Is carbon di,oxide a ‘good’ greenhouse gas?frst100.forestry.ubc.ca/files/2013/09/Discussion... · transform CO2 and water into sugars. All else being equal, augmenting the resources

Carbon dioxide (CO,) in the atmosphere, a greenhouse gas, may slso provide benefits for mankind: many plants grow better under increasing CO2 concentrations. Indlvlduals have speculated that agricultural yields will increase up to 30% under future CO2 concentrations, and natural ecosystems will become more lush and resilient. However, Infertile conditions and compkx ecological interactions often limit improved plant growth under increasing CO2 atmospheres. Future policies to adapt to a CO,-rich world must not overstate benefits from these conditions, nor ignore their usefulness for increasing future agricuttural yields or restoring degmded habitats.

The authors are with the Department of Organismic and Evolutionary Biology, Har- vard University, 16 Divinity Avenue, Cam- bridge, MA 02136, USA.

We thank the US Department of Energy for supporting research on the direct effects of elevated CO* concentrations on vegetation. Comments by S. Bassow of Harvard University and especially Dr C. Rosenz- weig of the Goddard Institute for Space Studies at Columbia University greatly improved the manuscript. Any remaining errors are the full responsibility of the authors.

Edltor’s note This article provides an ecologist’s per+ pective on the impacts and consequences of CO*-rich environments. It is a response to Sherwood B. Idso’s Viewpoint article

continued on page 302

Is carbon di,oxide a ‘good’ greenhouse gas?

Effects of increasing carbon dioxide on ecological systems

Eric D. Fajer and F.A. Bazzaz

As interest in the impacts of anticipated global climate change increases, policy makers and the public at large are being confronted with a wealth of information and misinformation about potential future impacts. From these inputs, hazy crystal balls have yielded global change forecasts ranging from the apocalyptic, in which warming temperatures and disrupted hydrological cycles threaten civilization with coastal inundation, agricultural disruption, and cataclysmic species extinctions,* to the sanguine, in which potential temperature increases are neutralized by negative feedbacks, and increasing atmospheric carbon dioxide (CO-J concentrations fertilize plant growth, thereby greatly increasing agricultural productivity and the resiliency of natural habitats.2 Sophisticated policy analysts often view the global climate change issue as intermediate between these two extreme scenarios, recognizing that profound negative environmental surprises can ensue from anthropogenic atmospheric alterations (ie stratospheric ozone loss), yet appreciating that the uncertainties of climate models and possible beneficial impacts of our changing atmosphere may render substantial expenditures to combat greenhouse gas emissions wasteful, if not inappropriate.’

How likely are these alternative scenarios of global change? The probability of the apocalyptic scenario, if it can be reasonably esti- mated, will help determine how much societies should invest to limit its occurrence. It may also be unwise to dismiss casually all elements of the sanguine scenario of a greening of Earth, even if it seems simplistic or merely too good to be true. Parts of the sanguine scenario may be realistic and should be incorporated into policies to adapt to changing climatic, as well as demographic, conditions.

In this paper we examine the ecological underpinnings of the sanguine scenario. We explore whether CO 2, in contrast to methane, ozone, chlorofluorocarbons (CFCs) and nitrous oxide, is a ‘good’ greenhouse gas, which, in increased atmospheric concentrations, will lead to a

OQ59-3760~92/040301-10 @ 1992 Butterworth-Heinemann Ltd 301

Is carbon dioxide a ‘good’ greenhouse gas?

continued from page 30 1 ‘Carbon dioxide and the fate of Earth’, Global Environmental Change, Vol 1, No 3, June 1991, pp 178-182.

‘Michael Oppenheimer and Robert H. Boyle, Dead Heat, Basic Books, New York, 1990; Stephen H. Schneider, Global basil: Are We Entering the Green- house Century?, Sierra Club Books, San Francisco, CA, 1989. %herwood 8. Idso, ‘Carbon dioxide and the fate of Earth’, Global Environmental Change, Vol 1, No 3, June 1991, pp 178 182. 3For example, ‘Count before you leap’, fhe Ec~omist, 7 July 1999, pp 21-24. ‘Idso, op tit, Ref 2. 5Fakhri A. Bazzaz, ‘The response of natural ecosystems to the rising global CO:! levels’, Annual Review of Ecology and Systematics, Vol 21, 1990, pp 167-196. H.A. Mooney, B.G. Drake, R.J. Luxmoore, W.C. Oechel, and L.F. Pitelka, ‘Predicting ecosystem responses to elevated CO* concentrations’, ffi&cience, Vol 41, No 2, 1991, pp 96-104; Boyd R. Strain and Jennifer D. Cure, Direct Effects of Incfeas- ing Carbon Dioxide on Vegetation, US Department of Energy, Washington, DC, 1985; Boyd R. Strain, ‘Direct effects of increasing atmospheric COP on piants and ecosystems’, Trends in Ecology and Evo/ution, Vol2, NO 1, 1987, pp 18-21. 6J.T. Houghton, G.J. Jenkins, and J.J. Ephraums, Climate Change.’ The lPCC Scientific Assessment, Cambridge Uni- versity Press, Cambridge, 1990. (Note that some General Circulation Models, such as the Goddard Institute for Space Studies (GISS) model, expect double equivalent atmospheric CO;! conditions to cause significant temperature increases in tropical regions - further, significant alterations in regional hydrological regimes (ie more frequent droughts and storms) may ensue even with reduced global warming.) J. Hansen, 0. Rind, A. DelGenio, A. Lacis, S. Lebedeff, M. Prather, R. Ruedy, and T. Karl, ‘Regional greenhouse climate effects’, in John C. Topping, ed, Coping with Climate Change, Proceedings of the Second North American Conference on Preparing for Climate Change, Climate In- stitute. Washinat~, DC. 1989, RR 68-81. ‘Ban&, op cit:Ref 5; Strain anb‘cure, op cit. Ref 5; Strain, op cit. Ref 5; Mooney et al, op tit, Ref 5. *Walter C. Oechel and Boyd R. Strain, ‘Native species responses to increased atmospheric carbon dioxide concentrations’, in Boyd R. Strain and Jennifer D. Cure, eds, Direct Effects of Increasing Carbon Dioxide on V~etation, US Depart- ment of Energy, Washington, DC, 1985, pp 118-154. ‘Houghton et al, op tit, Ref 6. “Ibid. “Bazzaz, op tit, Ref 5; Bruce A. Kimball, Carbon Dioxide and Agricultural Yield: An Assemblage and Analysis of 770 Prior


