Six decades of warming and drought in the world's top wheat-producing countries offset the benefits of rising CO2 to yield

Abstract

Future atmospheric carbon-dioxide concentration ([CO₂]) rise is expected to increase the grain yield of C3 crops like wheat even higher under drought. This expectation is based on small-scale experiments and model simulations based on such observations. However, this combined effect has never been confirmed through actual observations at the nationwide or regional scale. We present the first evidence that warming and drought in the world's leading wheat-producing countries offset the benefits of increasing [CO₂] to wheat yield in the last six decades. Using country-level wheat yield census observations, [CO₂] records, and gridded climate data in a statistical model based on a well-established methodology, we show that a [CO₂] rise of ~ 98 μmol mol^-1 increased the yield by 7% in the area of the top-twelve wheat-producing countries, while warming of 1.2 °C and water depletion of ~ 29 mm m^-2 reduced the wheat grain yield by ~ 3% and ~ 1%, respectively, in the last six decades (1961-2019). Our statistical model corroborated the beneficial effect of [CO₂] but contrasted the expected increase of grain yield under drought. Moreover, the increase in [CO₂] barely offsets the adverse impacts of warming and drought in countries like Germany and France, with a net yield loss of 3.1% and no gain, respectively, at the end of the sampling period relative to the 1961-1965 baseline. In China and the wheat-growing areas of the former Soviet Union-two of the three largest wheat-producing regions-yields were ~ 5.5% less than expected from current [CO₂] levels. Our results suggest shifting our efforts towards more experimental studies set in currently warm and dry areas and combining these with statistical and numerical modeling to improve our understanding of future impacts of a warmer and drier world with higher [CO₂].