At Tuesday’s House Energy & Commerce Committee hearing on Climate Science and EPA’s Greenhouse Gas Regulation, Dr. Christopher Field of the Carnegie Institution for Science, presented a scary assessment of global warming’s impact on U.S. grain yields. Field’s written testimony states, in pertinent part:
In the United States, the observed temperature sensitivity of three major crops is even more striking. Based on a careful county-by county analysis of patterns of climate and yields of corn, soybeans, and cotton, Schlenker and Roberts (Schlenker and Roberts 2009) concluded that observed yields from all farms and farmers are relatively insensitive to temperature up to a threshold but fall rapidly as temperatures rise above the threshold. For farms in the United States, the temperature threshold is 84˚F for corn, 86˚F for soybeans, and 90˚F for cotton. For corn, a single day at 104˚F instead of 84˚F reduces observed yields by about 7%. These temperature sensitivities are based on observed responses, including data from all of the US counties that grow cotton and all of the Eastern counties that grow corn or soybeans. These are not simulated responses. They are observed in the aggregate yields of thousands of farms in thousands of locations.
The testimony continues:
The temperature sensitivity observed by Schlenker and Roberts (Schlenker and Roberts 2009) suggests a challenging future for US agriculture. Unless we can develop varieties with improved heat tolerance, modest warming (based on the IPCC B1 scenario) by the end of the 21st century will reduce yields by 30-46%. With a high estimate of climate change (based on the IPCC A1FI scenario), the loss of yield is 63-82%. These three major crops, in some ways the core of US agriculture, are exquisitely sensitive to warming. This result is very clear. We may be able to breed warming tolerant varieties, and it is possible that some of the yield losses due to warming will be compensated by positive responses to elevated atmospheric CO2 (Long et al. 2006), but we will be trying to improve yields in a setting where warming is like an anchor pulling us back.
Well, I’m neither a scientist nor a farmer, but this sort of sky-is-falling alarm is suspicious. Field says his assessment is based on “thousands of farms in thousands of locations,” and that for U.S. corn farmers, the threshold beyond which corn yields fall is 84˚F.
Well, then, it must hardly if ever get warmer than 84˚F in Des Moines, Iowa, smack dab in the middle of corn country, right? Turns out, during 1970-2000, the average daily maximum July temperature in Des Moines was 86˚F. Monthly maximum temperatures were no doubt warmer still during 2001-2010. Due to flooding, U.S. corn production in 2010 was lower than in 2009. Nonetheless, the long term trend is up — from about 600 million bushels in 1958 to 2.2 billion bushels in 2010.
More pertinently (see p. 6 of U.S. Corn Production & Use Outlook), the long-established trend for U.S. corn yields is up, up, and up. In approximate terms, corn yields were 1,500 kilograms per hectare (kg/ha) in 1935, 2,200 kg/ha in 1945, 2,800 kg/ha in 1955, 4,500 kg/ha in 1965, 5,500 kg/ha in 1975, 6,500 kg/ha in 1985, 7,500 kg/ha in 1995, and 9,500 kg/ha in 2005.
The northwest corner of the Texas Panhandle, which includes Amarillo, is a major corn producing area. During 1970-2000, the average daily maximum July temperature in Amarillo was 91˚F — 7 degrees above the threshold at which yields supposedly fall.
In fact, in most of the places corn is grown in the United States, July daily maximum temperatures often reach 90˚F. As Alabama State Climatologist John Christy pointed out at the hearing, 84˚F is a very cool day in corn season in Alabama. Nationally, from 1980/81 to 2008/2009, average U.S. corn yield has increased from about 2.5 tons per acre to nearly 4 tons per acre.
My advice to corn growers — the end is not nigh, don’t sell the farm!