green plants have been identified: C3, C4, and CAM. Responses of these three photosynthetic mechanisms to carbon dioxide have been reviewed by Tolbert and Zelitch (1983). The biochemical pathway of photosynthetic carbon dioxide uptake was first determined for C3 plant photosynthesis. This pathway involves the use and subsequent regeneration of ribulose 1,5-biophosphate in a cyclic series of reactions, and it is frequently called the Calvin cycle. The first product of photoassimilation of carbon dioxide is 3-phosphoglyceric acid, a three-carbon sugar&mdashhence the term C3 pathway of photosynthesis.

The C4 plants begin their carbon dioxide uptake in a different process sometimes called the Hatch-Slack pathway. In mesophyll cells of leaves, these plants form a four-carbon molecule, oxalacetate, in the first step of incorporation of carbon dioxide. This four-carbon compound is changed into aspartic acid or malic acid and then transported immediately to bundle sheath cells. Here, the carbon dioxide is released and utilized in the C3 biochemical pathway. Thus, the C4 plant mechanism first traps carbon dioxide in the mesophyll cells, and then transports and concentrates the carbon dioxide in the bundle sheath cells, where it is utilized in C3 plant metabolism (Tolbert and Zelitch, 1983).

Crassulacean acid metabolism, or CAM, is a mechanism whereby plants typically take up and store carbon dioxide during the night and use it in photosynthetic carbon dioxide fixation during the day, when sunlight is available. Pineapple and ''air plants,'' such as Spanish moss and orchids, have this photosynthetic mechanism. Since few agricultural crops are CAM plants, they are not important in the process of managing water resources under conditions of climate uncertainty.

Since C4 plants have a mechanism for concentrating carbon dioxide in bundle sheath cells of leaves, their photosynthetic rates

will not respond to rising carbon dioxide levels to the same extent as C3 plants. Irrigated crop or turf plants that fit into the C4 category include maize (corn), sorghum, millet, sugar cane, and bermuda grass. Plants that fit into the C3 category include: wheat, rice, potato, soybean, sugar beet, alfalfa, cotton, tree and vine crops, and most vegetable crops and cool-season grasses.

Plant Growth Responses to Carbon Dioxide

Increasing atmospheric carbon dioxide levels have caused increasing photosynthetic rates, biomass growth, and seed yield for all of the globally important C3 food and feed crops (Acock and Allen, 1985 Enoch and Kimball, 1986 Warrick et al., 1986 Allen, 1990). Some plants, such as cucumber, cabbage, and perhaps tomato, have shown a tendency to first increase leaf photosynthetic rates in response to elevated carbon dioxide concentrations, and then to decrease photosynthetic rates after several days. This behavior is called "end-product inhibition of photosynthesis," and it is caused by the failure of translocation of photoassimilates to keep up with photosynthetic rates (Guinn and Mauney, 1980).

A few experiments have been conducted with carbon dioxide concentration maintained across a range of 160 to 990 ppm. Figure 7.3 shows the results of one study with soybean canopy photosynthetic rates across the 90 to 900 ppm carbon dioxide concentration range. A nonlinear hyperbolic model was used to fit soybean photosynthetic rate data to carbon dioxide concentration (Allen et al., 1987). Photosynthetic rates at the various carbon dioxide concentrations were divided by the photosynthetic rate at a carbon dioxide concentration of 330 ppm to normalize the data to a common condition. Data sets of biomass yield and seed yield from four locations over three years were also fit to the model (Allen et al., 1987). Relative yields with respect to yields at 330 to 340 ppm were used.

The form of the model fit to the experimental data was:

R = relative response of photosynthetic rate, biomass yield, or seed yield

Rmax = asymptotic upper limit for R from baseline Rint

FIGURE 7.3 Photosynthetic carbon dioxide uptake rate responses of a soybean crop canopy exposed to carbon dioxide concentrations ranging from 110 to 990 ppm. All data points are relative to the response obtained at 330 ppm.

SOURCE: Adapted from Allen et al., 1987.

C = carbon dioxide concentration (ppm)

Kc = Apparent Michaelis constant (ppm)

Rint = Y-axis intercept for zero C

From the parameters of this equation, photosynthetic rate, biomass accumulation, and seed yield changes of soybean due to carbon dioxide concentration changes can be estimated (Allen et al.,

1987). Table 7.1 shows the changes predicted across three time periods: from the last ice age (when the carbon dioxide concentration was at a minimum) to the preindustrial revolution era (about 1700), from 1700 to 1973, and from 1973 to about a century into the future. The modeled data show that there should have been large increases in productivity between the ice-age (when carbon dioxide concentration was about 200 ppm) and the beginning of the industrial revolution (when the carbon dioxide concentration was about 270 ppm). Likewise, there should have been a 12 percent increase in grain-yield productivity between 1700 and 1973, when the carbon dioxide concentration increased from about 270 to 330 ppm.

Most of the recent concerns about rising atmospheric carbon dioxide concentrations have been quantified by predicting changes for a doubling of the carbon dioxide concentration, usually from 330 to 660 ppm. Table 7.1 shows that soybean seed yields and biomass yields are predicted to increase 31 percent and 41 percent, respectively, from a doubling of carbon dioxide. Experimental studies have consistently showed a lower seed yield than biomass yield for soybean when grown under a doubled carbon dioxide concentration. If the harvest index&mdashthe ratio of seed yield to above-ground biomass yield (seed plus pod walls plus stems)&mdashwere 0.50 for soybean grown under a 330 ppm carbon dioxide concentration, then the harvest index would be 0.46 if the carbon dioxide concentration were doubled. This small decrease in soybean harvest index under elevated carbon dioxide conditions has been commonly observed (Allen, 1990 Jones et al., 1984). The relative midday maximum photosynthetic rates under carbon dioxide enrichment were consistently higher than relative biomass yields, probably because the photosynthetic response to elevated carbon dioxide levels is greater under high light conditions than it is under total daily solar irradiance conditions.

Transpiration Responses to Carbon Dioxide

The effect of carbon dioxide concentration on water use under field conditions has been discussed for many years. In the past, elevated carbon dioxide levels have been mentioned as the ideal antitranspirant. This conclusion seems reasonable, since elevated carbon dioxide has been observed to reduce stomatal conductance in numerous experiments. Morison (1987) reviewed 80 observations in the literature and found that a doubled carbon dioxide con-

TABLE 7.1 Percent increases of soybean midday photosynthesis rates, biomass yield, and seed yield predicted across selected carbon dioxide concentration ranges associated with relevant benchmark points in time.

1 IA, the Ice Age about 13,000 to 30,000 years before present. The atmospheric carbon dioxide concentrations that prevailed during the last Ice Age, and from the end of the glacial melt until preindustrial revolution times, were 200 and 270 ppm, respectively.

2 The first world energy "crisis" occurred in 1973, when the carbon dioxide concentration was 330 ppm. This concentration is used as the basis for many carbon dioxide&ndashdoubling studies. The carbon dioxide concentration is expected to double sometime within the twenty&ndashfirst century.

centration will reduce stomatal conductance of most plants by about 40 plus or minus 5 percent. Kimball and Idso (1983) calculated a 34 percent reduction in transpiration in response to a doubled carbon dioxide concentration in several short-term plant growth chamber experiments, which seems consistent with the review by Morison (1987). However, Morison and Gifford (1984) also showed that doubling carbon dioxide will cause a more rapid development of leaf area for many plants and hence an equal or greater transpiration rate in the early stages of plant growth, due to a more rapid development of transpiring surfaces. Therefore, increased rates of development of transpiring leaf surface offset the reduced stomatal conductance for water vapor.

