Earth greening and terrestrial water cycle change

Tuesday, December 10, 2019 - 13:40
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Long-term satellite measurements have established the significant greening of the Earth over the past four decades, indicating more “greening leaves” that can use energy to pump more water from land surfaces to the atmosphere. Terrestrial water cycle is likely to have been altered by this greening trend. The response of evapotranspiration plays a key role in the complex feedback loops of biosphere-atmosphere interactions. Thus, we first review the recent literature on the response of terrestrial evapotranspiration to Earth's greening and discuss how this response contribute to the observed global terrestrial evapotranspiration change over the past decades. A coupled model has been applied to quantify how other components of terrestrial water cycle (e.g., precipitation, runoff and soil moisture) have been influenced by the greening of the Earth. Both observation-based statistical analysis and observation-driven offline model simulations agree that more than 50% of the observed global intensification of terrestrial evapotranspiration is caused by the greening of the Earth (at 0.97 ± 0.16 trillion tones per year per decade). The coupled model simulation shows that the greening-induced precipitation has increased at the same magnitude as the evapotranspiration, and this contributes to ~30% of the observed increase in land precipitation. The impacts of greening on runoff and soil moisture differ from dry regions and wet regions. In wet regions, the greening does not significantly decrease runoff and soil moisture because it intensifies moisture recycling through a coincident increase of evapotranspiration and precipitation. But in dry regions, for example the Sahel, the greening is modeled to significantly decrease soil moisture through its coupling with the atmospheric water cycle. These results highlight the importance of accurate representation of vegetation dynamics in Earth system models for the prediction of terrestrial water cycle.

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