Greenland and Antarctica, Earth’s great ice sheets’ ice loss are now accelerating with losing mass six times faster than they were in the 1990s, thanks to global warming conditions.
The satellite data acquired at both poles for a comprehensive review is indisputable in its assessment of accelerating trends, say scientists.
Among them, Greenland and Antarctica lost 6.4 trillion tonnes of ice from 1992 to 2017.
This was sufficient to raise global sea-levels by 17.8mm.
“That’s not a good news story,” said Prof Andrew Shepherd from the University of Leeds in the UK.
“Today, the ice sheets contribute about a third of all sea-level rise, whereas in the 1990s, their contribution was actually pretty small at about 5%. This has important implications for the future, for coastal flooding and erosion,” he told BBC News.
The researcher co-leads a project called the Ice Sheet Mass Balance Intercomparison Exercise, or Imbie.
The team includes experts who have reviewed polar measurements obtained by observational spacecraft over nearly three decades.
These satellites have tracked the change in volume, flow and gravity of the ice sheets.
Imbie’s Antarctica assessment was lodged with the journal Nature in 2018. This week, its Greenland summary was published in the print edition of the periodical.
Latest milestones have been used by the team to offer some general remarks.
The key one is the recognition that ice losses are now running at the upper end of expectations when compared with the computer models used by the authoritative Intergovernmental Panel on Climate Change (IPCC).
The mid-range simulations (RCP4.5) in 2014 assessment of the panel suggested global sea-levels might rise by 53cm by 2100. But ice losses from Antarctica and Greenland are actually heading to much more pessimistic outcomes as per the Imbie team’s study and will likely add 17cm more to those forecasts at the end of the century.
“If that holds true it would put 400 million people at risk of annual coastal flooding by 2100,” said Prof Shepherd.
“What our latest estimates mean is that the timescales people are expecting will be shorter. Whatever town or coastal planning measures you’re intending to put in place, they need to be built sooner.”
Greenland and Antarctica are responding to climate change in slightly different ways.
The ice sheet losses of the southern polar region typically come from the melting effects of warmer ocean water that tend to attack its edges. Although the northern polar ice sheet experiences a more or less similar type of assault, it is also witnessing surface melt due to warmer air temperatures.
Of that combined 17.8mm sea-level rise, the contribution of Greenland ice losses was 10.6mm (60 %), and 7.2mm (40%) was due to Antarctica.
The combined rate of ice loss was at about 81 billion tonnes per year in the 1990s for the pair. However, by the 2010s, it had jumped to 475 billion tonnes per year.
The delivery time of the Imbie results was fixed so they could be incorporated into the IPCC’s next big assessment of the state of Earth’s climate or Sixth Assessment Report (AR6) due out next year.
The likely near-term demise of some dedicated polar satellites and in absence of clear and urgent plans to replace them, the future inter-comparisons risk being of poorer quality arise, Prof Shepherd warns.
His particular concern is to see successors to the European Space Agency’s CryoSat-2 satellite and the American space agency’s IceSat-2 platform.
In comparison to other satellites, these models observe more of the ice sheets as they fly orbits that reach very close to the north and south poles.
“I fear we will soon be back to the situation of the early 2000s when we had to make do with missions that were not really designed to look at polar regions. We’ll be doing our best despite the absence of the data we really require – unfortunately. But we’ve been there before.”