Increased water storage in North America and Scandinavia from GRACE gravity data

Space-borne gravity data from the Gravity Recovery and Climate Experiment (GRACE) have revealed trends in present-day continental water storage in many parts of the world. In North America and northern Europe, it has been difficult to provide reliable estimates because of the strong background signals of glacial isostatic adjustment(1). Attempts to separate the hydrologic signal from the background with numerical models(2) are affected by uncertainties in our understanding of the precise glacial history and mantle viscosity(3,4). Here we use a combination of GRACE data and measurements from the global positioning system to separate the hydrological signals without any model assumptions. According to our estimates, water storage in central North America increased by 43.0 +/- 5.0 Gt yr(-1) over the past decade. We attribute this increase to a recovery in terrestrial water storage after the extreme Canadian Prairies drought between 1999 and 2005. We find a smaller rise in water storage in southern Scandinavia, by 2.3 +/- 0.8 Gt yr(-1). In both North America and Scandinavia, our computed increases in water storage are consistent with long-term observations of terrestrial water level. We suggest that the detected mass gains in terrestrial water storage need to be taken into account in studies on global sea-level rise.