Asseng et al. (2014), a study published this week in Nature Climate Change, concludes that global warming “is already slowing yield gains at a majority of wheat-growing locations,” and estimates that worldwide wheat production will “fall by 6% for each °C of further temperature increase.” The study’s basic physical argument is that higher temperatures accelerate plant maturation, allowing fewer days for biomass accumulation and, thus, reducing yields.
The researchers acknowledge that “improvements in technology and management have led to increasing yields around the world.” Nonetheless, they contend, “wheat model simulations over the main global wheat-producing regions can isolate the climate signal by holding inputs and management constant with the exception of climate information.” Their model ensemble indicates that “Simulated yields declined between 1981 and 2010 (Fig. 2a) at 20 of the 30 representative global locations . . . owing to positive temperature trends over the same period.”
If I get their meaning, Asseng et al. claim that although global yields increased during 1981-2010, yields at those 20 locations would have been larger absent global warming. They also appear to be saying that absolute yield declines would have occurred at all 30 locations under a +2ºC warming scenario with even steeper declines under a +4ºC warming scenario.
A few observations spring to mind. First, the paper does not discuss Asseng et al.’s method for isolating the climate signal from other factors affecting yields. Climate economist Richard Tol cautions that the “signal” of recent climate change is “faint” and “drowned out by all the other things that have changed.” He elaborates:
If one tries to study the impacts of climate change on crops, for example, one must factor in the impact of new seeds, fertilizers, pesticides, and a host of other confounding variables such as air pollution and atmospheric deposition of nutrients. If one is interested in commercial agriculture, one needs to consider subsidies and international trade.
Second, there has not been much surface warming of the planet in 18 years, so it may be many decades before global average surface temperatures increase by 2ºC or more. The more gradual the rise in global temperatures, the greater the likelihood that management and technology will improve enough to prevent yield loss.
Third, management and technology have, in fact, boosted yields significantly during the current warm period. The world warmed 0.12ºC per decade during 1951-2012, according to the IPCC (AR5 Summary for Policymakers, p. 5), which implies an overall warming of about 0.72ºC. USDA’s Wheat Data Yearbook contains a chart showing, among other data, crop yield (tons per hectare) and total production (millions metric tons) over the 54-year period from 1960 to 2014.