Allen et al. (1985) and Allen (1990) also discussed the effect of reduction in stomatal conductance on foliage temperature. The cause-and-effect relationships can be summarized as follows: Any reduction in stomatal conductance due to increasing the carbon dioxide concentration will restrict transpiration rates per unit leaf area. A reduction in transpiration rates will result in less eva-

porative cooling of the leaves, and leaf temperatures will rise. As leaf temperatures rise, the vapor pressure inside the leaves will increase, and thus the leaf-to-air vapor pressure gradient, which is the driving force for transpiration, will increase. The increase in leaf vapor pressure will increase transpiration rates per unit leaf area thus, the transpiration rates will be maintained at only slightly lower values than would exist at ambient environmental carbon dioxide concentrations. In effect, all of the energy balance factors involved in canopy foliage energy exchange&mdashnot just stomatal factors&mdashmust be considered.

Controlled environment studies of soybean at Gainesville, Florida, showed that canopy transpiration rate changes ranged from negative 2 percent (Jones et al., 1985a) to plus 11 percent (Jones et al., 1985b) for carbon dioxide treatments of 800 and 330 ppm with corresponding LAI values of 6.0 and 3.3. In another experiment in which differences in the LAI of soybean between the 330 ppm and the 660 ppm treatments were small (3.36 and 3.46, respectively), the seasonal cumulative water use decreased by 12 percent for the doubled carbon dioxide treatments (Jones et al., 1985c). Decreases were similar for both water-stressed and nonstressed treatments.

Field weighing lysimeter and neutron-probe water balance studies of cotton at Phoenix, Arizona, have shown evapotranspiration reductions due to elevated carbon dioxide levels ranging from 0 up to 9 percent (Kimball et al., 1983).

In conclusion, although stomatal conductance may be reduced by about 40 percent for doubled carbon dioxide concentrations, water use by C3 crop plants under field conditions will probably be reduced by only about 0 to 12 percent. If leaf area increases due to doubled carbon dioxide concentrations are small (or can be controlled), then the transpiration reductions may be meaningful, albeit small. If leaf area increases due to doubled carbon dioxide concentrations are large, then no reductions in transpiration are to be expected, and increases may be possible.

Streamflow Responses to Carbon Dioxide

Several attempts have been made to predict changes in streamflow due to an increase in carbon dioxide (Aston, 1984), changes in climate (Revelle and Waggoner, 1983), or both (Brazel and Idso, 1984). Aston (1984) modeled streamflow changes from a New South Wales, Australia, watershed over the course of a year based on reduction of stomatal conductance to one-half of current

values. His model predicted a 40 to 90 percent increase in annual streamflow above the observed baseline of about 150 mm per year from the actual watershed. However, Aston (1984) did not consider any increase of the LAI, which is perhaps a justifiable assumption for C4 plants but probably not for C3 plants. Rosenberg et al. (1990) conducted a quantitative analysis of evapotranspiration sensitivity to several plant and environmental factors. Their analysis demonstrated that increasing the LAI could indeed partially offset the effects of decreasing stomatal conductance on transpiration.

For climate change only, Revelle and Waggoner (1983) predicted that western river streamflows could be reduced by about 40 to 76 percent from the combined effects of a 2°C rise in temperature and a 10 percent reduction in precipitation. Brazel and Idso (1984) considered that vegetation would reduce transpiration to about two-thirds of its current value with a doubling of the carbon dioxide concentration, which led to predictions of increasing Arizona streamflow from about 63 to 460 percent. When they included a temperature increase of 2°C and a precipitation decrease of 10 percent, the predictions were still a 4 to 326 percent increase in streamflow. However, Brazel and Idso's predictions did not include any likely increases in vegetation LAI due to increased carbon dioxide levels. Although efforts to relate carbon dioxide and climate change impacts on water resources are continuing (Waggoner, 1990), realistic integration of vegetation influences on the hydrologic cycle are lacking.

Changes in vegetation may be a moot point when streamflow depends largely on spring snowmelt from lower elevations and continuous warm season snowmelt from higher elevations in the mountains of the West. The combination of complex plant responses and complex terrain make accurate hydrologic modeling a difficult task.

Plant Water-Use Efficiency

Allen et al. (1985) compared water-use efficiencies of soybean canopies grown in outdoor, sunlit, controlled-environment chambers at 800 and 330 ppm carbon dioxide concentrations which had LAI values of 6.0 and 3.3, respectively. For each of these treatments (two replications), the exposure carbon dioxide levels were cross-switched for one day. The ratio of the WUE values (i.e., WUE at 800 ppm carbon dioxide exposure divided by the WUE at 330 ppm

carbon dioxide exposure) averaged 2.33. The relative contributions of photosynthesis and transpiration to the ratio of WUE values were 73 and 27 percent, respectively. These comparisons are valid only for plant canopies with equal LAI values, because the same canopy was used for both carbon dioxide exposure levels. However, when the treatment and exposure levels of 800 ppm carbon dioxide were compared with the treatment and exposure levels of 330 ppm, the WUE ratio was 1.80, and the relative contributions to this ratio were 104 percent for photosynthesis and negative 4 percent for transpiration. The negative contribution of transpiration arises from the fact that canopy transpiration rates for the 800 ppm carbon dioxide treatment were slightly greater than the rates from the 330 ppm carbon dioxide treatment, due to the much larger LAI of the canopy exposed to the higher carbon dioxide treatment. Clearly, higher LAI values under elevated carbon dioxide concentrations can increase transpiration rates to the point where all of the improved WUE arises from increased photosynthetic rates and none from decreased water use.

Finally, it should be pointed out that increases in WUE in a world with higher carbon dioxide levels do not necessarily imply any reduction in crop water requirements per unit area of land. However, farmers should be able to achieve higher crop yields per unit land area with similar amounts of water. If temperatures rise, however, the overall WUE could actually decrease, because warmer climates have higher water requirements (as illustrated by Figure 7.2) and higher temperatures may cause yield reductions. The crop response scenarios that may affect hydrology and water resources management will be determined by the carbon dioxide and climate change scenarios and will differ depending on photosynthetic types (C4 versus C3) and species.

CLIMATE CHANGE EFFECTS ON PHOTOSYNTHESIS, GROWTH, AND TRANSPIRATION

Leaf photosynthetic rates are known to be sensitive to temperature. Figure 7.4 shows possible responses of leaf photosynthetic carbon dioxide uptake rates to temperature for C3 plants (bottom curve) and C4 plants (top curve) when grown at a 330 ppm carbon dioxide concentration and exposed to high light levels, such as would occur under midday summer conditions. This figure shows that C4 plants have a higher maximum photosynthetic carbon dioxide uptake rate and a higher temperature maximum than C3

FIGURE 7.4 Examples of maximum photosynthetic (PS) rate responses to temperature of individual leaves of C3 plants under high light conditions when exposed to carbon dioxide concentrations of 330 ppm (lower curve) and 1,000 ppm or greater (upper curve). The upper curve is similar to the maximum PS response to temperature of C4 plant leaves, which have an internal mechanism for concentrating carbon dioxide for subsequent photosynthetic reactions. Various species differ widely, both in maximum leaf photosynthetic rates and in the distribution of leaf photosynthetic rates with temperature.

