Holding warming to 1.5 degrees Celsius above pre-industrial levels – rather than to 2 degrees, which up until now has been the most widely accepted target – would be extraordinarily difficult, if not outright impossible. Scientists have repeatedly suggested that to achieve such a powerfully ambitious target, with the world already at about 1 degree C and rising, one would need to overshoot 1.5 degrees and then come back down again using problematic “negative emissions” technologies.
So then why would the idea be coming on so strong right now?
The simple answer is that while the advocacy of small island nations on behalf of the 1.5 C goal has clearly been quite influential, in some ways just as persuasive has been, you know, science – particularly when it comes to the issue of sea level rise.
“The combination of small island states and the sea level commitment stuff is the big ball and hammer that has been taken out now,” says Anders Levermann, a researcher with the Potsdam Institute for Climate Impact Research who spoke here at the conference about the latest research on sea level rise and the planet’s ice sheets. “It’s saying, this is a moral imperative, we cannot rid people of their countries.”
So let’s explore some of the key science — when it comes to sea level commitment, or lock-in, and in other areas — that would appear to lend support to the 1.5 C temperature target:
– Even with about 1 degree C of warming so far, West Antarctica and Greenland are looking worrisome. Recent research by the University of California, Irvine’s Eric Rignot and his colleagues has, in the past two years, highlighted apparent “marine instabilities” in both the ice sheet of West Antarctica and parts of Greenland.
In each of these cases, Rignot is finding that the grounding line of these glaciers — or, the region where the glacier’s subsea base meets both the ocean and the bedrock — is not only retreating inland, but at the same time, retreating downhill. This means warm water can in effect chase the retreating glacier downhill and access more and more of its base.
It is possible that West Antarctica has already been destabilized through this process. As for Greenland, much of its ice is not below sea level, and so it is not vulnerable in the same way.
Nonetheless, recent work suggests that the ice sheet may have a temperature threshold around 1.5 degrees Celsius, Levermann says, or 1.6 according to one recent report.
“There is an uncertainty there, we have only one study that gives this number, but it’s the best study, it’s the only study that really takes the dynamics into account,” says Levermann. It is at this temperature, the researchers think, that a feedback kicks in where the ice sheet’s lowering in elevation will expose it to warmer temperatures, which will lead to more lowering in elevation — and so on. There’s uncertainty, to be sure, about precisely where this threshold lies, but that doesn’t take away the fact that we could be quite close to it.
– For every 1 degree Celsius of warming, we can expect about 2.3 meters of sea level rise. Shortly before Rignot and other researchers discovered retreating grounding lines at the bases of enormous marine-based glaciers that are perched on beds that slope downhill, Levermann found something else that was pretty troubling.
Using a combination of models to study Greenland, Antarctica and mountain glaciers, as well as research concerning sea levels during the planet’s past, he and his colleagues calculated that for every 1 degree Celsius of global temperature rise, we can expect roughly 2.3 meters (or 7.54 feet) of eventual sea level rise, playing out over several thousand years.
“We get 2.3 meters per degree of warming,” says Levermann. “All the components are quite nonlinear, but what comes out is a straight line.” Granted, this is thought to be a very long term process so it doesn’t matter, Levermann says, if you only spend a few decades above 2 degree C and then come back down again. What matters is the long term temperature average, and how oceans slowly adjust to it.
Using this logic, 2 degrees C implies 15 feet of very long term sea level rise. No wonder, then, that so many are now against letting warming get that bad.
– Ice sheets may be able to collapse faster than previously thought. But is it really long term sea level rise? Are we sure about that?
Modern and scientifically enabled humanity has never observed the major collapse of an ice sheet. We don’t know what it looks like, and what all the processes are.
Scientists have long thought that these collapses probably play out slowly — but earlier this year, David Pollard of Penn State and two colleagues added two new processes to an ice sheet model called “hydrofracturing” and “cliff collapse.” The first refers to how great ice shelves in Antarctica and Greenland develop large cracks before ultimately breaking off or disintegrating, as the Larsen B ice shelf so spectacularly did in 2002. The second refers to the notion that a sheer cliff of ice, extending over 100 meters above sea level, may collapse under strain, since ice is not a particularly durable material.
Adding these processes to their model, and then simulating a warm past climate representative of the so-called middle Pliocene era — when temperatures were 2 to 3 degrees Celsius above pre-industrial levels — the authors found that the retreat of West Antarctica was “much faster than might be expected from the previous work. The main cause is the new mechanisms of hydrofracture and cliff failure.” Indeed, the study found that the retreat occurred “on the order of decades.”
It’s worth pointing out that all of the research I’m citing above came out after the completion of the 2013 iteration of the U.N. Intergovernmental Panel on Climate Change’s assessment report on the state of climate science. That’s right — the research got more dire subsequent to the last articulation of the scientific consensus on this issue.
You will also note that I’m only focusing on recent research about what 2 and 1.5 degrees C means for our oceans and planetary ice — but there will be many other impacts of climate change that will differ at these different temperature levels.
So in sum, small island nations may have long supported a 1.5 degrees C temperature target. But in the past two years or so, quite a lot of science has backed them up by suggesting that we are already very near to thresholds where we could lock in major sea level rise — precisely what they’ve long been worried about.
That’s why 1.5 C remains in the latest draft text of the Paris climate change agreement and, well, may also be in the final one.
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