waters and atmosphere), which is as important to oceanographers as atmospheric pressure is to meteorologists. For example, measuring ocean pressure gradients allows scientists to estimate monthly changes in deep ocean currents. The limited resolution of GRACE is acceptable in this research because large ocean currents can also be estimated and verified by an ocean buoy network. Scientists have also detailed improved methods for using GRACE data to describe Earth's gravity field. GRACE data are critical in helping to determine the cause of sea level rise, whether it is the result of mass being added to the ocean - from melting glaciers, for example - or from thermal expansion of warming water or changes in salinity. High-resolution static gravity fields estimated from GRACE data have helped improve the understanding of global ocean circulation. The hills and valleys in the ocean's surface (ocean surface topography) are due to currents and variations in Earth's gravity field. GRACE enables separation of those two effects to better measure ocean currents and their effect on climate. GRACE data have provided a record of mass loss within the ice sheets of Greenland and Antarctica. Greenland has been found to lose of ice per year between 2003 and 2013, while Antarctica has lost per year in the same period. These equate to a total of 0.9 mm/yr of sea level rise. GRACE data have also provided insights into regional hydrology inaccessible to other forms of remote sensing: for example, groundwater depletion in India and California. The annual hydrology of the Amazon basin provides an especially strong signal when viewed by GRACE. A University of California, Irvine-led study published in Water Resources Research on 16 June 2015 used GRACE data between 2003 and 2013 to conclude that 21 of the world's 37 largest aquifers "have exceeded sustainability tipping points and are being depleted" and thirteen of them are "considered significantly distressed." The most over-stressed is the Arabian Aquifer System, upon which more than 60 million people depend for water. Geophysics GRACE also detects changes in the gravity field due to geophysical processes. Glacial isostatic adjustment—the slow rise of land masses once depressed by the weight of ice sheets from the last ice age—is chief among these signals. GIA signals appear as secular trends in gravity field measurements and must be removed to accurately estimate changes in water and ice mass in a region. GRACE is also sensitive to permanent