Where is the water coming from that cause current sea-level rise?

Sea-level rise is a well-studied climate change phenomenon. General explanation is that thermal expansion of water and water from melting ice caps and glaciers are responsible for the rise. Global sea-level rose about 1.8 millimetres every year in the last decades. 1.1mm from the 1.8 is explained by melting ice and ocean expansion combined which leaves 0.7mm unexplained. New study in Nature Geoscience suggests an additional component which can help to solve this equation: human use of terrestrial water.

The researchers state that human impacts on terrestrial water storage could account for as much as 0.77 millimetres per year, or 42%, of the observed sea-level rise between 1961 and 2003. They used an integrated model to estimate sea-level rise caused by human use of terrestrial water (e.g. irrigation, industrial use, reservoir management). Most of the extracted subsurface water is not restocked because it either evaporates or flows into rivers ending up in seas eventually. They found that unsustainable groundwater use, artificial reservoir water impoundment, climate-driven changes in terrestrial water storage and the loss of water from closed basins are the main drivers of rising sea levels. From those drivers, unsustainable groundwater use represents the largest contribution. Even Taikan Oki, one of the researchers, was surprised by their results: “I didn’t expect that human extraction of groundwater would matter so much”.

Groundwater provides up to 80% of public water supplies for large areas of England. The current drought and hosepipe bans in England already drew attention to this precious resource and the need for its sustainable use. Findings of this new research underline this need for the sake of mitigating sea-level rise.

Yadu N. Pokhrel, Naota Hanasaki, Pat J-F. Yeh, Tomohito J. Yamada, Shinjiro Kanae & Taikan Oki 2012 Model estimates of sea-level change due to anthropogenic impacts on terrestrial water storage. Nature Geoscience. doi:10.1038/ngeo1476