Over 230 million people under water

Submitted by AWL on 26 August, 2020 - 11:29 Author: Zack Muddle
Greenland ice sheet

The Greenland ice sheet had an average net loss of over one million tons of ice each minute in 2019, so research published on 20 August showed. One million tons is one cubic kilometre of water; across the year a record loss of over 500 billion tons.

That loss from 2019 could contribute well over 1mm of sea level rise globally, as well as diluting the Gulf Stream with fresh water. Antarctic ice loss is also progressing at an alarming rate. The cumulative impact is compounded by thermal expansion of the ocean’s water as the planet heats, and potentially accelerated by positive feedback mechanisms. As more surface snow is melted, darker ice is revealed, which absorbs more solar heat, and meltwater weakens ice sheets.

Modelling of sea level rise is complex and contested. The IPCC in 2018 stated (emphasis in original) that “It is very likely that the rate of global mean sea level rise during the 21st century will exceed the rate observed during 1971-2010 [even on the lowest carbon] scenarios due to increases in ocean warming and loss of mass from glaciers and ice sheets.”

“It is virtually certain that global mean sea level rise will continue beyond 2100, with sea level rise due to thermal expansion to continue for many centuries.”

Their “process-based models” predict roughly 1m rise for higher-carbon scenarios by 2100. However, “the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantially above” that.

Additionally, “Some semi-empirical models... project a median and 95th percentile that are about twice as large as the process-based models.”

“Semi-empirical” modelling, using statistical modelling based on past sea-level changes, is more contested than directly modelling currently-understood processes as in “process-based models”. But semi-empirical models have had “successful calibration and evaluation” against the record: more so than process-based.

The USA’s Fourth National Climate Assessment sees 2m by 2100 as the “[h]igh end of [the] very likely range under [a high carbon scenario] when accounting for possible ice cliff instabilities”.

Some sea level rise is now unavoidable, but, under all models, the bigger our reduction in greenhouse gas emissions, the lower the sea level rise.

An estimated 230 million people currently live below 1m above current high-tide lines, and so would be regularly under sea level with 1m rise. The (increased) number who would be hit by annual floods is much greater still, and then there’s the impact of flooding on agriculture and infrastructure,

With 2m, worse still.

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