Primer: Understanding sea level rise; ice sheet dynamics and melting feedbacks
[This primer introduces] a list of no less than 13 different (yet highly entangled, often synergistic) ice sheet melting feedbacks, feedbacks that can promote a rapid acceleration of global sea level rise:
Understanding sea level rise is understanding the two extremes of the story – and then properly tying them together. This article is about the processes that form that connecting part (and coincidentally that’s also where the scientific debate is at).
To develop a proper understanding of future sea level rise, you first have the two extremes of the story: the observational current speed (global average in the order of 2-3 millimetres/year) and the final sea level rise for a certain amount of created warming (in the order of 29 metres for +2 degrees, and going as high as 55 metres for a runaway warming scenario)...
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You have probably heard about James Hansen and his latest sea level publication, in which he – together with 18 co-authors – shows why the Greenland and Antarctic ice sheets might be far less stable than previously assumed, raising the possibility of 2-5 metres sea level rise, within this century.[1]
Perhaps a less familiar name (because he is a lot younger) is Robert DeConto, a climate scientist from the University of Massachusetts Amherst, who together with paleoclimatologist David Pollard of Pennsylvania State University this year also published a study suggesting a mechanism for accelerated melting of the Antarctic ice sheet – which could raise global sea levels to 2 metres by 2100.[2]
Now what’s with these studies? In one word: feedbacks. Positive feedbacks sadly – feedbacks that lead to acceleration of the climate response of large ice sheets and therefore to acceleration of sea level rise. In this article we’ll try to take a closer look.
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A research-based overview of melting feedbacks and other mechanisms for ice sheet collapse:
A sea level rise speed of multiple metres per century would be possible if positive melting feedbacks are activated. And throughout years of research many have already been suggested – and are all subject of ongoing investigation, through modelling, paleoresearch and in situ observations:
1: ALBEDO FEEDBACK
‘Albedo Effect’ – reflectivity-absorption-melting feedback. As the temperature goes up, more snow melts, decreasing summer albedo, increasing warming – accelerating melting.
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Formation of such darkening meltwater ponds is already being observed in Greenland and Antarctica. In May 2016 a group of researchers led by Marco Tedesco of Columbia University and NASA’s Goddard Institute of Space Studies (GISS) published evidence in The Cryosphere that on Greenland the overall albedo had already significantly declined between 1996 and 2012 – and that they found weaknesses in forecasts of continued albedo decline, stating these are underestimated, as models fail to reproduce (darkening) increases in water, grain size and aerosols on the ice sheet surface.[4]
And sadly, very recently, in August 2016, a group of researchers led by Emily Langley of Durham University presented evidence in a Geophysical Research Letters publication of a similar albedo feedback, already active on the East Antarctic ice sheet, where (based on satellite observations between 2000 and 2013) they had found a rapid increase of (dark-coloured, heat-absorbing) meltwater lakes (about 8000) on top of the ice sheet.[3]
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2: MELTWATER LUBRICATION FEEDBACK
‘Meltwater lubrication feedback’ – warming of the atmosphere causes ice sheet surface melting. Before this meltwater runs to the sea, it can also speed up glacier movement, accelerating sea level rise.
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3: ICE HEIGHT FEEDBACK
Diminishing ice sheet altitude feedback - working on temperature, melt season lengths & snow accumulation – easily overlooked, but a sensical Grand Final to full collapse(!)
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4 & 5: ICE SHELF COLLAPSE FEEDBACKS
Here we take two in one: the (basic) ocean warming and sea level rise* ice shelf feedback. (There are more complicated ice shelf feedbacks, as we’ll find out later!)
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6-8: BIPOLAR SEESAW FEEDBACKS
Greenland-Antarctic bipolar seesaw, Dansgaard-Oeschger & Heinrich Events: hemispheric asynchrony as a mechanism for accelerating net ice sheet disintegration and sea level rise.
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There is large paleoclimatic evidence against linear ice sheet-climate interaction – abrupt, non-linear and asynchronous responses of ice sheets, leading to net acceleration of warming-induced sea level rise should be considered.
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9: VOLCANIC ICE MELTING FEEDBACK
Isostatic rebound volcanology melting (and CO2) feedback.
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10 & 11: ICE CLIFF COLLAPSE (& HYDROFRACTURING OF ICE SHELVES)
Again, we pile together two ice sheet melting feedbacks (as they’re never as clearly defined) – both illustrating a mechanism for increased glacier flow and ice melting at the ice sheet marine margin.
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12 & 13: SOUTHERN OCEAN ‘COLD LID’ MELTWATER FEEDBACK
We get to the new Hansen study and can again pile two additional amplifying feedbacks together: a Southern Ocean meltwater surface ‘cold lid’ that promotes heat build up directly under ice shelves (and increases the Earth’s energy imbalance, amplifying the net warming)