SOURCE: Modified and adapted from the example of Pearcy and Björkman (1983). See also Berry and Björkman (1980) and Penning de Vries et al. (1989) for further examples of the variability of response among species and experimental conditions.

plants. The relative differences are smaller at lower temperatures. These curves were drawn to represent active crop plants in temperate zones. The actual photosynthetic carbon dioxide uptake rates

could be considerably different from those shown, and the temperature distribution of photosynthetic rates could be higher or lower, depending upon species, climate, or pretreatment temperature conditions (Berry and Björkman, 1980 Penning de Vries et al., 1989). In particular, the curves could be stretched to higher temperatures for species adapted to hot, desert environments (Pearcy and Björkman, 1983) or compressed to lower temperatures for species adapted to cool environments.

Nevertheless, from Figure 7.4 we can conclude that C4 plants could benefit more (or at least suffer less) than C3 plants from an increase in global temperatures. However, the differences for a whole canopy of leaves are somewhat reduced from the differences for individual leaves exposed perpendicularly to high light. First, a canopy of leaves generally has leaves oriented in all directions, so that much of the total leaf area is exposed to much less irradiance than in single leaf exposure systems. Under these conditions many of the individual leaves are limited by light, and the photosynthetic carbon dioxide uptake rates of the whole canopy become more similar. Second, solar irradiance levels are lower than midday values throughout much of the day. Nevertheless, the direction of the leaf-level differences, if not the magnitude, is maintained between C4 and C3 canopies.

Figure 7.4 also shows that C3 plant photosynthetic rates at elevated carbon dioxide levels may increase and resemble the rates of C4 plants (Pearcy and Björkman, 1983), but the extent of increase will vary widely among species. The photosynthetic rates of C3 plant leaves increase at elevated carbon dioxide levels because molecules of carbon dioxide compete more effectively with oxygen for binding sites on rubisco, the carboxylating enzyme (Bowes and Ogren, 1972).

When plants are well watered, leaf temperatures tend to rise more slowly than air temperatures throughout the daily cycle, so that foliage-to-air temperature differences become greater as air temperature rises (Idso et al., 1987 Allen, 1990). For soybean, Jones et al. (1985a) found no change in crop canopy photosynthetic rates across the air temperature set-point range of 28°C to 35°C. However, the transpiration rates increased 30 percent, which would lead to evaporative cooling of the leaves and larger foliage-to-air temperature differences. This 4 to 5 percent increase in transpiration rate per 1°C rise in temperature is close to the 6 percent per 1°C rise in saturation vapor pressure deficit over this temperature range.

Temperature affects growth of plants in several ways. The rate of development and expression of new nodes on plants increases

with increasing temperature. Leaves expressed at new nodes will grow larger, in general, if there is no concurrent water stress. Thus, plant size increases at a more rapid rate, and solar radiation capture occurs earlier in crop development. Once full ground cover is achieved, at a LAI of about 2 to 3, light capture becomes limiting, and the overall temperature effects on growth are muted but not eliminated. The duration of each ontogenic phase of plant growth decreases with increasing temperature, which is the most important effect of temperature within the upper and lower limits of survival.

INTERACTIVE EFFECTS OF CARBON DIOXIDE AND CLIMATE CHANGE

Photosynthetic and Productivity Interactions

As explained above, Figure 7.4 shows leaf photosynthetic carbon dioxide uptake rate versus temperature responses typical of C4 plants and C3 plants at carbon dioxide concentrations of 330 ppm. The upper curve can also represent C3 plants at high carbon dioxide levels of (1,000 ppm or greater). These curves suggest that a combination of rising carbon dioxide concentration and rising temperature should lead to greater photosynthetic rates and hence greater biomass growth rates.

S. G. Allen et al. (1988 1990a,b) conducted experiments in Phoenix, Arizona, on Azolla, water lily, and sorghum as seasonal temperatures were changing. They found that net photosynthetic rates were higher for Azolla and water lily during warmer times of year. Linear regressions on net photosynthetic rates for water lily versus air temperature at the time the measurements were taken showed a greater increase with temperature for plants grown at a 640 ppm carbon dioxide concentration than for those grown at a 340 ppm carbon dioxide concentration. However, there was also an interaction with solar radiation. The plants grown at 640 ppm of carbon dioxide also showed a much greater response to solar radiation than those grown at 340 ppm. Although the interactions among carbon dioxide treatment level, air temperature, and solar radiation were not resolved, the data show that all were interrelated in the carbon dioxide response. S. G. Allen et al. (1990a,b) also computed linear regressions of photosynthetic rate versus previous minimum air temperatures and previous maximum air tem-

peratures for periods of 1, 3, 6, and 9 days. They found that net photosynthetic carbon dioxide uptake rates were more sensitive to previous minimum air temperatures than to previous maximum air temperatures. (Of course, maximum temperatures are closely related to minimum temperatures.) The slope of the regression increased with the number of previous days included in the calculation of air temperature. Most importantly, Allen et al., also found that leaf photosynthetic rates were more sensitive to previous minimum temperatures than to air temperature at the time the measurements were being made.

Photosynthetic rates of sorghum leaves increased only slightly when the leaves were grown at a high carbon dioxide concentration: the rates for a 640 ppm carbon dioxide concentration were about 10 percent higher than the rates for a 340 ppm temperature (S. G. Allen et al., 1990b). The effect of temperature on photosynthetic rates was also quite small&mdasha response to be expected, since sorghum is a C4 plant. Leaves were about 1.0 to 1.5°C warmer under the 640 ppm carbon dioxide treatment.

Idso et al. (1987) compared growth rates (biomass accumulation rates) of carrot, radish, water hyacinth, Azolla, and cotton grown across a seasonal range of temperatures at carbon dioxide concentrations of 650 ppm with growth rates for these same crops grown at 350 ppm. They found that the ratio of biomass accumulation rates of plants grown at 650 ppm to plants grown at 350 ppm increased somewhat linearly with air temperature across the seasonal mean air temperature range of about 12°C to 36°C. The biomass growth ratio (biomass accumulation at specified elevated carbon dioxide concentration divided by biomass accumulation at a baseline carbon dioxide concentration) increased about 0.087 per 1°C over this temperature range and had a zero intercept at about 18.5°C mean daily air temperature. Under arid zone conditions at Phoenix, Arizona, daily minimum temperatures are 8 to 9°C lower than daily mean temperatures during the months of November to March, when mean air temperatures are well below 18.5°C. Low nocturnal temperatures, as well as low total solar irradiance, short photoperiod (day length), previous carbohydrate storage, and stage of growth of the plants may affect the biomass growth ratio during the winter months.

We may conclude that increasing both carbon dioxide concentrations and temperature will cause a greater increase in biomass productivity than increasing carbon dioxide levels alone. However, Baker et al. (1989) found different biomass growth ratios for both final harvest dry matter and seed yield for soybean grown at 330

and 660 ppm of carbon dioxide. Although early canopy vegetative growth rates suggest that the biomass growth ratio could increase with temperature, the final harvest data showed otherwise. The experiment was conducted with day/night air temperatures of 26/19, 31/24, and 36/29°C, which gave average air temperatures of about 22.8, 27.8, and 32.8°C under the 13/11 hour thermoperiod. The biomass growth ratios for final harvest dry matter were 1.50, 1.36, and 1.24 for the respective temperatures. The biomass growth ratios for final harvest seed yield were 1.46, 1.24, and 1.15 for the respective temperatures. The changes in the biomass growth ratio for dry matter and seed yield were -0.026 (r 2 = 0.98) and -0.031 (r 2 = 0.88) per 1°C, respectively. This cultivar of soybean, ''Bragg,'' has a determinate growth habit that causes vegetative growth to nearly cease when flowering begins. Furthermore, elevated temperatures tend to hasten maturity and shorten the life cycle of this soybean crop. These factors were different from the Arizona study. Furthermore, the study of Baker et al. (1989) had identical light conditions for all treatments, and photoperiod interaction with temperature was minimized by two weeks of supplemental lighting at the beginning of the season.

