Observed vegetation-climate feedbacks in the United States

Observed vegetation feedbacks on temperature and precipitation are statistically assessed across the United States using satellite-based fraction of photosynthetically active radiation (FPAR) and monthly climate data for the period of 1982-2000. This study represents the first attempt to spatially quantify the observed local impact of vegetation on temperature and precipitation over the United States, for all months and by season. Lead-lag correlations and instantaneous feedback parameters are computed to determine the regions where vegetation substantially impacts the atmosphere and to quantify this local forcing. Temperature imposes a significant instantaneous forcing on FPAR, while precipitation's impact on FPAR is greatest at one-month lead, particularly across the prairie. An increase in vegetation raises the surface air temperature by absorbing additional radiation and, in some cases, masking the high albedo of snow cover. Vegetation generally exhibits a positive forcing on temperature, strongest in spring and particularly across the northern states. The local impact of FPAR on precipitation appears to be spatially inhomogeneous and relatively weak, potentially due to the atmospheric transport of transpired water. The computed feedback parameters can be used to evaluate vegetation-climate interactions simulated by models with dynamic vegetation.