Long-term alterations in evapotranspiration—the movement of water from the land to the atmosphere—would be a clear sign of an accelerating hydrologic cycle, itself a potential effect of climate change with implications for sea level. One wide-ranging study relied on NASA’s long running, recently ended Tropical Rainfall Measuring Mission, which made microwave observations from orbit [Jung et al., 2010]. Combined with ground-based observations, modeling and a machine-learning algorithm, the microwave data provided a likely explanation for evapotranspiration changes on a decadal scale. The study found that yearly global evapotranspiration increased from 1982 to 1997, then plateaued, in concert with the significant El Niño event of 1998. It remained flat until 2008. The microwave data revealed a decrease in soil moisture during the same period, 1998 to 2008, particularly across large areas of Australia, East Africa and South America [Jung et al., 2010]. The study’s authors could not, however, determine whether the evapotranspiration changes were the product of short-term climate variability or a longer-term shift in the water cycle.
Data from the twin GRACE satellites have been used in many studies of water storage on land and of the exchange of water between land and ocean. One recent study analyzed gravitational trends in groundwater basins, finding that a large fraction of the world’s biggest aquifers are rapidly being depleted [Richey et al., 2015].
Another study more directly applicable to global sea level examined yearly changes in water volume in 27 large river basins around the world from 2003 to 2006 [Ramillien et al., 2008]. Among some of the basins with the largest shifts in water volume—the Amazon, Ganges, Mississippi, Nile, Parana and Zambezi—the study’s authors found a negative trend in water storage for the three-year period. For all the basins examined, they also found that a net loss of water from terrestrial reservoirs resulted in a net contribution to global sea level rise of about 0.19 millimeters per year.
A third study used GRACE to estimate monthly freshwater discharges from continents, drainage regions and global land surfaces from 2003 to 2005 [Syed et al., 2009]. The authors concluded that their method offered a promising means of monitoring freshwater discharge in near-real time, contributing to global water-mass estimates and improving hydrological modeling.