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Point of no return in West Antartica:
What's next?

by Eric Rignot

ThwaitesNASA-638x357.jpg
West Antarctica’s Thwaites glacier, one of those starting irreversible collapse (NASA).

NASA has just hosted a conference on the state of the West Antarctic ice sheet, which, it could be said, provoked something of a reaction. "This Is What a Holy Shit Moment for Global Warming Looks Like," ran a headline in Mother Jones magazine.

We announced that we had collected enough observations to conclude that the retreat of ice in the Amundsen sea sector of West Antarctica was unstoppable, with major consequences – it will mean that sea levels will rise one metre worldwide. What's more, its disappearance will likely trigger the collapse of the rest of the West Antarctic ice sheet, which comes with a sea level rise of between three and five metres. Such an event will displace millions of people worldwide.

Two centuries – if that is what it takes – may seem like a long time, but there is no red button to stop this process. Reversing the climate system to what it was in the 1970s seems unlikely; we can barely get a grip on emissions that have tripled since the Kyoto protocol, which was designed to hit reduction targets. Slowing down climate warming remains a good idea, however – the Antarctic system will at least take longer to get to this point.



The Amundsen sea sector is almost as big as France. Six glaciers drain it. The two largest ones are Pine Island glacier (30km wide) and Thwaites glacier (100km wide). They stretch over 500km.

Many impressive scientists have gone before us in this territory. The concept of West Antarctic instability goes back to the 1970s following surveys by Charles Bentley in the 1960s that revealed an ice sheet resting on a bed grounded well below sea level and deepening inland. Hans Weertman had shown in 1974 that a marine-based ice sheet resting on a retrograde bed was unstable. Robert Thomas extended his work to pursue the instability hypothesis. Terry Hughes suggested that the Pine Island sector of West Antarctica was its weak underbelly and that its retreat would collapse the West Antarctic ice sheet. Considerable uncertainty remained about the timescale, however, due to a lack of observation of this very remote area.

Things changed with the launch of the ERS-1 satellite which allowed glaciers in this part of Antartica to be observed from space. In 1997, I found that the grounding line (where the glacier detaches from its bed and becomes afloat) of Pine Island glacier had retreated five kilometres in the space of four years, between 1992 and 1996. Stan Jacobs and Adrian Jenkins had found a year earlier that the glacier was bathing in unusually warm waters, which suggested the ocean had a major influence on the glacier. Duncan Wingham and others showed that the glacier was thinning. In 2001, I found that Thwaites glacier was retreating too.

At that point, the scientific community took a different look at the region. Work by the British Antarctic Survey, Nasa and Chile led to more detailed observations, a monitoring programme was initiated, instruments were placed on the ice, in the ocean and scientific results started to pile up from a variety of research programmes. From that point, we all sought to find out whether this was really happening. Now, two decades after this process started, we have witnessed glacier grounding lines retreat by kilometres every year, glaciers thinning by metres every year hundreds of kilometres inland, losing billions of tons of water annually, and speeding up several percent every year to the flanks of topographic divides.

Thwaites glacier started to accelerate after 2006 and in 2011 we detected a huge retreat of the glacier grounding lines since 2000. Detailed reconstructions of the glacier bed further confirmed that no mountain or hill in the back of these glaciers could act as a barrier and hold them up; and 40 years of glacier flow evolution showed that the speed-up was a long story.

All these results indicate a progressive collapse of this area. At the current rate, a large fraction of the basin will be gone in 200 years, but recent modelling studies indicate that the retreat rate will increase in the future. How did this happen? A clue is that all the glaciers reacted at the same time, which suggested a common force that can only be the ocean. Ocean heat is pushed by the westerly winds and the westerlies have changed around Antarctica in response to climate warming and the depletion of the ozone. The stronger winds are caused by a world warming faster than a cooling Antarctica. Stronger westerlies push more subsurface warm waters poleward to melt the glaciers, and push surface waters northward.

Nerilie Abram and others have just confirmed that the westerlies are stronger now than at any other time in the past 1,000 years and their strengthening has been particularly prominent since the 1970s as a result of human-induced climate warming. Model predictions also show that the trend will continue in a warming climate.

What this means is that we may be ultimately responsible for triggering the fast retreat of West Antarctica. This part of the continent was likely to retreat anyway, but we probably pushed it there faster. It remains difficult to put a timescale on it, because the computer models are not good enough yet, but it could be within a couple of centuries, as I noted. There is also a bigger picture than West Antarctica. The Amundsen sea sector is not the only vulnerable part of the continent. East Antarctica includes marine-based sectors that hold more ice. One of them, Totten glacier, holds the equivalent of seven metres of global sea level.

Controlling climate warming may ultimately make a difference not only about how fast West Antarctic ice will melt to sea, but also whether other parts of Antarctica will take their turn. Several "candidates" are lined up, and we seem to have figured a way to push them out of equilibrium even before warming of air temperature is strong enough to melt snow and ice at the surface.

Unabated climate warming of several degrees over the next century is likely to speed up the collapse of West Antarctica, but it could also trigger irreversible retreat of marine-based sectors of East Antarctica. Whether we should do something about it is simply a matter of common sense. And the time to act is now; Antarctica is not waiting for us.

◊ Eric Rignot, a leading glaciologist at NASA is the senior author of the new landmark scientific paper on West Antartica. Publ. here 18.5.2014

 

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