Contribution of Semi-Arid Forests to the Climate System

The Long and Short of It Semi-arid forests cover close to 18% of Earth's land surface. If climate change were to stimulate carbon accumulation in these areas, resulting changes in the forests could both promote climate cooling and warming: On one hand, forest growth would draw CO 2 from the atmosphere, providing a cooling effect on climate; on the other, as forests grew and became more dense, their albedo would decrease, which would warm climate. Rotenberg and Yakir (p. 451 ; see the Perspective by Schimel ) now report that a shift in peak photosynthetic activities from summer to early spring would, indeed, cause carbon accumulation by the forests, but that a suppression of reflected longwave radiation effect would complement the better-known (shortwave) albedo effect, doubling the amount of potential warming. Several decades of carbon accumulation would thus be necessary to counteract these radiative changes.