Data show that sea levels have been rising significantly over the past half century. The contributions from thermal expansion of the oceans, the melting of glaciers, and loss of ice masses in Greenland and Antarctica are commonly studied, but together they do not account for the total sea level rise (SLR). The rate of SLR is ~1.8 mm yr–1, but the total contributions from these sources is estimated to be about ~1.1 mm yr–1, which leaves ~0.7 mm yr–1 unaccounted for. Pokhrel et al. (2012) examined terrestrial water storage, namely reservoir water impoundment and unsustainable groundwater irrigation, and found that these types of sources likely contribute about 0.77 mm yr–1 to SLR. Thus, anthropogenic water uses contribute greatly to changing sea levels, and can help close the gap in the global sea level rise.¾Olivia Jacobs
Pokhrel, Y.N., Hanasaki N., Yeh, P.J., Yamada, T.J., Kanae, S., Oki, T. Model estimates of sea-
level change due to anthropogenic impacts on terrestrial water storage. Nature
Geoscience 5, 389–392.
Pokhrel et al. (2012) used an integrated water resource assessment modeling framework to analyze the potential contributions of anthropogenic sources to global sea level change (SLC). They estimated many values such as the amount of water lost to seepage in reservoirs, the number of years to fill a reservoir, and the rate of change in loss over time, because no real values are currently available. Using these estimates and data from numerous supplementary sources, they concluded that the global reservoir capacity is about 8,000 km3. They also estimated unsustainable groundwater use based on total water demand and the availability of water from near-surface sources, which they concluded gave them an accurate estimate of groundwater contributions to SLR.
By comparing their estimated values with other model simulations, Pokhrel et al.were able to estimate the relative contributions from different types of terrestrial water storage (TWS) to SLC. They focused on artificial reservoir impoundment and unsustainable groundwater use and found that these two variables can close the gap in SLC quite successfully. Artificial reservoirs generally cause a drop in the sea level by holding water over land, and this study estimated that the cumulative contributions of reservoir capacity and storage to SLC was ~22 and ~15 mm, respectively. Further, when seepage from reservoirs was accounted for, the estimates of artificial reservoir capacity was ~31 mm, and the actual estimated contribution was ~21 mm. In both of these scenarios, the scientists noted a large discrepancy between reservoir capacity and reservoir storage.
Unsustainable groundwater use contributes to SLR because water is removed from the ground and eventually ends up in the oceans. Here, the scientists estimated groundwater depletion (GWD) has contributed ~48 mm to cumulative SLR. Also, climate-driven terrestrial water storage (TWS), including soil moisture and snow and river storage exclusive from Greenland and Antarctica, has a net contribution of ~8 mm to global SLR. Thus, while climate-driven TWS has significant decadal and annual variation, the long-term contributions to global SLR are relatively small and anthropogenic redirecting of water contributes to SLR much more significantly. Pokhrel et al. also considered the uncertainty in their estimates, and found that the uncertainty of net TWS, including groundwater depletion, climate-driven TWS, and reservoir storage, could be as high as 30%.
Overall, the annual estimated contribution of these TWS sources to SLC was +0.77 mm yr–1, which is close to the previously unexplained gap in SLR of 0.70 mm yr–1. Individually, the estimates were +1.05 mm yr–1 from groundwater, +0.08 mm yr–1 from climate-driven TWS, –0.39 mm yr–1 from reservoir storage, and +0.03 mm yr–1 from the Aral Sea, which was a main source of water diversion for irrigation. Comparatively, the contribution from thermal expansions of the oceans was ~0.42 mm yr–1, ~0.5 mm yr–1came from glaciers and ice caps melting, and ~0.19 mm yr–1 from ice-mass loss in Greenland and Antarctica.
The trends in TWS over the past 50 years indicate that groundwater depletion has been increasing significantly over time and may increase more in the future, and the climate-driven TWS has also been increasing in recent years. Reservoir impoundment has also recently leveled off, so there may be an even greater contribution to SLR from TWS sources in the future, and while there is obvious uncertainty in estimations used in this study, they are all within the plausible limits for many countries. The scientists note that some sources, such as the effects of deforestation and wetland drainage, were not considered, but these sources have previously been shown to contribute very little to global SLR. Thus, these TWS sources can help close the gap in estimations of global sea-level change, and can help shed light on the future forecasts for global SLR.