Food for Thought: Lower-Than-Expected Crop Yield Stimulation with Rising CO2 Concentrations
Abstract
Model projections suggest that although increased temperature and decreased soil moisture will act to reduce global crop yields by 2050, the direct fertilization effect of rising carbon dioxide concentration ([CO2]) will offset these losses. The CO2 fertilization factors used in models to project future yields were derived from enclosure studies conducted approximately 20 years ago. Free-air concentration enrichment (FACE) technology has now facilitated large-scale trials of the major grain crops at elevated [CO2] under fully open-air field conditions. In those trials, elevated [CO2] enhanced yield by ∼50% less than in enclosure studies. This casts serious doubt on projections that rising [CO2] will fully offset losses due to climate change.
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References and Notes
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Materials and methods for meta-analyses are available as supporting material on Science Online. Full results from the meta-analyses summarized in Table 1 are presented in table S2 with references in appendix S1. C4 crop yield responses to elevated [CO2] are presented in table S3 with references in appendix S2.
38
This work was supported by the Illinois Council for Food and Agricultural Research, Archer Daniels Midland Company, U.S. Department of Agriculture, U.S. Department of Energy (grant DE-FG02-04ER63849), and Illinois Agricultural Experiment Station.
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Published In
Science
Volume 312 | Issue 5782
30 June 2006
30 June 2006
Copyright
American Association for the Advancement of Science.
Submission history
Received: 1 March 2006
Accepted: 15 May 2006
Published in print: 30 June 2006
Notes
Supporting Online Material
www.sciencemag.org/cgi/content/full/312/5782/1918/DC1
Materials and Methods
Tables S1 to S3
References
Appendices S1 and S2
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Response to Ziska and Bunce
L. H. Ziska and J. A. Bunce state that the objective of Long et al. (2006) (1) was to show that chambered studies overestimate yield response to [CO2] relative to FACE. This was not our objective. It was to compare the yield response used to parameterize the major models currently used to project future grain supply with the yield response observed when [CO2] is increased under fully open air conditions in farm fields.
Ziska and Bunce claim there are significant contradictions between the data presented in Long et al. (2006) (1) versus Long et al. (2005) (2). Long et al. (2005) (2) is a review article that generates the hypothesis that the stimulation value used in models of future food supply is overestimated. It is based on earlier reviews of crop yield responses to [CO2]. Long et al. (2006) (1) is an original article that tests and upholds this hypothesis statistically. It is based on a new meta-analysis of the original literature, curve fitting, and also new FACE data, critically for C4 crops, none of which were part of the 2005 review. All of the display items, summarizing the analyses in Long et al. (2006) (1), are original with no overlap with any prior publication.
Despite the claim, there are no inconsistencies between Long et al. (2006) (1) and Long et al. (2005) (2). Some numbers differ because they are based on a much larger database and new analysis in Long et al. (2006) (1), or in other cases they do not differ. For example, the 17 to 19% yield enhancement of soybean in Long et al. (2005) (2) is, as we stated, from the review of Kimball (1983) (3). The 32% given in Long et al. (2006) (1) is calculated from 115 original studies as shown in Fig. 2A, most of which were published after 1983. The 15% yield enhancement reported by P. B. Morgan et al. (2005) (4) is not inconsistent with Long et al. (2006) (1) -- it is the exact figure we show for soybean in Fig. 2A and Table S2.
Ziska and Bunce suggest that the stimulation in soybean yield given in Long et al. (2006) (1) is inconsistent with the 24% stimulation at 689 ppm reported by E. A. Ainsworth et al. (2002) (5). However, the 24% stimulation that they extract from Ainsworth et al. (2002) (5) is the mean response for all chamber studies for all [CO2], ranging from 450 ppm to 1232 ppm. Because of the nonlinearity of the response of yield to [CO2], it is not valid to consider this a measure of the actual stimulation that would occur at 689 ppm as used by Ziska and Bunce. This is why we fitted a nonrectangular hyperbolic function to the data in Fig. 2A.
Finally, Ziska and Bunce note that the stimulation given for sorghum and maize in Long et al. (2005) (2) differs from Long et al. (2006) (1). Numbers in the 2005 review are from the review of Kimball (1983) (3), as we stated. Kimball (1983) (3) used just two prior studies of C4 crops. Long et al. (2006) (1) examined the stimulation in much greater depth by identifying all previous peer-reviewed research articles reporting on sorghum and maize yield responses in elevated [CO2]. This list of 14 studies was published as supplemental Table S3. Ziska and Bunce state that a 6 to 7% stimulation in corn is consistent with the small CO2 yield enhancement by FACE. This is incorrect. There has been no significant stimulation of yield at elevated [CO2] in any FACE study of either sorghum or maize to date. The average yield change for sorghum grown in FACE experiments was -1% and for maize was +1%, neither significantly different from zero (as shown in Table S2).
Stephen P. Long
Department of Plant Biology, Department of Crop Sciences, Institute for Genomic Biology, University of Illinois at Urbana Champaign, 1201 West Gregory Drive, Urbana, IL 61801, USA.