In summary, the biomass growth ratio of plants grown at elevated carbon dioxide concentrations may increase with increasing temperature for vegetative growth, as suggested by Figure 7.4. However, this response may be reversed for seed grain crops that have a determinate growth habit, such as "Bragg" soybean.

Evapotranspiration

Evapotranspiration refers to the combination of plant transpiration and evaporation directly from the soil surface. Much of the following discussion of evapotranspiration will refer largely to the effects of carbon dioxide and climate on the plant component, which is in general much larger than the soil component except when the LAI is small.

The best modeling studies to date on the simulated effects of climate change and carbon dioxide concentration increase on plant canopy evapotranspiration were conducted by Rosenberg et al. (1990), using the Penman-Monteith model. A similar approach was used by Allen and Gichuki (1989) and Allen et al. (1991) to estimate effects of carbon dioxide-induced climate changes on evapotranspiration and irrigation water requirements in the Great Plains from Texas to Nebraska. Rosenberg et al. (1990) examined the ef-

fects of temperature, net radiation, air vapor pressure, stomatal resistance, and LAI on three types of plant canopies: wheat at Mead, Nebraska grassland at Konza Prairie, Kansas and forest at Oak Ridge, Tennessee. Increasing the temperature by 3°C gave a 6 to 8 percent increase in transpiration per 1°C. This compares reasonably well with the 4 to 5 percent increase in transpiration per 1°C measured experimentally in soybean across the 28 to 35°C range by Jones et al. (1985a). Rosenberg et al. (1990) also reported that evapotranspiration decreased 12 to 17 percent for a 40 percent increase in stomatal resistance. This corresponds closely to a 12 percent decrease in seasonal transpiration obtained experimentally for soybean grown in controlled-environment chambers by Jones et al. (1985c) for doubled carbon dioxide concentration conditions when leaf area index was very similar for both the ambient and doubled carbon dioxide treatments. In another study, Jones et al. (1985b) showed that exposure of soybean canopies to a level of 800 ppm carbon dioxide decreased daily total transpiration by 16 percent in comparison to an exposure level of 330 ppm. Jones et al. attributed this reduction to an increase in stomatal resistance.

Increases in LAI of 15 percent caused increases in predicted evapotranspiration of about 5 to 7 percent according to the model of Rosenberg et al. (1990). These values were comparable to those extracted from Jones et al. (1985b). Their data showed a 33 percent increase in measured daily transpiration for a change in LAI from 3.3 to 6.0 (an 82 percent increase in LAI). However, the effect of LAI may not be linear. By use of a soil-plant-atmosphere model, Shawcraft et al. (1974) showed that the effect of LAI on transpiration would be highly nonlinear across a LAI range of 0 to 8. Most of the effect of changing leaf area occurred across the LAI range of 0 to 4. However, these simulations were conducted with a moist soil surface (having a water potential of -60 MPa) and relatively high soil surface-to-air boundary-layer conductance. Thus, predicted evapotranspiration rates at a LAI of 2 were maintained at 85 percent or more of the rates at a LAI of 8. Nevertheless, the modeling results of Shawcraft et al. (1974) for three solar elevation angles and three leaf elevation angle classes showed that predicted plant transpiration increased by an average of 27 plus or minus 8 percent for a LAI increase from 3.3 to 6.0.

Net radiation could increase under climate change conditions from both greater downwelling thermal radiation and increased solar radiation (decreased cloudiness), or it could decrease from increased cloudiness. Rosenberg et al. (1990) showed that evapotranspiration should change about 0.6 to 0.7 percent for each 1

percent change in net radiation. Likewise, they showed that evapotranspiration should change about -0.4 to -0.8 percent for each 1 percent change in vapor pressure of the air. A combination of several factors gave changes in evapotranspiration ranging from 27 percent (for a case of increased net radiation and decreased vapor pressure) to negative 4 percent (for a case of decreased net radiation and increased vapor pressure. These factors were: a temperature increase of 3°C, net radiation changes of plus or minus 10 percent, vapor pressure changes of plus or minus 10 percent, stomatal resistance increase of 40 percent, and leaf area index increase of 15 percent. Each factor related to climate change and plant response to carbon dioxide affects the predicted evapotranspiration.

ESTIMATING YIELD AND WATER REQUIREMENTS OF CROPS UNDER TWO CURRENT CLIMATE CHANGE SCENARIOS

General Circulation Models

Climate changes under conditions of doubled atmospheric carbon dioxide levels have been predicted using five atmospheric general circulation models (GCMs). These models are the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluids Dynamics Laboratory (GFDL) model developed at Princeton University (Manabe and Wetherald, 1986, 1987), the NASA Goddard Institute for Space Studies (GISS) model developed at Columbia University (Hansen et al., 1984, 1988), the Community Climate Models (CCM) developed at the National Center for Atmospheric Research (NCAR) (Washington and Meehl 1983, 1984, 1986), the Oregon State University (OSU) model (Schlesinger, 1984), and the United Kingdom Meterological Office (UKMO) model (Wilson and Mitchell, 1987 Mitchell, 1989). All of these models predict an increase of global average surface temperatures. The global mean surface temperature increases for recent modeling studies in which the carbon dioxide concentration was doubled were 2.8, 4.0, 4.0, 4.2 and 5.2°C for the OSU, CCM, GFDL, GISS, and UKMO models, respectively (Wilson and Mitchell, 1987). For a carbon dioxide concentration doubling, the global mean precipitation is also predicted to increase by 7.8, 7.1, 8.7, 11.0, and 15.0 percent for the above GCMs, respectively.

Grotch (1988) analyzed four of the GCM climate change scenarios for a climate with doubled carbon dioxide concentration.

Changes predicted across the United States were extracted and summarized, as were global, hemispherical, and other regional changes. The predicted June-July-August (JJA) median temperature increases for the United States were about 3.5, 3.0, 3.8, and 5.6°C for a carbon dioxide doubling from the OSU, CCM, GISS, and GFDL models, respectively. The predicted JJA changes in precipitation for the United States were 4, 10, 8, and -25 percent for the respective models. Thus, the GCM precipitation change scenarios are more variable for the United States than temperature change scenarios and may differ considerably among regions around the world. The UKMO model predictions for reduced precipitation for the United States are somewhat similar to the GFDL model scenario (Wilson and Mitchell, 1987). Higher JJA temperatures for the United States were associated with models with the lowest summer precipitation. As would be expected, both the GFDL model (Manabe and Wetherald, 1987) and the UKMO model (Wilson and Mitchell, 1987) predict serious decreases in soil wetness during the summer for the United States.

All of the GCMs predict a temperature increase for the Unites States for a doubling of atmospheric carbon dioxide. However, the predicted summer precipitation for the United States covers a range of 10 to -25 percent. The possibility of a significant reduction in summer precipitation, coupled with a temperature rise, could pose a serious problem for future agricultural productivity and water resources.

