How do we improve crop production in a warming world?

Future agricultural production will encounter multifaceted challenges from global climate change. Carbon dioxide (CO2) and other greenhouse gases are accumulating in the atmosphere at unprecedented rates, causing increased radiative forcing (Le Quéré et al., 2009; Shindell et al., 2009). Continued emissions of greenhouse gases will increase annual temperatures by 2.5 C to 4.3 C in important crop-growing regions of the world by 2080 to 2099, according to the Intergovernmental Panel on Climate Change (IPCC) A1B scenario (Christensen et al., 2007). Growing season temperatures are expected to warm more than the annual averages, with reduced precipitation expected to accompany higher temperatures in some regions. Additionally, heat waves are expected to increase in frequency, intensity, and duration (Tebaldi et al., 2006; Christensen et al., 2007), and end-of-century growing season temperatures in the tropics and subtropics may exceed even the most extreme seasonal temperatures measured to date (Battisti and Naylor, 2009). Despite these dramatic predictions for rising global temperatures and extreme temperature events, the latest IPCC assessment report predicts that adaptation of agriculture will result in increased yields of cereal crops (maize [Zea mays], wheat [Triticum spp.], and rice [Oryza sativa]) in midto high-latitude regions with modest increases in temperature across a range of CO2 concentrations and precipitation changes (Easterling et al., 2007). With warming temperatures of 1 C to 3 C, yields at lower latitudes are predicted to decrease, although global food production is predicted to increase (Easterling et al., 2007). The IPCC projections assume that yield improvements from the latter half of the 20th century will continue into the future; however, based on historical temperature-crop yield relationships, potential ceilings to crop yields, and limitations to expansion of agricultural lands, that assumption may not be sound (Long and Ort, 2010). In fact, the relative rates of yield increase for all of the major cereal crops are already declining (Fischer and Edmeades, 2010). In a global analysis of crop yields from 1981 to 2002, there was a negative response of wheat, maize, and barley (Hordeum vulgare) yields to rising temperature, costing an estimated $5 billion per year (Lobell and Field, 2007). An analysis of maize and soybean (Glycine max) production in the northern Corn Belt region of the United States found that productivity was adversely affected by rising growing season temperatures from 1976 to 2006 (Kucharik and Serbin, 2008). The response of maize and soybean to temperature is also nonlinear, and the decline in yields above the temperature optimum is significantly steeper than the incline below it (Schlenker and Roberts, 2009). Based on the nonlinearity of the temperature response, U.S. maize and soybean yields were predicted to decrease by 30% to 46% before the end of the century under the IPCC scenario with the slowest warming trend (Schlenker and Roberts, 2009). In addition to these historical trends, record crop yield losses were reported in 2003, when Europe experienced a heat wave with July temperatures up to 6 C above average and annual precipitation 50% below average (Ciais et al., 2005). Such extreme events are not well characterized in the IPCC assessment simulations (Easterling et al., 2007). Therefore, increased global temperatures and more frequent temperature extremes will greatly challenge agriculture in this century. Here, we identify regional priorities and biological targets for adaptation of agriculture to rising temperature.