greening of Earth and an improvement in the fate of humankind. If CO:! is unconditionally a good greenhouse gas, then efforts and expenditures to limit its atmospheric concentration will be counter-productive to improving the human condition. However, if the benefits resulting from increasing COz conditions, such as increased crop yields, are realized only under limited circumstances, and/or if certain environmental adverse impacts, such as biological diversity reductions, accompany the benefits, then we should not categorically promote the desirability of increasing atmospheric CO2 concentrations.4 Instead, we must incorporate scientific findings of when natural and man-made ecosystems might benefit from anticipated CO*-rich atmospheres into policies to adapt to future environments.

Here, we report what recent scientific evidence tells us about how anticipated COz-rich atmospheres will affect natural and man-made ecosystems, even in the absence of climate change. This is a legitimate line of enquiry because theoretical considerations and experimental evidence show that CO*-rich atmospheres have real and considerably direct impacts on vegetation, 5 and therefore on its wild, domesticated and human consumers. Further, only atmospheric CO:! concentrations, and not global temperatures, may increase significantly within the next 50 years or so, if the thermal capacitance of oceans delays global warming, if one accepts the low end of the ‘Business as Usual’ estimate of global warming of the Intergovernmental Panel on Climate Change (IPCC) - l.O”C increase every 50 years - or if one examines the effects on specific global regions, such as the tropics.”

CO2 fertilization and other positive effects

The fundamental biochemical process underlying life on Earth is photosynthesis: the ability of plants and algae to use light energy to transform CO2 and water into sugars. All else being equal, augmenting the resources necessary for photosynthesis, such as through the provi- sion of additional CO2 via fossil fuel burning and forest clearing, should increase the amount of sugars (food) plants make for themselves during photosynthesis, thereby increasing their growth.’ Further biochemical considerations also suggest that increasing atmospheric COz concentrations should ‘fertilize’ plant growth: at higher COz concentration, the rate of plant photosynthesis increases for most plant species such that more sugars are produced per unit (quanta) of light absorbed. In some experiments, photosynthetic efficiency increases 50% for plants grown in COz-rich envir0nments.s

Not surprisingly, then, copious short-term experiments have demonstrated that individual plants exposed to COz-rich atmospheres (around 650-700 ppm - atmospheric concentration expected around 2075 by IPCC’s Business as Usual scenario”) usually grow significantly larger than their counterparts grown under current atmospheric concentrations (ie about 350 ppm).‘* Both wild and agricultural plants show this response to COz-rich environments: under these conditions, wild plants tested increased their vegetative growth (ie shoots, leaves, stems, roots) by an average of 13%, and their reproductive output (ie flowers, seeds, etc) by an average of 31%, while grain yields increased, on average, by 34%.”

In addition io CO+nduced short-term growth enhancement, called the ‘CO2 fertilization effect’, other benefits accrue to plants growing in

302 GLOBAL ENVIRONMENTAL CHANGE December 1992

continued from page 302 Observations, Water Conservation Laboratory Report No 14, 1983, US Water Conservation Laboratory, Phoenix, AZ, 1983. “Boyd R. Strain and Fakhri A. Bazzaz, ‘Terrestrial plant communities’, in Edgar Ft. Lemon, ed, CO, and Plants: The Re- sponse of Plants to Rising Levels of Atmospheric Carbon Dioxide, Westview Press, Boulder, CO, 1983, pp 177-222; R.J. Luxmoore, ‘CO2 and phytomass’, Bioscience, Vol 31, No 9, 1981, p 828; R.B. Thomas, D.D. Richter, H. Ye, P.R. Heine, and B.R. Strain, ‘Nitrogen dyna- mics and growth of seedlings of an N-fixing tree (Gliricida sepium (Jacq.) Walp.) exposed to elevated carbon dioxide’, Oecolo- yia, Vol 88, No 3, 1991, pp 415421. 3S.C. Wong, ‘Elevated atmospheric par-

