Contribution of semi-arid forests to the climate system

Forests both take up CO2, and enhance absorption of solar radiation, with contrasting effects on global temperature. Based on a nine-year study in the forests' dry timberline, we show that significant carbon sequestration (cooling effect) is maintained in the large dry transition zone (precipitation 200-600 mm) by shifts in peak photosynthetic activities from summer to early spring, and this is counteracted by longwave radiation (L) suppression (warming effect), doubling the forestation shortwave (S) albedo effect. Several decades of carbon accumulation are required to balance the twofold S+L effect. Desertification over the past several decades, however, contributed negative forcing at the earth's surface equivalent to ~20% of the global anthropogenic CO2 effect over the same period, moderating warming trends.