Modeling Crop Responses to Carbon Dioxide and Climate Changes

Many years of experimental observations on the interactions of carbon dioxide and climate factors would be required to provide complete information on responses of plants to climate change. However, plant growth models have been developed that are sensitive to environmental factors such as photoperiod, temperature, soil water availability, and light interception. These models can provide projections of crop response to future climate change scenarios in comparison with baseline climate records of the recent past.

Peart et al. (1989) and Curry et al. (1990a,b) used a soybean crop growth model, SOYGRO (Wilkerson et al., 1983 Jones et al., 1989), and a maize growth model, CERES-maize (Jones and Kiniry, 1986), for predicting growth and yield responses to doubled carbon dioxide climate change scenarios in the southeastern United States.


Individual and interactive effects of ocean acidification, global warming, and UV radiation on phytoplankton

Rising carbon dioxide (CO2) concentrations in the atmosphere result in increasing global temperatures and ocean warming (OW). Concomitantly, dissolution of anthropogenic CO2 declines seawater pH, resulting in ocean acidification (OA) and altering marine chemical environments. The marine biological carbon pump driven by marine photosynthesis plays an important role for oceanic carbon sinks. Therefore, how ocean climate changes affect the amount of carbon fixation by primary producers is closely related to future ocean carbon uptake. OA may upregulate metabolic pathways in phytoplankton, such as upregulating ß-oxidation and the tricarboxylic acid cycle, resulting in increased accumulation of toxic phenolic compounds. Ocean warming decreases global phytoplankton productivity however, regionally, it may stimulate primary productivity and change phytoplankton community composition, due to different physical and chemical environmental requirements of species. It is still controversial how OA and OW interactively affect marine carbon fixation by photosynthetic organisms. OA impairs the process of calcification in calcifying phytoplankton and aggravate ultraviolet (UV)-induced harms to the cells. Increasing temperatures enhance the activity of cellular repair mechanisms, which mitigates UV-induced damage. The effects of OA, warming, enhanced exposure to UV-B as well as the interactions of these environmental stress factors on phytoplankton productivity and community composition, are discussed in this review.

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This study was supported by the National Key Rɭ Program (2016YFA0601400), National Natural Science Foundation (41430967, 41720104005, and 41721005), Joint Project of National Natural Science Foundation of China and Shandong Province (No. U1606404), and the U.S. National Science Foundation (OCE 1538525, 1638804, 1657757, and 1851222).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


References

Crutzen, P. J. The influence of nitrogen oxides on the atmospheric ozone content. Q. J. Royal Meteorol. Soc. 96, 320–325 (1970).

Molina, M. J. & Rowland, F. S. Stratospheric sink for chlorofluoromethanes: chlorine atomic-catalysed destruction of ozone. Nature 249, 810–812 (1974).

Farman, J. C., Gardiner, B. G. & Shanklin, J. D. Large losses of ozone in Antarctica reveal seasonal ClOx/NOx interaction. Nature 315, 207–210 (1985).

Watson, R. T., Prather, M. J. & Kurylo, M. J. Present State of Knowledge of the Upper Atmosphere 1988: An Assessment Report. NASA Reference Publication 1208 (NASA Office of Space Science and Applications, 1988).

Synthesis Report: Integration of the Four Assessment Panels Reports by the Open-Ended Working Group of the Parties to the Montreal Protocol (OEWG, 1989).

Solomon, S., Garcia, R. R., Rowland, F. S. & Wuebbles, D. J. On the depletion of Antarctic ozone. Nature 321, 755–758 (1986).

Solomon, S. Progress towards a quantitative understanding of Antarctic ozone depletion. Nature 347, 347–354 (1990).

Andersen, S. O. & Sarma, K. M. Protecting the Ozone Layer: The United Nations History (Earthscan, 2012).

Newman, P. A. et al. What would have happened to the ozone layer if chlorofluorocarbons (CFCs) had not been regulated? Atmos. Chem. Phys. 9, 2113–2128 (2009).

Mäder, J. A. et al. Evidence for the effectiveness of the Montreal Protocol to protect the ozone layer. Atmos. Chem. Phys. 10, 12161–12171 (2010).

Newman, P. A. & McKenzie, R. UV impacts avoided by the Montreal Protocol. Photochem. Photobiol. Sci. 10, 1152–1160 (2011).

Scientific Assessment of Ozone Depletion: 2018, Global Ozone Research and Monitoring Project. Report no. 58.88 (WMO, 2018).

Updating Ozone Calculations and Emissions Profiles for Use in the Atmospheric and Health Effects Framework Model (USEPA, 2015).

Myhre, G. et al. in IPCC Climate Change 2013: The Physical Science Basis (eds Stocker, T. F. et al.) 661–740 (Cambridge Univ. Press, 2013).

Garcia, R. R., Kinnison, D. E. & Marsh, D. R. ‘World Avoided’ simulations with the Whole Atmosphere Community Climate Model. J. Geophys. Res. Atm. 117, D23303 (2012).

Ripley, K. & Verkuijl, C. ‘Ozone family’ delivers landmark deal for the climate. Environ. Policy Law 46, 371 (2016).

Xu, Y., Zaelke, D., Velders, G. J. M. & Ramanathan, V. The role of HFCs in mitigating 21st century climate change. Atmos. Chem. Phys. 13, 6083–6089 (2013).

Chipperfield, M. P. et al. Quantifying the ozone and ultraviolet benefits already achieved by the Montreal Protocol. Nat. Commun. 6, 7233 (2015).

Velders, G. J., Andersen, S. O., Daniel, J. S., Fahey, D. W. & McFarland, M. The importance of the Montreal Protocol in protecting climate. Proc. Natl Acad.Sci. USA 104, 4814–4819 (2007).

Papanastasiou, D. K., Beltrone, A., Marshall, P. & Burkholder, J. B. Global warming potential estimates for the C1–C3 hydrochlorofluorocarbons (HCFCs) included in the Kigali Amendment to the Montreal Protocol. Atmos. Chem. Phys. 18, 6317–6330 (2018).

IPCC: Summary for Policymakers. In Global Warming of 1.5 °C. IPCC Special Report (IPCC, 2018).

Andrady, A. L., Pandey, K. K. & Heikkilä, A. M. Interactive effects of solar UV radiation and climate change on material damage. Photochem. Photobiol. Sci. 18, 804–825 (2019).

Lucas, R. M. et al. Human health in relation to exposure to solar ultraviolet radiation under changing stratospheric ozone and climate. Photochem. Photobiol. Sci. 18, 641–680 (2019).

Bornman, J. F. et al. Linkages between stratospheric ozone, UV radiation and climate change and their implications for terrestrial ecosystems. Photochem. Photobiol. Sci. 18, 681–716 (2019).

Williamson, C. E. et al. The interactive effects of stratospheric ozone depletion, UV radiation, and climate change on aquatic ecosystems. Photochem. Photobiol. Sci. 18, 717–746 (2019).

Sulzberger, B., Austin, A. T., Cory, R. M., Zepp, R. G. & Paul, N. D. Solar UV radiation in a changing world: roles of cryosphere–land–water–atmosphere interfaces in global biogeochemical cycles. Photochem. Photobiol. Sci. 18, 747–774 (2019).

Bais, A. F. et al. Ozone–climate interactions and effects on solar ultraviolet radiation. Photochem. Photobiol. Sci. 18, 602–640 (2019).