tial pressure of CO2 and plant growth’, Oecofogia, Vol44, No 1, 1979, pp 68-74; see also Strain and Cure, op tit, Ref 5; Strain, op tit, Ref 5; Bazzaz, op tit, Ref 5. IdPeter M. Ray, The Living P/ant, Sander College Publishing, Philadelphia, PA, 1972. 15Roger W. Carlson and Fakhri A. Bazzaz, ‘The effects of elevated CO2 concentrations on growth, photosynthesis, transpira- tion, and water use efficiency of plants’, in J.J. Singh and A. Deepak, eds, Environ- mental and Climatic Impact of Coal Utiliza- tion, Academic Press, New York, 1989, pp 609-623. “%arlson and Bazzaz, op tit, Ref 15. “Roger W. Carlson and Fakhri A. Bazzaz. ‘Photosynthetic and growth response to fumigation with SO2 at elevated CO2 for C3 and C4 plants’, Oecotogia, Vol 54, No 1, 1982, pp 5&54. “W D Bowman and B.R. Strain, ‘Interac- . . tion between CO2 enrichment and salinity stress in the Cq halophyte Andropogon glomeratus (Walter) BSP’, Plant, Cell and Environment, Vol 10, 1987, pp 267-270. “L.H. Ziska, B.G. Drake, and S. Chamber- lain, ‘Long-term photosynthetic response in single leaves of a C3 and Cq salt marsh species grown at elevated atmospheric do2 in situ’, Oecofogia, Vol 83, No 4, 1990. DD 469-472: P.D. Curtis. B.G. Drake, ‘P.W. Leadley, W. Arp, and D. Whigham, ‘Growth and senescence of plant communities exposed to elevated CO2 concentrations on an estuarine marsh’, Oecotogia, Vol 78, No 1, 1989. pp 2&26; Sherwood B. ldso and Bruce A. Kimball, ‘Effects of atmospheric CO2 enrichment on photosynthesis, respiration and growth of sour orange trees’, Plant Physkgy, Vol 99, No l,-1992, pp 341- 343: David T. Tissue and Walter C. Oe&el, ‘Response of Eriophorum vagina- turn to elevated CO2 and temperature in the Alaskan tussock tundra’, Ecology, Vol 68, No 2, pp 401410: Richard J. Norby, Carla A. Gunderson, Stan D. Wullschleger, E.G. O’Neil, and Mary K. McCracken, ‘Pro- ductivity and compensatory responses of yellow-poplar trees in elevated COP’. Na-



CO&h environments. Plants can use the additional sugars made under higher CO2 concentrations to feed symbiotic organisms associated with their roots which enhance plant nutrition.‘* The two most important symbiotic organisms are mycorrhizal fungi (eg many forest mushrooms), which attach themselves to plant roots and help plants forage the soil for nutrients, especially phosphorous, and nitrogen-fixing bacteria, associated with certain plants such as legumes (eg soybeans) and alders, which convert normally inaccessible nitrogen from the atmosphere into forms which plants can absorb and use, such as nitrate. In a CO*-rich atmosphere, plants associated with root symbionts will have more sugars to feed them and in exchange should receive additional nutrients, thereby improving their growth.

In addition to producing more sugars, virtually all plants consume less water per unit leaf area when grown in higher CO2 concentrations.‘” Normally, in the process of letting CO2 diffuse into the interior of leaves (where it is taken up as part of the photosynthetic process), water is lost to the outside air at a rate of 100-400 water molecules to every CO2 molecule taken in.14 Under CO*-rich conditions, this rate of water loss can be greatly curtailed (eg by 30% for soybeans and 50% for maize”), thereby reducing the amount of water a plant needs to develop: it has been suggested that this increased water-use efficiency will enable plants to expand their ranges into drier areas.16

A CO*-rich atmosphere also promotes a more subtle way to increase plant growth. The same physiological mechanism which limits water loss under CO*-rich conditions also protects plants from some pollutant damage. For example, for many plant species, damage from sulphur dioxide (SO,), a common emission from coal burning and an important antagonist causing acid rain, is reduced under increasing CO2 atmospheres because fewer SO2 molecules enter the interior of leaves.” In addition, CO*-rich environments may help alleviate the negative effects of other environmental stresses: increasing CO2 atmospheres enables some plants to overcome salinity stress, common to plants growing near arid, marine or formerly irrigated habitats.lx

An unresolved question and other ecological complications

A basic and still unresolved question is whether short-term COz- induced increases in photosynthesis and plant growth can be maintained for plants grown over the long term - an especially relevant query for the issue of whether forest trees will benefit from a CO*-rich atmosphere. Experimental evidence about the sustainability of CO*-induced photosynthesis and growth increases has yielded mixed results: marsh grasses from the nutrient-rich Chesapeake Bay region and sour orange trees from experimental plots significantly increase their growth over several growing seasons (at most, five years thus far) whereas the response of tundra sedges from nutrient-poor northern Alaska and US deciduous trees (ie oaks, birches, yellow-poplar, and some maples) from glasshouse experiments declines over time.”

Even if CO,-induced growth responses are sustainable over the long term, under many circumstances other complexities inherent in ecological systems will dampen potential beneficial aspects of increasing atmospheric CO2 concentrations. These complexities include the fertility of the local growing environment; the specific identity of the plant (ie species); the modification of plant growth by the presence of neighbour-

