ScienceWeek

ECOLOGY: CLIMATE CHANGE AND CROP YIELDS

The following points are made by David Schimel (Science 2006 312:1889):

1) The effects of increasing atmospheric carbon dioxide (CO2) concentrations on climate have been a source of worry for decades, but the positive effect of increasing CO2 levels on crop growth has been a silver lining in the climate change cloud. Hundreds of studies, some dating back decades, have shown that most major food crops respond positively to increasing CO2 concentrations, because of the direct stimulatory effect of CO2 on photosynthesis and the indirect effect of decreasing the water requirements of crops. The former effect should make crops more productive and the latter more drought-tolerant. Yet a new analysis (1) of recent and far more realistic studies based on the free-air concentration enrichment (FACE) technique sounds a cautionary note.

2) In FACE studies, a sophisticated, computer-controlled micrometeorological system enriches the air above a large plot to a target level, but does not otherwise alter the microclimate and growing environment. There is a large network of FACE studies around the world, but relatively few have used food crops; most have been in natural or seminatural ecosystems. Long et al (1) synthesized the results from agronomic FACE studies and find the effects of increased CO2 concentrations on crop yields to be no more than half of the levels expected from earlier laboratory and open-top chamber studies.

3) What is the significance of this result? In anticipating the impacts of global change on agriculture, crop models have been major tools. Modern crop models are widely used in agricultural research and planning and have been shown to be robust and accurate under a wide range of conditions in today's world. These models have been extended to study climate change scenarios, including simulated responses to climate and CO2 and a wide range of assumptions about technological change and adaptation by farmers and markets.

4) These modeling results suggest that, at least in some regions, the future of agriculture may be one of the brighter spots of climate change. Although guarded in their remarks, most assessments of climate change impacts on agriculture have concluded that the global impacts are relatively small (although they may be severe in some areas) (2). This is largely because the assumed effects of CO2 on crop growth and water use offset all or part of the negative impacts of warmer temperatures and rainfall changes (3). Depending on the region, this effect may ameliorate negative impacts of climate change or even allow for increases in crop yield. Long et al (1) find that most current models are based on literature suggesting 20% to 30% increases in photosynthesis and yield. This literature also suggests substantial effects in maize. Maize, a C4 photosynthesis pathway crop, should have no direct response to CO2 and should only respond via decreased water requirements (C4 photosynthesis uses a CO2-concentrating mechanism at the cellular level that is less sensitive to atmospheric CO2 than the more common C3 pathway). The extant FACE results in food crops are different from earlier lab and chamber results in a consistent and disturbing way. In surveying the effects of CO2 concentration in the more realistic FACE studies, Long et al (1) conclude that the actual effect of CO2 concentration is about half of that in the experiments used to develop climate change models. Despite earlier reports of substantial effects in C4 crops, Long et al (1) found no increase in yield at all in this case, although there was some evidence for increased drought tolerance.

References:

1. S. P. Long, E. A. Ainsworth, A. D. B. Leakey, J. Nösberger, D. R. Ort, Science 312, 1918 (2006)

2. J. J. MacCarthy et al., Eds., Climate Change 2001: Impacts, Adaptation and Vulnerability (Cambridge Univ. Press, New York, 2001)

3. J. Reilly, D. Schimmelpfennig, Clim. Change 43, 745 (1999)

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