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The rate of rise of sea level globally has lessened by 20 per cent during the last decade, according to a new study. This is because of 3.2 trillion tons of excess liquid water storage on land in aquifers, lakes and other water bodies. Though glaciers and ice bergs have been steadily melting, the earth has been behaving like a sponge and absorbing the water which should otherwise have flowed back to the oceans from the land, thus closing the hydrological cycle.
While increase in sea level can be attributed to glacier and ice-sheet melting and transfer of water from aquifers to ocean from human–driven pumping, the reason for the slowing down of the sea level rise has not been accurately attributed until this study, which states that climate driven hydrology has a major role.
The study also quantifies the contribution of land water storage to the offset to sea level rise. The study by NASA’s JPL and the University of California, Irvine, used data from the Gravity Recovery and Climate Experiment (GRACE) twin satellites orbiting the earth to calculate the amounts of water stored in the continents globally and the changes in these water quantities.
By measuring the distance between the two GRACE satellites to within the width of a strand of human hair as they orbit the Earth, researchers can detect changes in Earth’s gravitational pull that result from regional changes in the amount of water across Earth’s surface. With careful analysis of these data, JPL scientists were able to measure the change in liquid water storage on the continents, as well as the changes in ice sheets and glaciers.
IPCC estimates for the past decade of contributions to sea-level rise due to human-driven pumping were also used to account for the net sea level rise.
The study is the first to observe global patterns of changes in land water storage, with wet regions getting more wet and dry areas getting drier. These patterns are consistent with earlier observations of changing precipitation over both land and oceans, and with IPCC projections of changing precipitation under a warming climate. But a much longer data record is needed to fully understand the underlying cause of the patterns and whether they will persist. Dr. John T. Reager from JPL is the lead author of the study published in Science.
Human-induced changes in land water storage include the direct effects of groundwater extraction, irrigation, impoundment in reservoirs, wetland drainage, and deforestation. Several studies of large aquifers suggest that trends in regional and global land-water storage are now strongly influenced by the effects of groundwater withdrawal. Human activity (including groundwater pumping and impoundment in reservoirs) is estimated to have directly resulted in a net 15-25 per cent of observed sea level rise between 1993 and 2010. Climate-driven variability in rainfall, evaporation, and runoff also contributes to decadal rates of sea level change through changes in the total amount of water held in snow, soil, surface waters, and aquifers.
The study found that for the period 2002-2014, land water storage losses in water losing regions resulted in a gain in sea level of 0.97 mm per year. This included human–driven losses due to groundwater depletion. Land water storage gains in water gaining regions led to a gross decrease in sea level of 1.3 mm per year. The combined impacts of gaining and losing regions in land water storage were a net offset to sea level increase. Thus, land water storage acted as a net sea level sink during 2002-2014, resulting from a balance between human and climate-driven changes in hydrology.
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