GLOBAL ENVIRONMENTAL CHANGE December 1992 303


~ntinu~ from page 303 tore, Vol367, i8 May 1992, pp 322-324; F.A. Bazzaz and S.L. Mao. ‘Enhancement of growth in high COP environments declines in 2nd.year tree seedlings’, Nature, 1992, in press. “For example, Arthur Ft. Zangerl and Fakhri A. Bazzaz, ‘The response of plants to elevated CO,: II. Competitive interactions among annual plants under varying light and nutrients’, Oeco/ogia, Vol 62, No 3, 1984, pp 412-417; William E. Williams, Keith Garbutt and Fakhri A. Bazzez, ‘The response of plants to elevated CO,: V. Performance of an assemblage of serpen- tine grassland herbs’, ~n~fo~mentat Ex- perimental Botany, Vol28, No 2, 1988, pp 123-130. “For a more detailed description of the biochemical and anatomical differences between CB and Cq plants, see Fakhri A. Bazzaz and Eric D. iajer, ‘Plant life in a CO,-rich world’, ~~enti~c American. Voi 266: No 1, January 1992. pp 68-74. 22Bazzaz, op tit, fief 5; Strain and Cure, 0~ cit. Ref 5: Strain. oo cit. Ref 5: Moonev et al, op cit. -Ref 5. ’ . “For example, Fakhri A. Bazzaz and Ro- ger W. Carlson, ‘The response of plants to elevated CO,: I. Competition among an assemblage of annuals at two leveis of soil moisture’, Oeco@ia, Vol 62, No 2, pp 196-198; Ed G. Reekie and Fakhri A. Bazzaz, ‘Competition and patterns of re- source use among seedlings of five trooic- al trees grown $ ambient and elevated C02’, Oecologia, Vol 79, No 2, 1989, pp 212-222. 24For example, William E. Williams, Keith Garbutt, Fakhri A. Bauaz, and Peter M. Vitousek, ‘The response of plants to elevated COP: IV. Two deciduous tree communities’, Oecotoaia, Vol 69, No 3, 1986, pp 454-459; Williams et al, op tit, Ref 20. 25Lucy St Omer and Steven M. Horvath, ‘Elevated carbon dioxide concentrations and whole plant senescence’, Ecology, Vot 64, No 5,1983, pp 1311-1314; Keith Gar- butt and Fakhri A. Bazzaz, ‘The effects of elevated COz on plants: Ill. Flower, fruit and seed production and abortion’, New Phytologist, Vol 98, 1984, pp 433-446. =Garbutt and Bazzaz, op cit. Ref 25. “D E Lincoln, D. Couvet, and N. Sionit, *Re&&tse of an insect herbivore to host plants grown in carbon dioxide enriched atmospheres’, Oeco/ogia, Vol 69, No 4, 1986, pp 556-560; Eric D. Fajer, M. Deane Bowers, and Fakhri A. Bazzez, ‘The effects of enriched carbon dioxide atmospheres on plant-insect herbivore interactions’, Science, Vot 243, 1989, pp 1198- 1200.

ing plants; and the interactions with pollinators and herbivores. For example, experimental work has demonstrated that plants need a fertile growing environment to respond positively to increasing CO2 concentrations. In low nutrient habitats, the CO2 fertilization effect is greatly diminished, if not entirely lost. 20 This might partially explain the results mentioned above, in which marshland grasses from high nutrient Chesapeake sites maintained Cot-induced growth enhancement whereas sedges from the infertile Alaskan tundra did not.

Another complicating factor is the fact that not all plant species increase their growth in a C02-rich atmosphere. Ecologists call this phenomenon ‘species-specific growth responses’ to a high CO2 environment. One predictable factor influencing whether a plant species will grow larger in C02-rich conditions is the type of biochemical machinery a plant uses to capture CO2 for photosynthesis. Plants possessing C3 biochemistry, such as wheat, rice and all trees, usually increase their growth much more in C02-rich conditions than those possessing C4 biochemistry, such as corn, sugarcane and many dryland grasses.*’ Yet, for reasons we do not yet understand, even certain C3 plants species grow much better than others in response to an increased CO2 atmosphere.22

These species-specific growth responses are accentuated when plants are grown together in competition, as they commonly do in nature. Plants which are more responsive to CO?-rich environments grow better, often at the expense of less responsive plant species.2” More responsive species grow faster and co-opt other resources necessary for growth (light, water and nutrients), thereby restricting the growth of species less responsive to increasing CO2 atmospheres. Therefore, under competitive conditions, the total ‘biomass’ (ie the amount of living tissue) of plant communities frequently does not increase under COz-rich ~onditions;2~ rather, only certain plant species grow better, producing hierarchies which may have different dominant species from communities grown under current ambient CO2 conditions.

The interactions between plants and their animal pollinators and herbivores (ie plant eaters) are likely to change under CO*-rich conditions in ways which might limit the importance of the CO2 fertilization effect. In response to these conditions, the timing of plant development, called ‘phenology’, often changes. For example, many annual plant species, those which set seed and die after one growing season, often produce flowers earlier when grown in C02-rich conditions.25 Ecologists have speculated that shifting flowering times might disrupt the synchrony between peak flowering times and peak pollinator abundance.26 Under circumstances in which the number of pollinators limits plant seed set, shifting flowering times could severely reduce the number of seeds produced by certain species. Clearly, one generation of increased plant growth under C02-rich conditions becom- es less relevant for species maintenance if reduced seed numbers limit the population of the next generation of plants.

In response to CC&-rich atmospheres, plants not only change the amount and rate at which they grow and flower, but also their quality - ie the chemical composition of their tissues. Specifically, plants grown in increasing CO2 atmospheres have lower concentrations of proteins in their leaves than those grown under today’s CO2 concentrations.” From the perspective of an herbivorous animal, this means that the nutritional quality of plants grown in C02-rich conditions is lawer. Studies using



insect herbivores, such as grasshoppers and caterpillars of moths and butterflies, have demonstrated that insects eat dramatically more leaves of plants grown in CO*-rich conditions, presumably to compensate for their lower protein concentration. 28 For example, in three separate experiments, a common western grasshopper, Melanoplus differentialis, ate from 3674% more sagebrush leaves which grew in a C02-rich atmosphere. 29 Given these remarkable increases in insect consumption under C02-rich conditions, it seems reasonable to wonder whether C02-induced increases in plant growth will simply end up in the stomachs of herbivores.

