Response of terrestrial evapotranspiration to Earth's greening

The greening of the Earth has been unequivocally observed in 30 years of satellite measurements from NOAA-AVHRR. Here, we review the recent literature on the response of terrestrial evapotranspiration (ET) to Earth's greening, including the contribution of greening to the global terrestrial ET change over recent decades and its role in the regulation by vegetation of changes in Earth's climate system. Although large uncertainties remain in the observation-based reconstructions of global terrestrial ET, all products established a significant increase in terrestrial ET over the past three decades (P < 0.05). The ensemble of all reconstructions over the period 1982-2011 provided a relatively robust estimate of 0.97 ± 0.16 trillion tonnes per year per decade, or 7.65 ± 1.26 mm per year per decade averaged over the terrestrial area. More than 50% of this global intensification of terrestrial ET was caused by the greening of the Earth, as evidenced by observation-based statistical analysis and observation-driven model simulations. Earth system model simulations further showed that this response is a key determinant of the complex feedback loops of Earth's greening with Earth's water and climate systems. These results highlight the need for much more accurate representation of vegetation dynamics and the sensitivity of ET to vegetation changes in Earth system models, which will ultimately improve the strategies developed for water resource management and climate change mitigation via ecosystem management.