Wilson, S. R., Madronich, S., Longstreth, J. D. & Solomon, K. R. Interactive effects of changing stratospheric ozone and climate on composition of the troposphere, air quality, and consequences for human and ecosystem health. Photochem. Photobiol. Sci. 18, 775–803 (2019).

IPCC Climate Change 2014: Synthesis Report (eds Core Writing Team, Pachauri, R. K. & Meyer L. A.) (IPCC, 2014).

Arblaster, J. et al. In Scientific Assessment of Ozone Depletion: 2014. Global Ozone Research and Monitoring Project Report No. 55, Ch. 4 (WMO, 2014).

Langematz, U. et al. In Scientific Assessment of Ozone Depletion: 2018. Global Ozone Research and Monitoring Project Report No. 58, Ch. 4 (WMO, 2018).

Clem, K. R., Renwick, J. A. & McGregor, J. Relationship between eastern tropical Pacific cooling and recent trends in the Southern Hemisphere zonal-mean circulation. Clim. Dyn. 49, 113–129 (2017).

Lim, E. P. et al. The impact of the Southern Annular Mode on future changes in Southern Hemisphere rainfall. Geophys. Res. Lett. 43, 7160–7167 (2016).

Holz, A. et al. Southern Annular Mode drives multicentury wildfire activity in southern South America. Proc. Natl Acad. Sci. USA 114, 9552–9557 (2017).

Kostov, Y. et al. Fast and slow responses of Southern Ocean sea surface temperature to SAM in coupled climate models. Clim. Dyn. 48, 1595–1609 (2017).

Oliveira, F. N. M. & Ambrizzi, T. The effects of ENSO-types and SAM on the large-scale southern blockings. Int. J. Climatol. 37, 3067–3081 (2017).

Robinson, S. A. et al. Rapid change in East Antarctic terrestrial vegetation in response to regional drying. Nat. Clim. Change 8, 879–884 (2018).

Robinson, S. A. & Erickson, D. J. III Not just about sunburn—the ozone hole’s profound effect on climate has significant implications for Southern Hemisphere ecosystems. Glob. Change Biol. 21, 515–527 (2015).

Morgenstern, O. et al. Review of the global models used within phase 1 of the Chemistry–Climate Model Initiative (CCMI). Geosci. Model Dev. 10, 639–671 (2017).

Williamson, C. E. et al. Solar ultraviolet radiation in a changing climate. Nat. Clim. Change 4, 434–441 (2014).

IPCC Climate Change 2013: The Physical Science Basis (eds Stocker, T. F. et al.) (Cambridge Univ. Press, 2013).

López, M. L., Palancar, G. G. & Toselli, B. M. Effects of stratocumulus, cumulus, and cirrus clouds on the UV-B diffuse to global ratio: experimental and modeling results. J. Quant. Spectrosc. Radiat. Transf. 113, 461–469 (2012).

Feister, U., Cabrol, N. & Häder, D. UV irradiance enhancements by scattering of solar radiation from clouds. Atmosphere 6, 1211–1228 (2015).

Williamson, C. E. et al. Sentinel responses to droughts, wildfires, and floods: effects of UV radiation on lakes and their ecosystem services. Front. Ecol. Environ. 14, 102–109 (2016).

Gies, P., Roy, C., Toomey, S. & Tomlinson, D. Ambient solar UVR, personal exposure and protection. J. Epidemiol. 9, S115–S122 (1999).

Xiang, F. et al. Weekend personal ultraviolet radiation exposure in four cities in Australia: influence of temperature, humidity and ambient ultraviolet radiation. J. Photochem. Photobiol. B 143, 74–81 (2015).

Cuthill, I. C. et al. The biology of color. Science 357, eaan0221 (2017).

Mazza, C. A., Izaguirre, M. M., Curiale, J. & Ballaré, C. L. A look into the invisible. Ultraviolet-B sensitivity in an insect (Caliothrips phaseoli) revealed through a behavioural action spectrum. Proc. R. Soc. B 277, 367–373 (2010).

IPCC Climate Change 2014: Impacts, Adaptation, and Vulnerability (eds Field, C. B. et al.) (Cambridge Univ. Press, 2014).

Steinbauer, M. J. et al. Accelerated increase in plant species richness on mountain summits is linked to warming. Nature 556, 231–234 (2018).

Urmy, S. S. et al. Vertical redistribution of zooplankton in an oligotrophic lake associated with reduction in ultraviolet radiation by wildfire smoke. Geophys. Res. Lett. 43, 3746–3753 (2016).

Ma, Z., Li, W., Shen, A. & Gao, K. Behavioral responses of zooplankton to solar radiation changes: in situ evidence. Hydrobiologia 711, 155–163 (2013).

Leach, T. H., Williamson, C. E., Theodore, N., Fischer, J. M. & Olson, M. H. The role of ultraviolet radiation in the diel vertical migration of zooplankton: an experimental test of the transparency-regulator hypothesis. J. Plankton Res. 37, 886–896 (2015).

Fischer, J. M. et al. Diel vertical migration of copepods in mountain lakes: the changing role of ultraviolet radiation across a transparency gradient. Limnol. Oceanogr. 60, 252–262 (2015).

Cohen, J. M., Lajeunesse, M. J. & Rohr, J. R. A global synthesis of animal phenological responses to climate change. Nat. Clim. Change 8, 224–228 (2018).

Predick, K. I. et al. UV-B radiation and shrub canopy effects on surface litter decomposition in a shrub-invaded dry grassland. J. Arid Environ. 157, 13–21 (2018).

Kauko, H. M. et al. Windows in Arctic sea ice: light transmission and ice algae in a refrozen lead. J. Geophys. Res. Biogeosci. 122, 1486–1505 (2017).

Williamson, C. E. et al. Climate change-induced increases in precipitation are reducing the potential for solar ultraviolet radiation to inactivate pathogens in surface waters. Sci. Rep. 7, 13033 (2017).

Arnold, M. et al. Global burden of cutaneous melanoma attributable to ultraviolet radiation in 2012. Int. J. Cancer 143, 1305–1314 (2018).

van Dijk, A. et al. Skin cancer risks avoided by the Montreal Protocol—worldwide modeling integrating coupled climate–chemistry models with a risk model for UV. Photochem. Photobiol. 89, 234–246 (2013).

Flaxman, S. R. et al. Global causes of blindness and distance vision impairment 1990–2020: a systematic review and meta-analysis. Lancet Glob. Health 5, e1221–e1234 (2017).

Sandhu, P. K. et al. Community-wide interventions to prevent skin cancer: two community guide systematic reviews. Am. J. Prev. Med. 51, 531–539 (2016).

Gordon, L. G. & Rowell, D. Health system costs of skin cancer and cost-effectiveness of skin cancer prevention and screening: a systematic review. Eur. J. Cancer Prev. 24, 141–149 (2015).

Hodzic, A. & Madronich, S. Response of surface ozone over the continental United States to UV radiation. Nat. Clim. Atmos. Sci. 1, 35 (2018).

Ballaré, C. L., Caldwell, M. M., Flint, S. D., Robinson, S. A. & Bornman, J. F. Effects of solar ultraviolet radiation on terrestrial ecosystems. Patterns, mechanisms, and interactions with climate change. Photochem. Photobiol. Sci. 10, 226–241 (2011).

Uchytilova, T. et al. Ultraviolet radiation modulates C:N stoichiometry and biomass allocation in Fagus sylvatica saplings cultivated under elevated CO2 concentration. Plant Physiol. Biochem. 134, 103–112 (2018).