Lower leaf protein concentrations do more than stimulate herbivores to eat more. Performance usually declines when insects are reared an entire lifecycle on low-protein, high C02-grown plants.“’ ‘Fitness’, measured as the insect’s survival rate, how long it takes to mature, and its final size and potential fecundity (ie how many offspring it is likely to produce), often declines for insect herbivores reared on plants grown in C02-rich conditions. Conceivably, these fitness reductions may result in smaller populations of certain insect herbivore species - perhaps good news for some previously consumed plants, but less beneficial for butter- fly populations or predatory animals which eat caterpillars and grasshoppers.

Implications

Agriculture in a C02-rich world

Perhaps the best way to examine agricultural productivity under future atmospheric conditions is to determine when the benefits of a C02-rich environment might be garnered. Essentially, crops grown at 650-700 ppm can be expected to increase their yields by lO-35%, provided that plants grow in fertile conditions, water is supplied in adequate amounts, competition from weeds is minimized, increased consumption by insect pests is minimized, and unpredictable climatic extremes (eg drought, hot spells, frosts) are rare. When might these circumstances occur?

As mentioned earlier, scientists do not expect atmospheric concentrations to reach 650-700 ppm until around the year 2075. Because energy use and population growth will probably grow exponentially, the presence of higher atmospheric CO2 concentrations will be skewed closer toward 2075 than to the present (ie according to IPCC’s Business as Usual energy-use scenario, the atmosphere will reach 500 ppm CO2 around 2040 and 600 ppm CO2 around 2060);3’ lower atmospheric CO2 concentrations, even around 500 ppm, often have less dramatic effects on plant growth.32 In other words, crop productivity increases in response to a C02-rich environment will not reach lO-35% during most lifetimes of today’s agricultural scientists and policy analysts (though even small yield increases will be welcomed). Unfortunately, the global scientific consensus expects significant climate change (a mean global temperature increase of 1.2”C) by 2030 or so, when atmospheric CO2 concentrations are expected to be only 450 ppm.33 Conceivably, however, crop breeders and experts in biotechnology can select for strains which are more responsive to lower atmospheric CO2 concentrations (ie between 350 and 450 ppm) - perhaps a suitable goal for agricultural research.

Assuming that the climatic extremes do not negate the CO2 fertilization effect (but see Hansen et al”), we can only assume that crop



2*W.L.A. Osbrink. J.T. Trumble, and R.E. Wagner, ‘Host suitability of Phase&s lunata for Trichop/usia ni (Lepidoptera: Nootuidae) in controlled atmospheres’, En- vironmental Entomology Vol 16, 1967, pp 639-644; R.H. Johnson and D.E. Lincoln. ‘Sagebrush and grasshopper responses to atmospheric carbon dioxide concentration’, &co/ogia, Vol 64, No 1, 1990, pp 103-l 10; Lincoln et al, op tit, Ref 27; Fajer et a/, op cif, Ref 27. =Johnson and Lincoln, op tif, Ref 26. 30For examele. Faier et al. OD cif. Ref 27. 31Houghton’ et’al, bp cif, def’6. =For example, R. Hunt, D.W. Hand, M.A. Hannah. and A.M. Neal. ‘Resuonse to CO, enrichment in 27 herbacedus species’: Functional Ecology, Vol5, No 3, 1991, pp 410-421. %Houghton et al, op tit, Ref 6. 34Hansen et al, op cif, Ref 6. 35J.S. Boyer, ‘Plant productivity and environment’, Science, Vol 216, 1992, pp 443446. %Data from David Pimental, Cornell Uni- versity, cited in Peter Weber, ‘A place for pesticides’, World Watch, Vol 5, No 3, May/June 1992, pp 16-25. 37David T. Patterson, Elizabeth P. Flint, and Jan L. Beyers, ‘Effects of CO* enrichment on competition between a C4 weed and a C3 crop’, Weed Science, Vol 32, 1964, pp 369-394. %Lincoln et al, op cif, Ref 27; Fajer et al, op tit, Ref 27. %Data from George P. Georghiou, Uni- versity of California, Riverside, cited in Weber, op tit, Ref 36. 40Fajer et al, op tit, Ref 27. 41Peter W. Price, Carl E. Bouton, Paul Gross, Bruce A. McPheron, John N. Thompson, and Arthur E. Weis, ‘Interac- tions among three trophic levels: Influence of plants on interactions between insect herbivores and natural enemies’, Annual Review of Ecology and Systematics, Vol 11, 1960, pp 41-65.

productivity will increase on fertile sites, or for farmers with adequate access to fertilizers (given the other caveats mentioned above and described in detail below). Low-input farmers, or poorer farmers in the developing world, are less likely to benefit from a C02-rich atmosphere. However, for tropical development projects, the incorporation of nitrogen-fixing legume trees into agro-forestry projects may be even more successful in the future: increasing CO2 atmospheres are likely to lead to greater nitrogen capture by legumes. This nitrogen (as ammonia or nitrate) will then diffuse into agricultural soils and fertilize them. Under these circumstances, food crops from agro-forestry plots may show a CO2 fertilization effect.

Water availability is also a crucial feature to increasing yields in an increased CO2 atmosphere.35 Although drought tolerance of plants generally increases under C02-rich conditions, presence or absence of adequate water supplies will have a much greater influence on crop yields than will atmospheric CO* concentration. CO*-rich conditions are likely to increase yields on fertile, dryland sites; however, yield increases from increased water supplies (ie ‘via irrigation) would likely dwarf yield increases in response to CO*-rich conditions. Conversely, CO*-rich conditions will not save farmers from the ravages of drought, although crop yield depressions occasionally might not be as severe.

