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Long on Hype, Short on Facts: FACE vs. non-FACE Studies of Plant Growth Responses to CO2
Volume 9, Number 52: 27 December 2006

In a paper designed to strike a telling blow against the views of the many people who believe the positive effects of the ongoing rise in the air's CO2 content will temper - or even negate - the catastrophic consequences that climate alarmists claim the biosphere will suffer as a result of climate-model-predicted CO2-induced global warming, Long et. al. (2005) claimed there would be lower-than-expected CO2-induced crop yield increases in a high-CO2 world of the future, because of problems associated with all atmospheric CO2 enrichment techniques other than the free-air CO2 enrichment or FACE technique, which they said yields CO2-induced growth enhancements that are only half as large as those measured in the non-FACE studies upon which prior predictions of future crop yields had been based. Receiving little flack (indeed, receiving rave reviews) following the publication of their paper in Britain's prestigious Philosophical Transactions of the Royal Society, Long et. al. (2006) had a similar paper accepted for publication in the similarly-prestigious American journal Science, wherein they again contended that crop models based on non-FACE studies "have overestimated future yields," once again by the familiar factor of two.

Believing this claim to be grossly in error, we discussed it at some length in our editorial of 5 July 2006, wherein we accepted Long et. al.'s quantitative evaluations of the CO2-induced growth responses derived from FACE and non-FACE assessment techniques, but where we argued it was the FACE technique that was the more likely of the two approaches to be in error, citing two aspects of it - (1) rapidly-varying atmospheric CO2 concentrations around the targeted concentration in CO2-enriched treatments and (2) the fact that many FACE studies have only employed atmospheric CO2 enrichment during daylight hours - both of which phenomena have been shown to dramatically reduce CO2-induced growth responses in some studies. In giving Long et. al. the benefit of the doubt with respect to their quantitative evaluations, however, we apparently were far too generous, for an international team of ten specialists in the field has recently published a comprehensive and persuasive debunking of all of Long et. al.'s negative contentions about non-FACE experiments (Tubiello et. al., 2006).

The researchers' detailed criticisms of the Long et. al. (2006) paper focus on the latter's inattention to certain important "technical inconsistencies," as Tubiello et. al. call them, plus the fact that Long et al.'s findings were "lacking [in] statistical significance." When the pertinent data were properly analyzed, for example, the ten-member four-country team demonstrated that "the meta-analysis of Long et. al. (2006) does not [our italics] show significantly lower crop yield response to elevated CO2 in FACE compared to non-FACE experiments."

As further evidence for the validity of their findings, Tubiello et. al. note that when the air's CO2 concentration was raised to a value of 550 ppm in various prior experiments, "mean yields increased 17-20% in FACE, compared to 19-23% in non-FACE experiments," as they report has also previously been demonstrated by Amthor (2001), Kimball et. al. (2002), Gifford (2004), Long et. al. (2004) and Ainsworth and Long (2005). In addition, they demonstrated that "simulated yield responses to elevated CO2, as implemented in most crop models used for climate change impact assessment, are [our italics] consistent with FACE results," and that "any remaining differences in CO2 response based on FACE results would not significantly alter projections of world food supply in the 21st century."

Another of Tubiello et. al.'s important conclusions about atmospheric CO2 enrichment studies is the fact that "controlled environmental chamber, greenhouse, closed-top or open-top field chambers, or gradient tunnel approaches can continue to be used with reliable results," which has always been our view as well. The only questions remaining, therefore, are why some studies employing rapidly-fluctuating CO2 concentrations and/or daylight-only CO2 enrichment have yielded CO2-induced growth enhancements that have been much smaller than those of experiments where the atmosphere's CO2 concentration has not been subject to rapid fluctuations and where the CO2 enrichment has been applied for a full 24 hours per day.

With respect to FACE studies, the dichotomy in the first of these situations may be due to the fact that in most of them the large size of the plots employed may damp out the rapid CO2 concentration fluctuations that occur at the points of release of the CO2 before it reaches the plants. In the case of daylight-only vs. 24-hour-per-day CO2 enrichment, however, the answer is not so obvious. In any event, the important point of the Tubiello et. al. paper is that it clearly refutes the erroneous contentions of Long et. al. (2005, 2006) that FACE studies are far superior to studies that employ other types of atmospheric CO2 enrichment, and that studies based on all types of non-FACE techniques have overestimated the growth-promoting effects of elevated CO2 by an approximate factor or two. These contentions are absolutely false.

Sherwood, Keith and Craig Idso

Ainsworth, E.A. and Long, S.P. 2005. What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analysis of the responses of photosynthesis, canopy properties and plant production to rising CO2. New Phytologist 165: 351-372.

Amthor, J.S. 2001. Effects of atmospheric CO2 concentration on wheat yield: review of results from experiments using various approaches to control CO2 concentration. Field Crops Research 73: 1-34.

Gifford, R.M. 2004. The CO2 fertilising effect - does it occur in the real world? New Phytologist 163: 221-225.

Kimball, B.A., Kobayashi, K. and Bindi, M. 2002. Responses of agricultural crops to free-air CO2 enrichment. Advances in Agronomy 77: 293-368.

Long, S.P., Ainsworth, E.A., Leakey, A.D.B. and Morgan, P.B. 2005. Global food insecurity treatment of major food crops with elevated carbon dioxide or ozone under large-scale fully open-air conditions suggests recent models may have overestimated future yields. Philosophical Transactions of the Royal Society B 360: 2011-2020.

Long, S.P., Ainsworth, E.A., Leakey, A.D.B., Nosberger, J. and Ort, D.R. 2006. Food for thought: Lower-than-expected crop yield stimulation with rising CO2 concentrations. Science 312: 1918-1921.

Long, S.P., Ainsworth, E.A., Rogers, A. and Ort, D.R. 2004. Rising atmospheric carbon dioxide: plants FACE the future. Annual Review of Plant Biology 55: 591-628.

Tubiello, F.N., Amthor, J.S., Boote, K.J., Donatelli, M., Easterling, W., Fischer, G., Gifford, R.M., Howden, M., Reilly, J. and Rosenzweig, C. 2006. Crop response to elevated CO2 and world food supply: A comment on "Food for Thought ..." by Long et. al., Science 312: 1918-1921. European Journal of Agronomy: 10.1016/j.eja.2006.10.002.