Amazon Forests Modulate Continental Precipitation: What If We Lose Them?

Abstract:

Forests interact with the atmosphere via transpiration, a physiological process that actively pumps water from the soil to the atmosphere. Through this process, a series of ecoclimatic mechanisms are triggered, that define climate in many regions of the world. Recently, it has been proposed that forests might also exert a “pump-like” mechanism that drives moisture flux from the ocean. If this holds true, forest losses might have a much larger impact on climate than previously thought. In this work we analyze how the presence of the Amazon forests affects precipitation along streamlines in tropical South America. Long term monthly wind streamlines were computed using 850 hPa wind data from ERA-Interim. Long term monthly precipitation along each streamline was obtained from TRMM, UDEL and GPCC. Land cover type was obtained from MODIS. Streamlines were segmented based on the presence of forest and precipitation on land versus distance from the ocean was analyzed on each segment (n = 2706). We found that precipitation on segments before (after) the forests increase (decrease) exponentially with distance to the ocean. Although precipitation also increases in the forest, it does not follow an exponential model. Remarkably, forests function as precipitation attractors: precipitation grows exponentially towards the forest independently of the streamline orientation. Our results provide empirical evidence of a fundamental ecoclimatic and ecohydrological function of tropical forests that links physiological processes with larger scale mechanisms controlling regional-to-global climate processes. In consequence, current forest loss can alter this mechanism and fundamentally chance ecohydrological dynamics at multiple scales.