For wheat, maize and barley, negative yield impacts for the 1980s and 1990–2002 indicate that recent climate trends have, unless addressed through adaptation measures, suppressed global yield progress for these three crops. Effects are less pronounced for other crops and decades, though with significant yield suppression for soybean and sorghum since 1990, and wheat in the 1970s. All instances of significant yield effects were attributable mainly to warming temperature trends, as precipitation trends had only minor effects on yields (not shown).
While small when expressed as a percentage of current yields, the absolute losses in global production due to warming trends since 1981 were substantial. Wheat, maize and barley production in 2002, for example, would have been roughly 2–3% higher without climate trends since 1981.
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The results suggest that recent climate trends, attributable to human activity [22], have had a discernible negative impact on global production of several major crops. The impact of warming was likely offset to some extent by fertilization effects of increased CO2 levels
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If each additional ppm of CO2 results in ~ 0.1% yield increase for C3 crops (a yield increase of 17% for a concentration increase from the current 380 ppm to the frequently studied 550 ppm) [23, 24], then the ~ 35 ppm increase since 1981 corresponds to a roughly 3.5% yield increase, about the same as the 3% decrease in wheat yield due to climate trends over this period. Thus, the effects of CO2 and climate trends have likely largely cancelled each other over the past two decades, with a small net effect on yields. This conclusion challenges model assessments that suggest global CO2 benefits will exceed temperature related losses up to ~ 2° warming (1).