All of the challenges identified in these papers will need to be addressed in a context of changing climates. Global circulation models tell us much about the large-scale, long-term changes that may occur, but they 8346 | www.pnas.org/cgi/doi/10.1073/pnas.1208054110 Sayer and Cassman are very uncertain tools when applied at local levels to address day-to-day realities of smallholder farmers who will bear the brunt of changes. Four case studies are presented that illustrate the uncertainties of global climate models and their limitations in addressing the short-term needs of poor farmers. Farmers have extensive capacity for local short-term adaptation. Global models have to be drawn upon carefully to enhance longer-term transformational changes in the ability of farmers to cope with the uncertainties of climate change