Assuming ideal atmospheric and soil conditions for CO2 fertilization of crops, CO*-rich environments will also impact crop pests and weeds, thereby potentially minimizing farm yield increases under these conditions. For 1986 US farmers, crop competitors (weeds, diseases and insects) led to over a 35% harvest 10~s.~~ How might this change in a COz-rich world? When a C02-responsive C3 crop, such as soybean, was grown together with C4 weed johnsongrass, soybean actually outgrew the weed more under C02-rich conditions.37 Hypothetically, however, if one grew C4 crop, such as corn, with a C02-responsive C3 weed, we would anticipate the opposite result - that the weed would outperform the crop under COz-rich conditions. Under competitive conditions between crops and weeds in which the physiological dice are not loaded one way or the other, the expectation would be that the inference of competing weeds under COz-rich conditions would limit crop yield enhancement. To benefit from increasing CO2 conditions, then, it may be necessary to eliminate the competitor weeds, in all likelihood, via herbicide application. Again, poorer farmers may be unable to afford needed herbicides to benefit from a C02-rich world.

Insect pests may present even greater problems under future CO;?- rich conditions. If wild pest species behave comparably to those studied experimentally, they are likely to eat dramatically more leaves under increasing CO2 atmospheres, 38 thereby reducing CO*-induced yield increases. Insecticide applications may become even more essential to gain the advantage of a COz-rich environment. Due to the environmental and economic costs of insecticides, and the growing resistance of insect pests to these chemicals (ie currently there are 500 pest species resistant to insecticides worldwide”‘), it may be difficult to eliminate anticipated additional crop loss. However, due to the slower development of insect pests on low-protein, high CO*-grown plants,40 inte- grated pest management (IPM) techniques may be more effective: slower development times render insects more susceptible to natural or introduced predators and parasitoids.41

In summary, it is incorrect to assume automatically that all crops will


‘*Cynthia Rosenzweig, ‘Potential effects of climate change on agricultural production in the Great Plains, a simulation study’, in J.B. Smith and D.A. Tirpak, eds, The Potential Effects of Climare Chanae on the US, US Environmental Protect& A$ncy, Washington, DC, 1990, pp 3-9-3-

‘%Vorld Resources Institute, World Re- sources 15X72-93, Oxford University Press, Oxford, 1992. “Ibid. 45Norby et al, op tit, Ref 19; Bazzaz and Mao, op cir, FM 19.

Is carbon dioxide a ‘good’ greenhouse gav

increase their yield by l&35% in a CO*-rich atmosphere. Only under specific conditions of soil fertility, water supply, climate moderation, and weed and insect pest control can we attain these yield increases. High input ‘industrialized’ agriculture can probably create many of the conditions necessary to benefit from a COz-rich atmosphere, although the economic and environmental costs of developing CO*-responsive varieties and IPM techniques, applying chemical fertilizers, herbicides and pesticides, and diverting and consuming water from rivers and aquifers, have yet to be rigorously assessed.

Further, the assumption that ‘full’ CO2 fertilization will occur concur- rently with climate change is unsupportable: we do not yet understand how higher temperatures, altered hydrological regimes and increased atmospheric CO* concentrations interact to affect crop yields. How- ever, it is likely that increased COZ concentrations will not totally compensate, let alone overcompensate, for lost crop productivity resulting from other sub-optimal growth conditions. Similar conclusions were drawn by Rosenzweig in her assessment of future agricultural production in the US Great Plains under CO*-rich conditions.42

Assuming crop productivity does increase by 35% worldwide (in addition to other assumed crop improvements and innovations), is this necessarily a big deal? Are the benefits of an increased CO2 atmosphere worth the other risks of dramatic climatic changes? By 2075, when we are more likely to garner a crop yield of 35%, the population of the globe will be between 10 and 11 billion people (given a medium fertility projection with a replacement-level fertility of 2.06).43 This is approx- imately double our current global population (about 5.4 billion people). 44 Crop yield increases due to CO2 fertilization would facilitate feeding more people and, by this logic, would be ‘a big deal’; however, these potential C02-induced crop yield increases alone would be drastically insufficient to feed a burgeoning population.

Natural ecosystems in a C02-rich world

When will natural ecosystems and their component species benefit from an increasing CO2 atmosphere? Here, we use a simple anthropocentric viewpoint of natural ecosystem ‘benefits’ because there is not an equivalent ecological perspective. Thus, ‘benefits’ accrue (to humans) when natural ecosystems store more carbon (especially if they store excess carbon added to the atmosphere by humans), maintain a greater number and/or variety of plant and animal species, and are more resilient in the face of human-induced disturbances, such as pollution and climate change.

By this definition, only plant communities from fertile sites, such as marsh grasslands or habitats on volcanic soils, will have the resources to respond positively, in terms of growth, to a C02-rich world. Even under these conditions, recent experimental evidence demonstrates that plants are not guaranteed to increase their growth over the long term. Yellow-poplar (Liriodendron tulipifera) seedlings grown for three years under elevated CO2 conditions were no larger than their ambient C02-grown counterparts, and red oak seedlings could not maintain higher relative growth rates under C02-rich conditions, suggesting that their ability to store more carbon in their tissues than ambient C02- grown plants diminishes as they age.45 Although scientists have speculated that increased forest productivity, especially in temperate regions, has been sequestering and will continue to sequester ‘extra’ carbon



