Rising sea levels have slowed over the past decade because of climate-induced and human-driven changes in land water storage, National Taiwan University (NTU) atmospheric science professor Lo Min-hui (羅敏輝) said.
Lo joined an international team of scientists who found that between 2002 and 2014, the increase in land water storage decelerated the rise in sea levels by about 20 percent, he told a news conference held by the Ministry of Science and Technology on Wednesday.
The study was the first to could gauge the effects of terrestrial groundwater storage on the rise in sea levels and was published in the academic journal Science in February.
In past decades, sea levels rose by an average 3.3mm every year due to warmer sea temperatures, melting ice sheets and human activity, Lo said, adding that between 2002 and 2014 — a period of global warming hiatus — the rise in sea levels slowed to an average increase of 2.5mm every year.
It had been hypothesized that an increase in land water storage could cause a slowdown in rising sea levels, but it remained unproven until the team’s research.
The team was able to quantify land water storage variations by measuring regional gravitational differences with NASA’s gravity recovery and climate experiment satellites.
While the Greenland and Antarctic ice sheets continue to melt, net groundwater storage has been rising because of an increase in rainfall and floods due to climate-driven variabilities in precipitation, such as a strong La Nina phenomenon that caused catastrophic floods in Australia in 2010 and 2011, Lo said.
That water has been trapped in inland waterway systems and cannot enter the sea, thereby mitigating the rise in sea levels, he said, adding that it has been predicted that the frequency of La Nina events could increase in the future.
“That was not the first observed slowdown in rising sea levels. A similar phenomenon occurred in the 1950s and 1960s, and it was likely due to more water being stored on land,” he said.
“However, additional water stored on land will eventually return to the sea, and the pace of sea-level rise is expected to speed up,” he said.
“Although a slowdown tends to occur about once every 20 years, it is still unknown whether it is a recurring phenomenon,” Lo added.
Human activity also affects sea level change, as damming rivers reduces the amount of water entering the sea, while draining groundwater does the opposite, he said.
The satellites data has revealed the reduction in groundwater levels in the North China Plain, northern India, Central Asia, south-central US and California due to large-scale utilization of groundwater.
Rising sea levels have a dramatic impact on island nations and low-lying countries, but climate-driven and human-driven changes in land water storage and their effects on sea levels have been absent from Intergovernmental Panel on Climate Change sea level budgets.
“The ability to measure a rise in sea levels and their contributing factors is extremely important to help us build disaster mitigation strategies,” Lo said. “It remains to be seen when sea level rise will reach a critical point of no return, but the melting of glaciers is irreversible.”
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