Complete melting of the Greenland ice sheet under climate feedback of declining ice altitude

Heard about Sunday’s big climate news? We are afraid it is actually twice as bad as most media reports suggest:

Even in the purely theoretical scenario in which we immediately reduced all anthropogenic CO2 emissions to the point that the atmospheric concentration would flatline*, it is already likely that the entire Greenland ice sheet will melt as a consequence of the current human-induced climate change.

[*) A common misunderstanding: this does not require 100% reduction, as ocean CO2 uptake lags – it does require very steep reductions though: the atmospheric CO2 concentration is semi-cumulative, rather than cumulative.]

This worrying conclusion one can reach when adding the conclusions from a new report (1) by the Potsdam Institute for Climate Impact Research (PIK) – published in Nature Climate Change – to.an often-overlooked characteristic of our climate system (2).

1. Yet another positive feedback

Their climate scientists have incorporated* a positive climate feedback for ice sheet melting which hasn’t received much attention [unlike for instance glacier velocity and albedo changes]: the decline in height.

[*) The conclusions are derived from a specialised climate model, which performs calculations of ‘physical systems including the most important processes, for instance climate feedbacks associated with changes in snowfall and melt under global warming. The simulation proved able to correctly calculate both the observed ice-sheet of today and its evolution over previous glacial cycles, thus increasing the confidence that it can properly assess the future. All this makes the new estimate of Greenland temperature threshold more reliable than previous ones,’ the PIK press release states.]

At its centre the Greenland ice sheet is about three kilometres thick, which means it forms a serious mountain range in itself.

As the ice melts at the glacier fronts, so it does at the ice cap surface. This means the Greenland ice cap is not just shrinking in size, but also in altitude – and as a consequence the air above it rises exponentially faster in temperature.

Twice as bad times two

“We estimate that the warming threshold leading to a monostable, essentially ice-free state is in the range of 0.8–3.2 °C, with a best estimate of 1.6 °C,” the researchers write.

That is about twice as bad as model studies that do not take ice height into account calculate (3.1 degrees Celsius – 1.9–5.1, 95% confidence interval).

Today, already 0.8 degrees of global warming has been observed, the researchers state. That suggests we may be halfway – fast-forwarding through centuries and millennia of melting inertia half the ice on Greenland would have already disappeared and sea levels on average would be some 3 meters higher. Because although it is a delayed process, it would also be inevitable. Well, 3 meters of committed sea level rise, that’s bad enough you would say.

2. Earth’s climate inertia

Now we say it is twice as bad as twice as bad. Ice sheet melting is not the only inert climate process. Ocean warming is another. Because of such processes there is a thermal delay in Earth’s climate system, which at present is estimated to be around 4 decades. The current warming would therefore be the result of combined CO2 emissions before 1970, or rather the CO2 concentration of that year (326 ppm). Since then emissions have (more than) doubled, which implies warming will, well, almost double – at least an additional 0.6 degrees Celsius on decades timescale (again, assuming current CO2 concentrations as a future constant – in reality they keep rising) and probably more than a doubling of current warming on a much longer timescale.

Indeed, doubling 0.8 gets you to 1.6 degrees – the very estimate of the PIK researchers at which all Greenland ice would melt, and global sea levels would be on average about 6 meters higher. We are already there. We just don’t feel it yet. That inertia is the real climate killer.

If the new study about the Greenland ice height feedback implications is correct, then the Eemian may serve as Greenland analogue after all.

And if emission reductions can no longer do the job, would trying to geoengineer Greenland snowfall be a last resort? How can one ever turn 6 meters times the world’s oceans’ entire surface in water into snowflakes, drop them at the exact same site and then tell them to stick this time? Ice sheet tipping points don’t have a safety button. The word ‘irreversible’ has a very strong meaning here…

© Rolf Schuttenhelm | www.bitsofscience.org