Groundwater‑use big contributor to sea-level rise

Unsustainable groundwater use contributed 48 mm to the amount the global sea-level rose between 1951 and 2000, according to Japanese research published recently on human contributions to sea-level change.

In a paper, Model estimates of sea-level change due to anthropogenic impacts on terrestrial water storage, published in Nature Geoscience , the team of six centred on the University of Tokyo say that, while the global sea-level is rising, only part of that rise can be attributed to the effects of climate change.

They estimate sea-level change in response to human impacts on terrestrial water storage by using an integrated model that simulates global terrestrial water stocks and flows (exclusive of Greenland and Antarctica) and especially accounts for human activities such as reservoir operation and irrigation.

Tidal measurements have revealed that the global sea level rose by 1.8 mm per year between 1951 and 2000. But, the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) estimated that thermal expansion and loss of ice accounted for just 1.1 mm per year of sea level rise, leaving 0.7 mm unaccounted for.

Searching for the reason for this, the researchers used their model to calculate that artificial reservoirs caused the average sea-level to fall 15 mm over 50 years, by withholding water on land. When seepage from reservoirs was included, this figure rose to 21 mm.

However, more than counteracting this drop in sea-level, the researchers found that unsustainable groundwater use resulted in a sea level rise of 48 mm, as most water extracted from the ground for domestic, industrial and agricultural uses eventually makes its way to the sea. Groundwater depletion has been increasing, say the team, and may continue to do so, heightening concerns about sea-level rise in the 21st century.

When the model results were compared to the gap in sea-level rise reported by the IPCC, covering 1961‑2003, they found that net effects of terrestrial water storage sources accounted for an extra 0.77 mm per year of sea-level rise. This closely matched the 0.7 mm difference between total sea-level rise and the effects of climate change.

The 0.77 mm per year rise consisted of a fall of 0.39 mm due to reservoir storage and a rise of 1.05 mm from groundwater, 0.08 mm from climate-driven factors, and 0.03 mm irrigation losses from the Aral Sea.

This research should provide valuable support for agencies promoting artificial aquifer recharge.