Local Plant Physiological Responses to Increasing CO2 Contribute to a Zonally Asymmetric Pattern of Precipitation Change over

Friday, December 16, 2016 - 15:10
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Understanding how anthropogenic CO2 emissions may impact future precipitation patterns is a critical question for earth science and society, especially over tropical forests where changes affect drought conditions, ecosystem health, and the availability of freshwater. While there remains significant uncertainty about how tropical precipitation will change in the future, CMIP5 models robustly project a consistent zonally asymmetric pattern over land, amplifying differences between the Maritime Continent and Amazon. This pattern cannot be explained by mechanisms describing zonal mean changes to the hydrological cycle (e.g. Hadley strength, ITCZ shift, or wet-get-wetter response). Here we show, in CESM1(BGC), that the pattern is largely controlled by plant physiological responses to increased CO2, which setup local dynamic anomalies over each continent, rather than global-scale radiative forcing. Regional precipitation and associated circulation changes that manifest with global CO2 increases are also captured when CO2 increases are isolated to the land-surfaces of individual continents. Increased CO2 throttles stomatal conductance, reducing local transpiration and increasing sensible heating and surface temperature. Changes in heating over land drive regional circulations that influence vertical mixing and moisture fluxes over each continent, leading to greater moisture transport into the upper atmosphere and more precipitation over Indonesia, Central Africa and the west coast of South America, and less precipitation over the Amazon.

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