Global Adaptation Potential and Uncertainties. Aggregated globally, expansion of irrigation agriculture has the potential to increase production on current cropland. However, model projections indicate that even under the most optimistic assumptions about freshwater distribution and transportation within FPUs, the beneficial effect would be exhausted by detrimental climate change effects on crop yields by 2070 at the latest, for irrigation scenario IWDcrop and crop yields estimated with the inclusion of the effects of increasing [CO2] (Fig. 5). By 2090, 57% of the median 730-Pcal reduction due to climate change with effects of increasing [CO2] could be ameliorated by the net expansion of irrigation according to the more optimistic irrigation scenario (IWDcrop). Under the more pessimistic irrigation scenario (IWDhydro), the limitations on irrigation water supply availability further constrain the potential ameliorating effect of expanded irrigation to only 12% of the 1,840-Pcal reduction in 2090 due to climate change without effects of increasing [CO2], highlighting the need to improve agricultural productivity by other means. This general mechanism is valid for all GCM × (GGCM or GHM) combinations, although there is considerable variation among the projections of individual ensemble members (Fig. 5). Our analysis is subject to considerable uncertainties which we address in part here. Agricultural PIrrUse and corresponding increases in productivity have been simulated by the GGCMs
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