These new results and observations indicate a significant risk of high-temperature thresholds being crossed that could substantially undermine food security globally in a 4°C world.

The projected impacts on water availability, ecosystems, agriculture, and human health could lead to large-scale displacement of populations and have adverse consequences for human security and economic and trade systems. The full scope of damages in a 4°C world has not been assessed to date. Large-scale and disruptive changes in the Earth system are generally not included in modeling exercises, and rarely in impact assessments. As global warming approaches and exceeds 2°C, the risk of crossing thresholds of nonlinear tipping elements in the Earth system, with abrupt climate change impacts and unprecedented high-temperature climate regimes, increases.

The report also says drought-affected areas would increase from 15.4% of global cropland today, to around 44% by 2100. The most severely affected regions in the next 30 to 90 years will likely be in southern Africa, the United States, southern Europe and Southeast Asia, says the report. In Africa, the report predicts 35% of cropland will become unsuitable for cultivation in a 5°C world.

http://documents.worldbank.org/curated/en/865571468149107611/pdf/NonAsciiFileName0.pdf