A study published in Nature Geoscience concluded that the global demand for fresh water is contributing to the oceans’ rise faster than the impact of global warming on melting glaciers. The trillions of tons of fresh water pumped out of underground aquifers, and then used for irrigation and to keep cities watered and fed is seeping into oceans faster than those underground water supplies can be replenished. The researchers leading the study insist that the impact of humans’ unquenchable usage of water over the past 50 years has been grossly overlooked.
University of Tokyo Ph.D. Yadu Pokhrel was among the group of researchers who studied the impact that groundwater extraction has had on rising sea levels. Since 1961, scientific evidence generally agrees that the oceans levels have risen an average of 1.8 millimeters a year. The combination of melting ice caps and glaciers, along with the expansion of water due to the oceans’ slow heating, contributes about 1.1 mm to that figure. Melting ice caps and glaciers contribute around 0.4 mm each year. Meanwhile 0.7 mm of the oceans’ rise had long been unexplained.
Pokhrel’s group developed an integrated statistical model that measured global groundwater supplies and matched them to human activities such as reservoir maintenance and irrigation. The result was a sea-level rise of 0.77 mm a year increase the last half century that the researchers say can be attributed to humans’ rapid consumption of groundwater.
The consumption of groundwater across the world, from China to Saudi Arabia, in both rural areas and urban areas like Mexico City, has therefore become unsustainable. Since the early 1960s an estimated 18 trillion tons of water, most of which took millenia to collect, have been pumped out of underground reservoirs. That water eventually flows into the world’s oceans at a much faster rate than rainfall can eventually replenish the ancient aquifers. The study’s conclusions add to the growing concern among scientists that even if climate change is to be stalled or even reversed, oceans will still continue to rise because of human activity.