As the planet warms, sea levels are set to rise for two main reasons: glaciers and ice sheets on land will melt, adding extra volumes of water to the planet’s oceans. Meanwhile, seawater also expands in warmer temperatures as a result of a phenomenon called thermal expansion.

The melting of the ice sheets is well under way. Greenland alone lost a record 532 billion tons last year, according to a team of scientists. That translates into 3 million tons of extra water every day, or six Olympic-sized swimming pools every second.

Between 2003 and 2016, the melting of Greenland’s massive ice sheet, which is up to 3km thick in some parts, was one of the largest contributors to a rise in sea levels globally with a loss of about 255 gigatons of ice a year.

At the same time, last year melting glaciers and torrents of melt-water served as the biggest source of sea level rise globally, accounting for 40% of it, which was equivalent to 1.5 millimeters. Greenland’s ice sheet lost 1.13 trillion tons, of which 45% was from glaciers sliding into the sea, and the rest came from melted ice.

Local ice sheets gained only about 600 billion tons in volume from rain and snow in 2019, which means the loss was nearly double the gain.

Worse: there were four other years this decade with a record loss of sea ice on Greenland. Should all of Greenland’s ice sheet melt, the level of oceans worldwide would rise by 7 meters, which would have cataclysmic consequences for all human communities living on and near seashores.

And it seems the melting of Greenland’s ice sheet has already reached a “tipping point” and is set to melt over time, although it remains uncertain at what speed that will happen, according to the authors of another study.

“The Greenland Ice Sheet is losing mass at accelerated rates in the 21st century, making it the largest single contributor to rising sea levels,” they write. “Faster flow of outlet glaciers has substantially contributed to this loss, with the cause of speedup, and potential for future change, uncertain.”

Even a partial melting of all the ice stored in Greenland would remake coastlines worldwide and inundate islands and low-lying coastal areas, which would force hundreds of millions of people to relocate.

However, because the rate of thermal expansion, which accelerates with warming temperatures, will be uneven worldwide, sea levels will rise at different rates, experts say. The hardest hit, they say, will be areas where changes from ocean thermodynamics and other climate variability processes align because coastal flooding is driven both by rising sea levels and ocean variability.

“Whereas it is well understood that the rate of global mean sea level rise will accelerate with future warming, in part due to the oceans expanding faster at higher temperatures, it was previously unexplored how this nonlinear thermal expansion property of seawater will affect future sea level variability,” says Matthew Widlansky, associate director of the UH Sea Level Center who headed a new study on the matter.

“Forecasting can potentially provide alerts months in advance if sea levels are likely to cause tides to be more extreme than otherwise expected,” he adds.

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