Climatic classification and future global redistribution of agricultural land

Future global carbon (C) cycle dynamics under climates altered by increased concentrations of greenhouse gases (GHGs) will be defined in part by processes which control terrestrial biospher~c C stocks and fluxes. Current research and modeling activities which involve terrestrial C have focused on the response of unmanaged vegetation to changing climate and atmospheric chemistry. A common conclusion reached from applying geographically explic~t terrestrial carbon models is that more C would be stored by equilibrium vegetation controlled by a stable GHG-warmed climate than by equihbrium vegetation under the current (stable) climate. We examined the potential Impact on the terrestrial C cycle if global agriculture were to increase to the limits permitted by future GHG-induced climates. Climatic limits to global agricultural zones were determined, the new climatic limits to agricultural zones projected, and the amount of C the terrestrial biosphere would store under the new climate and agricultural conditions was calculated. We conclude that following a warming loss of C from agnculture could be as important as gain of C by climate effects. As much or less C would be stored by a terrestrial biosphere in which agriculture reached its new climatic limits as is stored by the current biosphere in which agriculture reaches its climatic l im~ts. We project that agnculture alone could produce a C source of 0.3 to 1.7 Pg yr-' if doubling of GHGs required 50 to 100 yr The gains in agnculture would occur almost entirely in the developed countries of high latitudes, and the losses, in the less developed countries of the lower latitudes.