Expansion of the world's deserts due to global warming and vegetation-albedo feedback

Many subtropical and mid-latitude regions are expected to become drier due to climate change. This will lead to reduced vegetation which may in turn amplify the initial drying due to positive feedbacks such as albedo change in response to biome redistribution, an effect rarely accounted for in climate projections. Using a coupled atmosphere-ocean-land model with a dynamic vegetation component that predicts surface albedo change, here we simulate the climate change from 1901 to 2099 with CO 2 and other forcings. In a standard IPCC-style simulation with interactive atmosphere, ocean and soil moisture, the model simulated an increase in the world's 'warm desert' area of 2.5 million km 2 or 10% at the end of the 21 st century, mainly due to subtropical rainfall decrease and enhanced evaporative demand for soil moisture in response to greenhouse warming. In a more realistic simulation where the vegetation-albedo feedback was allowed to influence the atmospheric energy balance, the 'warm desert' area expands by 8.5 million km 2 or 34%. This occurs mostly as an expansion of the world's major subtropical deserts such as the Sahara, the Kalahari, the Gobi, and the Great Sandy Desert. The desiccation of these marginal zones will have major implication for the world's food supply and other ecosystem services. A consensus has emerged in recent climate projections from the Intergovernmental Panel on Climate Change (IPCC) that rainfall in the subtropical regions of the world may become scarcer (1-4), driven by atmospheric circulation changes in response to greenhouse warming (5). Signs of such changes have already emerged in recent decades in regions such as the Mediterranean, southwestern US and other northern subtropical regions (6-8). However, the IPCC models have not addressed satisfactorily some of the potentially important feedbacks that could generate large changes in the climate system, such as ice sheet instability, permafrost carbon, and from land and vegetation processes. Here we address the role of one such feedback, the vegetation-albedo feedback in projected climate change. Albedo is a leading component in vegetation-atmosphere interaction (9). Theoretical estimates suggest that an albedo increase of 0.1, typical for the conversion of forest to savanna (even larger for savanna to desert), will lead to a decrease of about 20 W m-2 in surface absorbed solar energy. Although acting on regional scale, such a large negative radiative forcing can significantly reduce upward motion, resulting in less moisture convergence and a reduction in rainfall of approximately …