Methods summary for food figure
For broad regions, yield losses per ºC of local warming were taken from Figure 5.2 in the Working Group 2 reports of the Fourth Assessment Report of the IPCC (Easterling et al., 2007). These estimates include estimates of CO2 effects but without explicit modeling of adaptation. The mean and one standard error for each level of warming were approximated from the figure. Local temperature changes were converted to global temperature levels using a value of 1.5ºC local per global ºC for mid-to-high latitudes and 1.2ºC local per global ºC for low latitudes.
Note that since several of these studies are based on experiments where climate is allowed to equilibrate with doubled CO2levels, while others were taken from transient simulations (e.g., based on SRES scenarios), the CO2levels for different amounts of warming likely varied by study, with the equilibrium studies likely underestimating CO2levels for a given warming amount.
Yield losses per ºC of local warming were taken from the following studies: U.S. maize and soybean (Schlenker and Roberts, 2009); Asia rice (Matthews et al., 1995); India wheat (Lal et al., 1998); Africa maize (Schlenker and Lobell, 2010). For each region, global temperatures were converted to local temperature change based on the patterns in Section 4.2. The yield effects of higher CO2 were estimated based on a recent metaanalysis (Ainsworth et al., 2008). CO2 levels for each temperature value were based on the values reported in Section 3.2, and assuming a ratio of CO2 to CO2-equivalent equal to the average of the SRES scenarios (ratio is 1.05 for 1ºC, 0.93 for 2ºC, and 0.8 for 3ºC and warmer).
Standard errors were estimated by propagating estimates of standard errors for (1) local temperature change for a given global temperature; (2) crop yield response to temperature; (3) CO2 levels for a given global temperature; and (4) yield response at a CO2 level. Propagation was done with the standard equations: