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Global warming only partially responsible for Greenland ice sheet’s record shrinking

Last year was one of the worst on record for the shrinking of the Greenland ice sheet, US scientists have said.

Hundreds of billions of tons of ice melted over the course of the year, and the loss wasn’t caused by warm temperatures alone, according to a new study from Columbia University, New York.

“Exceptional” atmospheric circulation patterns were discovered that contributed in a major way to the ice sheet’s rapid loss of mass.

Current climate models do not account for these atmospheric patterns, meaning they could be underestimating future melting by about half, lead author Marco Tedesco said.

High pressure conditions above Greenland were found to have inhibited the formation of clouds in the southern part of the country.

The resulting clear skies let in more sunlight to melt the surface of the ice sheet and with fewer clouds, there was about 50 billion fewer tons of snowfall than usual to add to the mass of the ice sheet.

The lack of snowfall also left dark, bare ice exposed in some places, and because ice doesn’t reflect as much sunlight as fresh snow, it absorbed more heat and exacerbated melting and runoff.

The study used satellite data, ground measurements, and climate models to analyse changes in the ice sheet during the summer of 2019.

The researchers found that while 2019 saw the second-highest amount of runoff from melting ice (2012 was worse), it brought the biggest drops in surface mass balance since record-keeping began in 1948.

Surface mass balance takes into account gains in the ice sheet’s mass — such as through snowfall — as well as losses from surface meltwater runoff.

“You can see the mass balance in Greenland as your bank account,” said Tedesco. “In some periods you spend more, and in some periods you earn more. If you spend too much you go negative. This is what happened to Greenland recently.”

“We’re destroying ice in decades that was built over thousands of years. What we do here has huge implications for everywhere else in the world.”

The ice sheet’s 600 billion tonne of water loss last year would contribute about 1.5 millimetres of sea level rise, according to the study. Greenland’s ice sheet covers 80 per cent of the island and could raise global sea levels by up to 7 metres if it melted entirely.

Greenland contributed 20-25 per cent of global sea level rise over the last few decades, Tedesco said. If carbon emissions continue to grow, this share could rise to around 40 per cent by 2100.

In 2018 researchers from Denmark expressed hope that the West Antarctic Ice Sheet, which is at risk of floating away due to global warming, may be halted in its tracks by bedrock that is rising as the weight of the melting ice lowers. 

The melting of polar icecaps and the consequent rise in global sea level is a deeply troubling concern for many coastal towns and cities. Scientists have warned that some coastal areas of the US could see the kind of devastating floods that previously occurred only twice a century happening on an almost daily basis by the year 2100, as ocean levels rise because of planetary warming and ice melt.

By 2050, almost 70 per cent of 200 coastal areas studied could see such major floods at least once a year, said Sean Vitousek, an oceanographer and engineer and a researcher with the US Geological Survey (USGS) in California.

For low-lying cities such as Miami or Honolulu, current "nuisance" flooding - which temporarily blocks roads or backs up through stormwater drains - could soon become the norm, requiring residents to adapt or even leave, he said.

Without efforts to curb climate-changing emissions or put in place new protective measures, flooding "will continue and get worse until it almost makes some areas impractical to live in because it’s happening so frequently", Vitousek told the Thomson Reuters Foundation. "When what we consider now as the extreme happens at every high tide - that’s no joke".

Vitousek's research, carried out with colleagues at the USGS, the University of Illinois at Chicago and the University of Hawaii, looked at historical records of sea level fluctuations from 200 tidal gauges around the U.S. coastline.

Those records were paired with models of expected sea level rise driven by climate change, to predict how often flooding was likely to occur in each location in coming decades.

The findings, published in the journal Scientific Reports, showed expected sea level rise of at least 1m (3ft) by 2100 was likely to bring flooding on a near-daily basis in 93 per cent of locations studied.

"You take high tide, add on an extra metre, and you're exceeding thresholds at every high tide," Vitousek said.

This story was updated on April 17 to include the information from the US Geological Survey.

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