James Hansen and 18 leading climate experts have published a peer-reviewed version of their 2015 discussion paper on the dangers posed by unrestricted carbon pollution. The study adds to the growing body of evidence that the current global target or defense line embraced by the world — 2 °C (3.6 °F) total global warming — “could be dangerous” to humanity.
That 2 °C warming should be avoided at all costs is not news to people who pay attention to climate science, though it may be news to people who only follow the popular media. The warning is, after all, very similar to the one found in an embarrassingly under-reported report last year from 70 leading climate experts, who had been asked by the world’s leading nations to review the adequacy of the 2 °C target.
Specifically, the new Hansen et al. study — titled “Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming could be dangerous” — warns that even stabilizing at 2 °C warming might well lead to devastating glacial melt, multimeter sea level rise and other related catastrophic impacts. The study is significant not just because it is peer-reviewed, but because the collective knowledge about climate science in general and glaciology in particular among the co-authors is quite impressive.
Besides sea level rise, rapid glacial ice melt has many potentially disastrous consequences, including a slowdown and eventual shutdown of the key North Atlantic Ocean circulation and, relatedly, an increase in super-extreme weather. Indeed, that slowdown appears to have begun, and, equally worrisome, it appears to be supercharging both precipitation, storm surge, and superstorms along the U.S. East Coast (like Sandy and Jonas), as explained here.
It must be noted, however, that the title of the peer-reviewed paper is decidedly weaker than the discussion paper’s “Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming is highly dangerous.” The switch to “could be dangerous” is reminiscent of the switch (in the opposite direction) from the inaugural 1965 warning required for cigarette packages, “Caution: Cigarette Smoking May Be Hazardous to Your Health” to the 1969 required label “Warning: The Surgeon General Has Determined that Cigarette Smoking Is Dangerous to Your Health.”
And yes I’m using the analogy to suggest readers should not be sanguine about the risks we face at 2 °C warning. Based on both observations and analysis, the science is clearly moving in the direction that 2 °C warming is not “safe” for humanity. But as Hansen himself acknowledged Monday on the press call, the record we now have of accelerating ice loss in both Greenland and West Antarctica is “too short to infer accurately” whether the current exponential trend will continue through the rest of the century.
Hansen himself explains the paper’s key conclusions and the science underlying them in a new video:
The fact that 2 °C total warming is extremely likely to lock us in to sea level rise of 10 feet or more has been obvious for a while now. The National Science Foundation (NSF) itself issued a news release back in 2012 with the large-type headline, “Global Sea Level Likely to Rise as Much as 70 Feet in Future Generations.” The lead author explained, “The natural state of the Earth with present carbon dioxide levels is one with sea levels about 70 feet higher than now.” Heck, a 2009 paper in Science found the same thing.
What has changed is our understanding of just how fast sea levels could rise. In 2014 and 2015, a number of major studies revealed that large parts of the Antarctic and Greenland ice sheets are unstable and headed toward irreversible collapse — and some parts may have already passed the point of no return. Another 2015 study found that global sea level rise since 1990 has been speeding up even faster than we knew.
The key question is how fast sea levels can rise this century and beyond. In my piece last year on Hansen’s discussion draft, I examined the reasons the Intergovernmental Panel on Climate Change (IPCC) and scientific community have historically low-balled the plausible worst-case for possible sea level rise by 2100. I won’t repeat that all here.
The crux of the Hansen et al. forecast can be found in this chart on ice loss from the world’s biggest ice sheet:
Antarctic ice mass change from GRACE satallite data (red) and surface mass balance method (MBM, blue). From Hansen et al.
Hansen et al. ask the question: if the ice loss continues growing exponentially how much ice loss (and hence how much sea level rise) will there be by century’s end? If, for instance, the ice loss rate doubles every 10 years for the rest of the century (light green), then we would see multi-meter sea level rise before 2100? On the other hand, it is pretty clear just from looking at the chart that there isn’t enough data to make a certain projection for the next eight decades.
The authors write, “our conclusions suggest that a target of limiting global warming to 2 °C … does not provide safety.” On the one hand, they note, “we cannot be certain that multi-meter sea level rise will occur if we allow global warming of 2 °C.” But, on the other hand, they point out:
There is a possibility, a real danger, that we will hand young people and future generations a climate system that is practically out of their control.
We conclude that the message our climate science delivers to society, policymakers, and the public alike is this: we have a global emergency. Fossil fuel CO2 emissions should be reduced as rapidly as practical.
I have talked to many climate scientists who quibble with specific elements of this paper, in particular whether the kind of continued acceleration of ice sheet loss is physically plausible. But I don’t find any who disagree with the bold-faced conclusions.
Since there are a growing number of experts who consider that 10 feet of sea level rise this century is a possibility, it would be unwise to ignore the warning. That said, on our current emissions path we already appear to be headed toward the ballpark of 4-6 feet of sea level rise in 2100 — with seas rising up to one foot per decade after that. That should be more than enough of a “beyond adaptation” catastrophe to warrant strong action ASAP.
The world needs to understand the plausible worst-case scenario for climate change by 2100 and beyond — something that the media and the IPCC have failed to deliver. And the world needs to understand the “business as usual” set of multiple catastrophic dangers of 4 °C if we don’t reverse course now. And the world needs to understand the dangers of even 2 °C warming.
So kudos to all of these scientists for ringing the alarm bell: James Hansen, Makiko Sato, Paul Hearty, Reto Ruedy, Maxwell Kelley, Valerie Masson-Delmotte, Gary Russell, George Tselioudis, Junji Cao, Eric Rignot, Isabella Velicogna, Blair Tormey, Bailey Donovan, Evgeniya Kandiano, Karina von Schuckmann, Pushker Kharecha, Allegra N. Legrande, Michael Bauer, and Kwok-Wai Lo.
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