- …without drastic cuts in global greenhouse gas emissions–more drastic than any being discussed ahead of the critical climate meeting in Paris later this year—a rise of 20 metres will soon be unavoidable.
I’ve been exasperated by the last two IPCC Reports in their treatment of sea level rise.
Michael Le Page in a special report for the New Scientist (paywalled) takes a look at some recent research and comes up with very different numbers.
The IPCC Working Party on The Physical Science Basis published in 2013 says that over the next 2000 years we can expect a rise of about 2.3 metres for each sustained 1°C increase in global temperature. As Le Page says:
- This means a 5-metre rise could happen only if the world remains at least 2°C warmer than in pre-industrial times up to the year 4100. That doesn’t sound so bad: it suggests that if we found some way of cooling the planet, we could avoid that calamity.
Since then two massive glaciers in West Antarctica have passed a point of no return. If they go, there is little to stop the rest of West Antarctica. “The West Antarctic ice sheet is gone”, said Anders Levermann of the Potsdam Institute for Climate Impact Research in Germany. Of significance here is that Levermann was one of the authors of the sea level chapter in the IPCC report.
However, the Totten Glacier in East Antarctica is also on the move. It forms the gateway to the vast Aurora Basin, which contains as much ice as all of West Antarctica. Like West Antarctica, the underlying rock is below sea level. Almost as large is the Wilkes Basin and similarly placed, but not yet losing ice. Nevertheless the plug holding it is quite thin. This unfortunately truncated image gives some idea:
The Aurora Basin is marked “3” on the map. The Wilkes Basin is immediately to the left of it.
If these basins and West Antarctica go, there won’t be much left of Greenland.
Time to start adding up the numbers:
- Mountain glaciers – 0.4 metres
- Ocean expansion – 1.6m
- West Antarctica – 3.3m
- Wilkes Basin – 3.5m
- Aurora Basin – 5.1m
- Greenland – 6m
That gets you to 19.9 metres. It doesn’t include any additional loss from the huge East Antarctic ice sheet, which from memory comes in at 59 metres worth of sea level rise.
During the Pliocene with warming 2–3°C higher than today, global sea level was 25 metres higher.
This should be no surprise. I repeat here a graph made by David Archer as long ago as 2006:
The big question is how fast the sea will rise. The short answer is, no-one knows.
The IPCC had things happening rather gently (see the Synthesis for Policymakers pp 29-30)
They predicted sea level rise by 2100 of 52 to 98 centimetres. The rate from 2081 to 2100 was foreseen at 8 to 16 mm/yr compared to 3.2mm now. James Hansen speculated on the effect of a doubling of ice sheet loss every 10 years:
Almost certainly it won’t be 5 metres. Richard Alley headed a team that recently ran a model of sea level rise that included ice sheet decay. Le Page concludes with his results:
- Yet in the improved ice model that Alley’s team ran, Antarctica alone added 5 metres to sea level in the first two centuries. That model was run with warm Pliocene-like conditions from the start, not where we are at now.
It might not take too long to reach a similar point, though. We’re in danger of soaring past Pliocene levels of warmth as early as the middle of the century if we don’t slash emissions soon. In the study, the West Antarctic ice sheet collapsed in mere decades in response to this kind of warmth.
What’s more, the model might still leave out some melting processes, Alley says. “It is possible that this rather short timescale is not the worst possible case.”
It’s highly likely, I think, that things will start to go distressingly pear-shaped during this century. The scenario in prospect is very different from the gentle progression outlined by the IPCC.
Elsewhere Justin Bowles at Risk and Well-Being has an excellent post on the article. He suggests that:
- climate change has already committed the world to the destruction of human heritage many orders of magnitude greater than anything ISIS is capable of doing.
* Update: I’ve changed the title from Scientists take sea level rise seriously out of recognition that the scientists who wrote the IPCC report took sea level rise very seriously indeed. It’s just that now their work needs updating, which was the point of the New Scientist article.