What Has Caused the Recent Changes in Global Evapotranspiration from 1982-2010?

Abstract:

Terrestrial evapotranspiration (ET) may change due to external climate forcing and internal climate variability. Some recent studies have shown considerable changes in terrestrial ET since the early 1980s, but the causes of these changes are unclear. In this study, the relative contributions of the external forcing and the internal climate variability to the recent ET changes since 1982 are examined. Three datasets of global terrestrial ET (estimated from satellite observations and land-model simulations) and the CMIP5 multi-model ensemble mean ET are analyzed, respectively, for the actual and externally forced ET changes, while the unforced ET variations are estimated as the actual ET minus the forced component. Large discrepancies and uncertainties of the ET, in terms of its trend, variability, and the temperature- and precipitation-dependence, are found among the three datasets. Results show that the global-mean forced ET exhibits an upward trend of 0.08 mm day^-1 century^-1 for the period from 1982-2010, which results primarily from rising evaporative demands in response to warmer temperatures and secondarily from increased precipitation that are caused by increasing anthropogenic emissions including greenhouse gases (GHGs) and anthropogenic aerosols. The forced ET also contains considerable multi-year to decadal variations during the latter half of the 20th century that are caused mainly by volcanic aerosol forcings. For the unforced ET, the global-mean trend during 1982-2010 ranges from -0.07 to 0.06 mm day^-1 century^-1 with varying spatial patterns among the three datasets. Furthermore, linkages between the unforced ET and internal climate modes are examined. The Interdecadal Pacific Oscillation (IPO) is found to be correlated with the global ET, but significance of the correlation varies among the three datasets. The results suggest that there are large uncertainties in our current estimates of global terrestrial ET for the recent decades, and the GHGs and aerosol external forcings seem to account for a large part of the apparent trend and variations in the recent terrestrial ET, although the impact of the IPO and other internal climate variability are also significant.