Elizabeth A. Ainsworth
Photosynthesis Research Unit, USDA/ARS, 1201 W. Gregory Drive, Urbana, IL 61801, USA, and Department of Plant Biology, Institute for Genomic Biology, University of Illinois at Urbana Champaign, 1201 West Gregory Drive, Urbana, IL 61801, USA.
Andrew D. B. Leakey
Institute for Genomic Biology, Department of Plant Biology, University of Illinois at Urbana Champaign, 1201 W. Gregory Drive, Urbana, IL 61801, USA.
Josef Nösberger
Institute for Plant Sciences, ETH, 8902 Zurich, Switzerland.
Donald R. Ort
Photosynthesis Research Unit, USDA/ARS, 1201 West Gregory Drive, Urbana IL 61801, USA, and Department of Plant Biology, Department of Crop Sciences, Institute for Genomic Biology, University of Illinois at Urbana Champaign, 1201 West Gregory Drive, Urbana, IL 61801, USA.
References
1. S. P. Long et al., Science 312, 1918 (2006).
2. S. P. Long, E. A. Ainsworth, A. D. B. Leakey, P. B. Morgan, Philos. Trans. R. Soc. Ser. B 360, 2011 (2005).
3. B. A. Kimball, Agron. J. 75, 779 (1983).
4. P. B. Morgan, G. A. Bollero, R. L. Nelson, F. G. Dohleman, S. P. Long, Global Change Biol. 11, 1856 (2005).
5. E. A. Ainsworth et al., Global Change Biol. 8, 695 (2002).
Does FACE Significantly Increase Grain Crop Yield?
We recently read a Research Article published in Science (30 June 2006, p. 1918) by Steve Long et al.The purpose of the paper was to demonstrate that enclosure, or chamber experiments, compared with "free-air CO2 enrichment" (FACE) experiments, greatly overestimate the effect of increasing atmospheric CO2 concentration on five major crops: corn, rice, sorghum, soybean, and wheat. The paper states that the FACE experiments "elevated [CO2] enhanced yield by ~50% less than in enclosure studies." We see why Science would publish these data, given their widespread implications.
However, in reviewing these data, we discovered a number of inconsistencies; sufficient in nature as to question the given conclusions.
First, it should be noted that the same hypothesis was published a year previously in Philosophical Transactions of the Royal Society (Biology) (1), and the Science article is simply a reiteration of the previous article's conclusions. That is, the objective of both articles is to show that chambered (enclosed) studies have overestimated the yield response of these same five crops, relative to FACE systems, with subsequent consequences for model projections of future food supply and security.
Second, there are significant contradictions between the data presented by the same authors in the Science and Philosophical Transactions articles. For example, the enhancement effect of soybean yield (at 550 ppm) for enclosure studies is given as 32% in Table 1 (relative to ambient CO2) in Science. However in the Philosophical Transactions paper, the relative enhancement of soybean yield for enclosure studies is given as 17 to 19% at 550 ppm CO2, (depending on whether a quadratic or linear function was used for interpolation).
Dr. Long and co-authors have published a meta-analysis (cited in the Science paper) for the effects of CO2 on soybean yield enhancement (2) that lists an average yield enhancement of 24% at 689 ppm relative to ambient CO2 for 57 enclosure studies. This would interpolate to 14 to 16% at 550 ppm, again, depending on whether it was interpolated using a linear or quadratic function. This would be consistent with the Philosophical Transactions paper, but not the Science article. In fact, the enhancement value for soybean yield in the FACE methodology is 15% (3). The FACE value is therefore not ~50% less than (but in fact equal to) results from enclosure studies, contrary to what is given in Science. Note that Long is a coauthor of (3) and is presumably fully aware of the results.
The Science article makes similar claims in Table 1 for wheat yield that also significantly contradict the publication cited as the source of wheat yield data used by Long et al. (4). Disturbing too, is the fact that C4 species examined (corn and sorghum) are lumped together in Science, but are listed as 6 to 7% and 23 to 34%, respectively, in the Philosophical Transactions paper (i.e., 6 to 7% stimulation in corn is consistent with the small CO2 yield enhancement projected by FACE).
Overall, we found the data in the Science article to be incompatible with the author's previous publications. Although we checked the supplemental material, we could find no explanation as to why the relative yield enhancement CO2 effect in enclosure studies could differ so greatly between Table 1 in the Science paper and the previously published articles by the same authors.
Lewis H. Ziska
James A. Bunce
Crop Systems and Global Change Laboratory, USDA-ARS, 10300 Baltimore Avenue, Beltsville, MD 20705, USA.
References
1. S. P. Long, E. A. Ainsworth, A. D. B. Leakey, P. B. Morgan, Philos. Trans. R. Soc. Ser. B 360, 2011 (2005).
2. E. A. Ainsworth et al., Global Change Biol. 8, 695 (2002).
3. P. B. Morgan, G. A. Bollero, R. L. Nelson, F. G. Dohleman, S. P. Long, Global Change Biol. 11, 1856 (2005).
4. J. S. Amthor, Field Crops Research 71, 1 (2001).