46Wilfred M Post. Tsung-Hung Peng, Wil- liam Ft. Emanuel, Anthony W. King, Virgi- nia H. Dale, and Donald L. DeAngelis, ‘The global carbon cycle’, American Scientist, Vol78, 1990, pp 31g-326; Pieter Y. Tans, lnez Y. Fung, and Taro Takahashi. ‘Obser- vational constraints on the global atmospheric COP budget’, Science, Vol 247, 1990. pp 1431-1438. 47H.H. Shugart, AM Y.A. Antonovsky, P.G. Jan&, and A.P. Sandford, ‘COP, climate change, and forest ecosystems’, in Bert Bolin, Bo Ft. Doos, Jill Jager, and Richard A. Warrick, eds, The Greenhouse Effect, Climate Change and Ecosystems, Scope 29, John Wiley and Sons, Chichester, 1986, pp 476-521. 92tis ef al, op tit, Ref 19. %.P. Smith, B.R. Strain, and T.D. Shar- key, ‘Effects of COP enrichment on four Great Basin grasses’, Functional Ecology, Vol 1, No 2,1987, pp 139-143. %Jerry M. Melillo, John D. Aber, and John F. Muratore, ‘Nitrogen and lignin control of hardwood leaf litter decomposition dyna- mics’, Ecology, Vol 63, No 3, 1982, pp 621-626.

liberated into the atmosphere from anthropogenic sources,46 seedling responses from simplified experimental conditions warn us that additional carbon storage (via greater plant growth) is not guaranteed in a future high-CO2 world.

Further, under more complex natural settings, competitive interactions between neighbouring plants are likely to diminish growth improvements in response to enriched CO2 environments.47 At the species level, the disproportionate response of some plant species to a COz-rich environment, often at the expense of less responsive species, means that all species will not benefit from future atmospheric conditions, or that plant communities in total will not increase their. carbon storage capacity. The decline of less responsive species, such as many C4 plants, seems likely in certain plant communities, with rarer less responsive species becoming endangered, if not extinct over time. The loss of C4 species seems especially likely in low-diversity habitats, such as the salt marsh grasslands of the Chesapeake Bay.48

Changes in species composition can induce second-order changes in the functioning of entire ecosystems. For example, when exposed to a C02-rich environment, Bromu.s rectorurn, a weedy grass, grew much better than three other grass species which share its rangeland Great Basin habitat.49 Because Bromus predisposes land to burning, its potential increase in future Great Basin communities may lead to more frequent and/or severe wildfires. Increased wildfire frequency would change which species (both plants and animals) could survive in this habitat, potentially leading to local extinctions, as well as release more CO2 and other pollutants into the atmosphere.

Biodiversity reductions, the less benign element of a COz-rich world, may also result from the decline in the protein concentration of leaves from high CO*-grown plants. Herbivorous insect performance is greatly determined by the nutritional quality of its diet; therefore, if a general lowering of plant nutritional quality occurs in a COz-rich world, populations of herbivorous insects are likely to decline. Again, currently rare insect species, as well as those specialist insect herbivores which depend upon less CO*-responsive plant species for food (as leaves, pollen or nectar), may become endangered if not extinct over time. The ramifications of declining herbivorous insects for the plants that they pollinate or the predators they nourish are unknown but the consequences are potentially damaging for complex ecological interactions and food webs.

Declining leaf protein (ie nitrogen) concentrations in high CO*-grown plants may also impact future soil fertility. In a COz-rich environment, if dead plant matter, called ‘litter’, has reduced protein concentrations in parallel to its living precursors, it is likely that future soil fertility will decline. Lack of nitrogen sources constrains the rate at which soil bacteria and fungi decompose dead material.50 Under CO*-rich conditions, reductions in litter nitrogen concentration will slow down the rate of decomposition - the rate of nutrient liberation by these micro- organisms. More nutrients will therefore remain trapped in litter, and this will be inacessible to plants. Clearly, if soil fertility declines in a COz-rich world, growth increases under these conditions will be minimized. However, if temperatures do increase and soil moisture is maintained, decomposition rates will increase, potentially negating the effect of reduced nitrogen in litter. Biologists are actively researching to ascertain whether effects of higher temperatures increasing decomposi-


51Gaius R. Shaver, W. Dwight Billings, F. Stuart Chapin, AMe E. Giblin, Knute J. Nadelhoffer, Walter C. Oechel, and EB. Rastetter, ‘Global change and the carbon balance of arctic ecosystems’, Bioscience, Vol42, No 6,1992, pp 433-441. (Note that increased respiration rates from higher temperatures may lead to increases in soil fertility as formerly trapped nutrients are liberated and made available to growing plants. This increase in soil fertility could increase plant productivity, thereby increasing an ecosystem’s capacity to ‘scrub’ COP from the atmosphere. Further, plants sometimes respond to increasing CO2 atmospheres by lowering their dark respiration rates (ie the rates at which they maintain their life processes while liberating CO*). Under these conditions, living plants could release less COP from their own metabolic processes while simul- taneously increasing their rate of CO2 up- take via photosynthesis. Under this scenario, future ecosystems would have a greater capacity to scrub CO* from the atmosphere than they do presently (see ldso and Kimball, op tit, Ref 19; J.S. Amthor, ‘Respiration in a future higher- CO, world’, Plant Cell and Environment. Vol-14, No 1, 1991, pp 13-20). 52Bazzaz and Fajer, op cif, Ref 20; Robert L. Peters and Joan D.S. Darling, ‘The Greenhouse Effect and nature reserves’, Bioscience, Vol35, No 11, 1985, pp 707- 71 7. -Houghton 81 al, op cit. Ref 6. “For example, Norman J. Rosenberg, ‘The increasing CO, concentration in the atmosphere and its implication on agricultural productivity: II. Effects through CO,-induced climate change’, Climatic Change, Vol 4, 1962, pp 239254; R.A. War&k, R.M. Gifford, with M.L. Parry, ‘CO*, climatic change and agriculture’, in Bert Bolin, Bo R. Does, Jill Jager, and Richard A. Warrick, eds, The Greenhouse Effect, Climatic Change, and Ecosystems, Scope 29, John Wiley and Sons, Chiches- ter, 1986, pp 395-473.