Robson, T. M., Hartikainen, S. M. & Aphalo, P. J. How does solar ultraviolet-B radiation improve drought tolerance of silver birch (Betula pendula Roth.) seedlings? Plant Cell Environ. 38, 953–967 (2015).

Jenkins, G. I. Photomorphogenic responses to ultraviolet-B light. Plant Cell Environ. 40, 2544–2557 (2017).

Šuklje, K. et al. Effect of leaf removal and ultraviolet radiation on the composition and sensory perception of Vitis vinifera L. cv. Sauvignon Blanc wine. Aust. J. Grape Wine Res. 20, 223–233 (2014).

Escobar-Bravo, R., Klinkhamer, P. G. L. & Leiss, K. A. Interactive effects of UV-B light with abiotic factors on plant growth and chemistry, and their consequences for defense against arthropod herbivores. Front. Plant Sci. 8, 278 (2017).

Ballaré, C. L., Mazza, C. A., Austin, A. T. & Pierik, R. Canopy light and plant health. Plant Physiol. 160, 145–155 (2012).

Wargent, J. J. in The Role of UV-B Radiation in Plant Growth and Development (ed. Jordan, B. R.) 162–176 (CABI, 2017).

Zagarese, H. E. & Williamson, C. E. The implications of solar UV radiation exposure for fish and fisheries. Fish. Fish. 2, 250–260 (2001).

Tucker, A. J. & Williamson, C. E. The invasion window for warmwater fish in clearwater lakes: the role of ultraviolet radiation and temperature. Divers. Distrib. 20, 181–192 (2014).

Neale, P. J. & Thomas, B. C. Inhibition by ultraviolet and photosynthetically available radiation lowers model estimates of depth-integrated picophytoplankton photosynthesis: global predictions for Prochlorococcus and Synechococcus. Glob. Chang. Biol. 23, 293–306 (2017).

Garcia-Corral, L. S. et al. Effects of UVB radiation on net community production in the upper global ocean. Glob. Ecol. Biogeogr. 26, 54–64 (2017).

Cory, R. M., Ward, C. P., Crump, B. C. & Kling, G. W. Sunlight controls water column processing of carbon in arctic fresh waters. Science 345, 925–928 (2014).

Austin, A. T., Méndez, M. S. & Ballaré, C. L. Photodegradation alleviates the lignin bottleneck for carbon turnover in terrestrial ecosystems. Proc. Natl Acad. Sci. USA 113, 4392–4397 (2016).

Almagro, M., Maestre, F. T., Martínez-López, J., Valencia, E. & Rey, A. Climate change may reduce litter decomposition while enhancing the contribution of photodegradation in dry perennial Mediterranean grasslands. Soil Biol. Biochem. 90, 214–223 (2015).

Lindholm, M., Wolf, R., Finstad, A. & Hessen, D. O. Water browning mediates predatory decimation of the Arctic fairy shrimp Branchinecta paludosa. Freshw. Biol. 61, 340–347 (2016).

Cuyckens, G. A. E., Christie, D. A., Domic, A. I., Malizia, L. R. & Renison, D., Climate change. and the distribution and conservation of the world’s highest elevation woodlands in the South American Altiplano. Glob. Planet. Change 137, 79–87 (2016).

Poste, A. E., Braaten, H. F. V., de Wit, H. A., Sørensen, K. & Larssen, T. Effects of photodemethylation on the methylmercury budget of boreal Norwegian lakes. Environ. Toxicol. Chem. 34, 1213–1223 (2015).

Tsui, M. M. et al. Occurrence, distribution, and fate of organic UV filters in coral communities. Environ. Sci. Technol. 51, 4182–4190 (2017).

Corinaldesi, C. et al. Sunscreen products impair the early developmental stages of the sea urchin Paracentrotus lividus. Sci. Rep. 7, 7815 (2017).

Fong, H. C., Ho, J. C., Cheung, A. H., Lai, K. & William, K. Developmental toxicity of the common UV filter, benophenone-2, in zebrafish embryos. Chemosphere 164, 413–420 (2016).

Willenbrink, T. J., Barker, V. & Diven, D. The effects of sunscreen on marine environments. Cutis 100, 369 (2017).

Clark, J. R. et al. Marine microplastic debris: a targeted plan for understanding and quantifying interactions with marine life. Front. Ecol. Environ. 14, 317–324 (2016).

UNEP Frontiers: 2016 Report. Emerging Issues of Environmental Concern (UNEP, 2016).

Frank, H., Christoph, E. H., Holm-Hansen, O. & Bullister, J. L. Trifluoroacetate in ocean waters. Environ. Sci. Technol. 36, 12–15 (2002).

Solomon, K. R. et al. Sources, fates, toxicity, and risks of trifluoroacetic acid and its salts: relevance to substances regulated under the Montreal and Kyoto Protocols. J. Toxicol. Environ. Health B 19, 289–304 (2016).

Fleming, E. L., Jackman, C. H., Stolarski, R. S. & Douglass, A. R. A model study of the impact of source gas changes on the stratosphere for 1850–2100. Atmos. Chem. Phys. 11, 8515–8541 (2011).

Eyring, V. et al. Long-term ozone changes and associated climate impacts in CMIP5 simulations. J. Geophys. Res. Atm. 118, 5029–5060 (2013).

Montzka, S. A. et al. An unexpected and persistent increase in global emissions of ozone-depleting CFC-11. Nature 557, 413–417 (2018).

Crutzen, P. J. Albedo enhancement by stratospheric sulfur injections: a contribution to resolve a policy dilemma? Clim. Change 77, 211–220 (2006).

Tilmes, S. et al. Impact of very short-lived halogens on stratospheric ozone abundance and UV radiation in a geo-engineered atmosphere. Atmos. Chem. Phys. 12, 10945–10955 (2012).

Nowack, P. J., Abraham, N. L., Braesicke, P. & Pyle, J. A. Stratospheric ozone changes under solar geoengineering: implications for UV exposure and air quality. Atmos. Chem. Phys. 16, 4191–4203 (2016).

Madronich, S., Tilmes, S., Kravitz, B., MacMartin, D. & Richter, J. Response of surface ultraviolet and visible radiation to stratospheric SO2 injections. Atmosphere 9, 432 (2018).

Kayler, Z. E. et al. Experiments to confront the environmental extremes of climate change. Front. Ecol. Environ. 13, 219–225 (2015).

Pecl, G. T. et al. Biodiversity redistribution under climate change: impacts on ecosystems and human well-being. Science 355, eaai9214 (2017).

Millenium Ecosystem Assessment. Ecosystems and Human Well-being: Our Human Planet Summary for Decision-makers, Vol. 5 (Island, 2005).

NASA Institute for Space Studies. GISS Surface Temperature Analysis (GISTEMP) (GISTEMP, accessed 24 July 2018) https://data.giss.nasa.gov/gistemp/

Hansen, J., Ruedy, R., Sato, M. & Lo, K. Global surface temperature change. Rev. Geophys. 48, RG4004 (2010).


Ask the Experts: Does Rising CO2 Benefit Plants?

