Sensitivity of climate to changes in NDVI

The sensitivity of global and regional climate to changes in vegetation den sity is investigated using a coupled biosphere-atmosphere model. The magnit ude of the vegetation changes and their spatial distribution are based on n atural decadal variability of the normalized difference vegetation index (N DVI). Different scenarios using maximum and minimum vegetation cover were d erived from satellite records spanning the period 1982-90. Albedo decreased in the northern latitudes and increased in the Tropics wit h increased NDVI. The increase in vegetation density revealed that the vege tation's physiological response was constrained by the limits of the availa ble water resources. The difference between the maximum and minimum vegetat ion scenarios resulted in a 46% increase in absorbed visible solar radiatio n and a similar increase in gross photosynthetic CO, uptake on a global ann ual basis. This increase caused the canopy transpiration and interception f luxes to increase and reduced those from the soil. The redistribution of th e surface energy fluxes substantially reduced the Bowen ratio during the gr owing season, resulting in cooler and moister near-surface climate, except when soil moisture was limiting. Important effects of increased vegetation on climate are a cooling of about 1.8 K in the northern latitudes during the growing seaso n and a slight warming during the winter, which is primarily due to the mas king of high albedo of snow by a denser canopy; and a year-round cooling of 0.8 K in the Tropics. These results suggest that increases in vegetation density could partially compensate for parallel increases in greenhouse warming. Increasing vegetat ion density globally caused both evapotranspiration and precipitation to in crease. Evapotranspiration, however, increased more than precipitation, res ulting in a global soil-water deficit of about 15%. A spectral analysis on the simulated results showed that changes in the state of vegetation could affect the low-frequency modes of the precipitation distribution and might reduce its low-frequency variability in the Tropics while increasing it in northern latitudes.