Is carbon diaxide a ‘good’ greenhouse gas?

tion rates, or increasing CO* concentration decreasing these rates, will predominate in future natural ecosystems.

To summarize, current ecological evidence suggests that increasing COz atmospheres will induce changes in the components and functioning of natural ecosystems. However, many of these changes may not be beneficial, let alone benign, as suggested by the sanguine scenario of a COz-rich world. We will not witness a greening of Earth under these future atmospheric conditions. Rather, due to the limits imposed by low soil fertility and water supply, most natural areas will not become more lush. Hypothesized changes in the frequency of fires or the rates at which nutrients are liberated could further affect ecosystems in ways which benefit some species at the expense of others, or conceivably, reduce total ecosystem productivity.

Only pockets of fertile natural areas will increase their productivity, potentially at the cost of some elements of plant species diversity, and a reduction in herbivorous insect diversity and density. The implications of reduced herbivorous insect numbers for upper trophic level organisms (ie those that eat herbivorous insects, and so on), as well as reduced pollinator numbers for certain plant species, are unclear although conceivably destabilizing. Finally, all bets are off on ecosystem response if rapid climate change occurs. Fertile marsh grasslands cannot grow more if they are under 30 cm of water from rising seas; temperature increases and especially changing rainfall patterns will compromise plant responses to increasing CO* atmospheres; and finally, under warmer conditions, microbial respiration rates and the decay of dead matter will increase, liberating additional CO2 into the atmosphere; thereby adding a potential critical positive feedback loop to the global atmosphere-biosphere system.51

From the perspective of natural ecosystem response, it is premature at best, and irresponsible at worst, to assume that CO*-rich environments will benefit natural ecosystems. Further, if COz-induced climate change is rapid, we expect major disruptions in the functioning of terrestrial natural ecosystems, species.52

including possible severe losses of Under those circumstances, CO? clearly would not be a ‘good’

greenhouse gas.

Conclusions - evaluating the sanguine scenario

In evaluating the validity or likelihood of the sanguine scenario for global climate change, we do not feel qualified to dispute its first part: the range of global warming rates generally accepted by expertss3 However, we feel more than justified in asserting that, in the absence of dramatic climatic changes, increasing concentrations of atmospheric CO* will not provide (1) a widespread agricultural panacea; (2) a world replete with more lush natural habitats; or (3) guaranteed additional forest growth to absorb carbon added to the atmosphere by fossil fuel burning. Put simply, according to the best experimental evidence, CO* is not, unconditionally. a good greenhouse gas: only under certain limited circumstances will humans benefit from the fertilizing effect of increasing CO2 atmospheres on crops and wild plants, including the condition that climate change does not occur too rapidly.

Further, we fear that recent models and discussions concerning the agricultural responses to C02-rich conditions, which, for example, routinely assume lO-30% crop yield increases,54 do not adequately

GLOBAL ENVIRONMENTAL CHANGE December 1992


-Richard M. Adams, Cynthia Rosenz- weig, Robert M. Pear-t, Joe T. Ritchie, Bruce A. &Carl, J. David Glyer, R. Bruce Curry, James W. Jones, Kenneth J. Boote. and L. Hartwell Allen, ‘Global climate change and US agriculture’, Nature, Vol 345. 1990. DD 214-223: ML. Parrv. J.H. Porter, and T.R. Carter, ‘~gr~u~ure~~lirna- tic change and its implications’, Trends in Ecology and Evolution, Vol5, No 9, 1990, pp 318-322; for a detailed regional and global treatment of potential world agri- cuftural productivity under climate change in which the ~~~ti~ of the CO, ferti- liiation effect is carefully delineated, see Cynthia Rosenzweig, Martin Parry, Gun- ther Fishcer, and Klaus Frohberg, C/&ate Change and World Food Supply: A Pre- liminary Report, Environmental Change Unit, University of Oxford, May 1992, manuscript. ssldso, op tit, Ref 2.

consider the ecological circumstances which could severely limit a COTfertilization effect. Consequently, these provide little guidance as to how human societies should adapt to these future environments. Instead, we welcome analyses which outline limitations of their assump- tions, and include the careful delineations of when the benefits of a CO*-rich atmosphere accrue, plus the potential economic and environmental costs (ie fertilizer and pesticide use) necessary to gamer these benefits.”

Fortunately, when planning to adapt to our future environment, we can incorporate the knowledge of when increasing atmospheric CO2 concentrations enhances plant growth. Benefits to agriculture will occur under the proper conditions, especially if we gear current agricultural development, management and research efforts to this end. For example, we recommend breeding or engineering crops to respond positively to atmospheric concentrations the world is likely to experience within 50 years, rather than 100 years. In the case of natural habitats, restoration of degraded and/or desertified sites may proceed more rapidly under C02-rich conditions, especially if more CO*-responsive and drought tolerant species are initially used. These benefits from our anticipated C02-rich world will be most welcomed by the 10-11 billion future human inhabitants, as well as whatever semblance of wildlife survives the intensification of land use on our planet. That is no excuse for overstating the benefits, nor for belittling the potential risks of CO*-rich atmospheres.56


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