Climate change skeptics have an arsenal of arguments for why humans need not cut their carbon emissions. Some assert rising CO2 levels benefit plants, so global warming is not as bad as scientists proclaim. &ldquoA higher concentration of carbon dioxide in our atmosphere would aid photosynthesis, which in turn contributes to increased plant growth,&rdquo Rep. Lamar Smith (R&ndashTexas) wrote in an op-ed last year. &ldquoThis correlates to a greater volume of food production and better quality food.&rdquo Scientists and others calling for emission cuts are being hysterical, he contends.

So is it true rising atmospheric CO2 will help plants, including food crops? Scientific American asked several experts to talk about the science behind this question.

There is a kernel of truth in this argument, experts say, based on what scientists call the CO2 fertilization effect. &ldquoCO2 is essential for photosynthesis,&rdquo says Richard Norby, a corporate research fellow in the Environmental Sciences Division and Climate Change Science Institute of Oak Ridge National Laboratory. &ldquoIf you isolate a leaf [in a laboratory] and you increase the level of CO2, photosynthesis will increase. That&rsquos well established.&rdquo But Norby notes the results scientists produce in labs are generally not what happens in the vastly more complex world outside many other factors are involved in plant growth in untended forests, fields and other ecosystems. For example, &ldquonitrogen is often in short enough supply that it&rsquos the primary controller of how much biomass is produced&rdquo in an ecosystem, he says. &ldquoIf nitrogen is limited, the benefit of the CO2 increase is limited&hellip. You can&rsquot just look at CO2, because the overall context really matters.&rdquo

Scientists have observed the CO2 fertilization effect in natural ecosystems, including in a series of trials conducted over the past couple decades in outdoor forest plots. In those experiments artificially doubling CO2 from pre-industrial levels increased trees&rsquo productivity by around 23 percent, according to Norby, who was involved in the trials. For one of the experiments, however, that effect significantly diminished over time due to a nitrogen limitation. That suggests &ldquowe cannot assume the CO2 fertilization effect will persist indefinitely,&rdquo Norby says.

In addition to ignoring the long-term outlook, he says, many skeptics also fail to mention the potentially most harmful outcome of rising atmospheric CO2 on vegetation: climate change itself. Its negative consequences&mdashsuch as drought and heat stress&mdashwould likely overwhelm any direct benefits that rising CO2 might offer plant life. &ldquoIt&rsquos not appropriate to look at the CO2 fertilization effect in isolation,&rdquo he says. &ldquoYou can have positive and negative things going at once, and it&rsquos the net balance that matters.&rdquo So although there is a basic truth to skeptics&rsquo claim, he says, &ldquowhat&rsquos missing from that argument is that it&rsquos not the whole picture.&rdquo

Scientists have also looked specifically at the effects of rising CO2 on agricultural plants and found a fertilization effect. &ldquoFor a lot of crops, [more CO2] is like having extra material in the atmosphere that they can use to grow,&rdquo says Frances Moore, an assistant professor of environmental science and policy at the University of California, Davis. She and other experts note there is an exception for certain types of plants such as corn, which access CO2 for photosynthesis in a unique way. But for most of the other plants humans eat&mdashincluding wheat, rice and soybeans&mdash&ldquohaving higher CO2 will help them directly,&rdquo Moore says. Doubling CO2 from pre-industrial levels, she adds, does boost the productivity of crops like wheat by some 11.5 percent and of those such as corn by around 8.4 percent.

A lack of nitrogen or other nutrients does not affect agricultural plants as much as wild ones, thanks to fertilizer. Still, research shows plants &ldquoget some benefits early on from higher CO2, but that [benefit] starts to saturate&rdquo after the gas reaches a certain level, Moore says&mdashadding, &ldquoThe more CO2 you have, the less and less benefit you get.&rdquo And while rising carbon dioxide might seem like a boon for agriculture, Moore also emphasizes any potential positive effects cannot be considered in isolation, and will likely be outweighed by many drawbacks. &ldquoEven with the benefit of CO2 fertilization, when you start getting up to 1 to 2 degrees of warming, you see negative effects,&rdquo she says. &ldquoThere are a lot of different pathways by which temperature can negatively affect crop yield: soil moisture deficit [or] heat directly damaging the plants and interfering with their reproductive process.&rdquo On top of all that, Moore points out increased CO2 also benefits weeds that compete with farm plants.

Rising CO2&rsquos effect on crops could also harm human health. &ldquoWe know unequivocally that when you grow food at elevated CO2 levels in fields, it becomes less nutritious,&rdquo notes Samuel Myers, principal research scientist in environmental health at Harvard University. &ldquo[Food crops] lose significant amounts of iron and zinc&mdashand grains [also] lose protein.&rdquo Myers and other researchers have found atmospheric CO2 levels predicted for mid-century&mdasharound 550 parts per million&mdashcould make food crops lose enough of those key nutrients to cause a protein deficiency in an estimated 150 million people and a zinc deficit in an additional 150 million to 200 million. (Both of those figures are in addition to the number of people who already have such a shortfall.) A total of 1.4 billion women of child-bearing age and young children who live in countries with a high prevalence of anemia would lose more than 3.8 percent of their dietary iron at such CO2 levels, according to Meyers.

Researchers do not yet know why higher atmospheric CO2 alters crops&rsquo nutritional content. But, Myers says, &ldquothe bottom line is, we know that rising CO2 reduces the concentration of critical nutrients around the world,&rdquo adding that these kinds of nutritional deficiencies are already significant public health threats, and will only worsen as CO2 levels go up. &ldquoThe problem with [the skeptics&rsquo] argument is that it&rsquos as if you can cherry-pick the CO2 fertilization effect from the overall effect of adding carbon dioxide to the atmosphere,&rdquo Myers says. But that is not how the world&mdashor its climate&mdashworks.


UK National Curriculum Links

KS3 chemistry – the carbon cycle

KS3 chemistry – the production of carbon dioxide by human activity and the impact on climate.

KS3 biology – the dependence of almost all life on Earth on the ability of photosynthetic organisms, such as plants and algae, to use sunlight in photosynthesis to build o/a
rganic molecules that are an essential energy store and to maintain levels of oxygen and carbon dioxide in the atmosphere

KS3 biology – how organisms affect, and are affected by, their environment, including the accumulation of toxic materials.

GCSE chemistry/ combined science – evaluate the evidence for additional anthropogenic causes of climate change, including the correlation between change in atmospheric carbon dioxide concentration and the consumption of fossil fuels, and describe the uncertainties in the evidence base

GCSE chemistry/combined science – describe the potential effects of increased levels of carbon dioxide and methane on the Earth’s climate and how these effects may be mitigated, including consideration of scale, risk and environmental implications

GCSE physics – explain that all bodies emit radiation and that the intensity and wavelength distribution of any

GCSE physics – explain how the temperature of a body is related to the balance between incoming radiation absorbed and radiation emitted illustrate this balance using everyday examples and the example of the factors which determine the temperature of the earth.

GCSE Biology/ Combined Science

– explain the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis

– explain the interaction of these factors in limiting the rate of photosynthesis.

– explain the importance of the carbon cycle and the water cycle to living organisms

– evaluate the evidence for the impact of environmental changes on the distribution of organisms, with reference to water and atmospheric gases.



Comments:

  1. Volkree

    It is a pity, that now I can not express - I am late for a meeting. But I will return - I will necessarily write that I think on this question.

  2. Garrson

    It is the usual conditionality

  3. Hadar

    Your phrase is very good

  4. Bressal

    Is this a prank?

  5. Nastas

    Not logically

  6. Mazuzahn

    You have hit the spot. I like this idea, I completely agree with you.

  7. Takinos

    funny on sunday



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