James Hansen and 16 other authors have a new paper, in review, at Atmospheric Chemistry and Physics Discussions. It discusses possible short-term climate change, sea level rise, and land ice melt. It’s a few different things at once so I’ll talk about different parts of it beneath some headers.


Their own summary of their paper

We suggest that ice sheet disintegration is a highly nonlinear process and poses a danger of rapid sea level rise. We find evidence in paleoclimate observations and in global climate simulations supporting the existence of amplifying feedback processes that would contribute to nonlinear ice sheet response. Modern observations reveal that these processes are already underway, including cooling of the Southern Ocean surface. We conclude that a 2°C limit on global warming is not a safe “guardrail”.

This isn’t even its final form

ACPD is a bit different from usual scientific journals in that it’s open access (yay!) and it’s interactive (wha?) in the sense that peer review happens after the initial publication of the paper. So the manuscript was received by the journal back in mid June, It was accepted for publication two weeks ago, and it’s now published. … but it’s not in its final form; reviewers can still make very large suggestions for edits, and it might look very different in the near future.

This makes it tough to talk about

Any discussion about a scientific paper is ephemeral at a long-enough timeframe; since science is always building on itself, a paper published sort of decays in strength over time (and papers from different fields have different half-lives…), as we learn more and more things that the authors could not have known about. But Hansen et al. 2015 isn’t a peer-reviewed paper, yet, and some climatologists have stated that they will not discuss this paper because it’s not peer-reviewed science. Chris Mooney, writing for the Washington Post, has been discussing this paper the last few days because he claims it is worth talking about, and I agree with him. So…

So let’s (correctly) assume that the ice is melting

Our ability to detect actual land ice (e.g., components of the cryosphere that are present on land. Informally: glaciers and ice sheets) mass has been improving as Earth-observing satellite technology has improved. These observations lead anyone to conclude, even if it’s based only a fairly short decades-long time span for which we have satellite data, that land ice in Greenland and Antarctica is melting at a much larger rate than it is accumulating, and that this rate is increasing. Hansen et al. 2015 assume, as their starting point for modeling 21st century climate, that this melt rate is doubling every 10 years, which roughly corresponds to a doubling in the rate of mean sea level increase every 10 years.

What this leads to

This increasing melt rate leads to two kinds of responses; a global surface air temperature response and a sea level rise response.

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The global surface air temperature sees a slight reduction depending on the severity of ice melt, following Figure 19 of this paper. A lot of ice melt (if 3.8 meters of sea level rise happens in the 21st century) results in global temperatures hitting a peak around 2050, a drop in global temperatures (to those of the 1880s-1920s) until ~2075, and then a quick rise in global temperatures to, in 2100, around +2°C relative to a 1880-1920 benchmark. A little ice melt (if 1 meter of sea level rise happens) results a similar ~2050 peak, a very small pause, and then a resumption of warming by 2060 to produce a 2100 around +2.5°C relative to a 1880-1920 benchmark.

Sea level rise also depends a lot on how much the land ice melts, and this is where the Hansen et al. 2015 paper gets most speculative. Global mean sea level rise (following Figure 29 of this paper) is happening at an increasing rate: while sea level increased by about 10 cm between 1900 and 1990, about 7.5 cm of increase has occurred since 1990. But then the Hansen et al. 2015 paper assumes much larger rates of sea level increase, roughly corresponding to:

Sea level 60 cm above present: 2065

Sea level 170 cm above present: 2080

Sea level 500 cm above present: 2096

Assuming a linear sea level rise trend (which the Hansen et al. 2015 paper says not to, on multiple occasions), that translates to:

Stated rate of sea level rise between 1993 and 2015: 3.3 mm/yr

Rate of sea level rise between 2015 and 2065: 600 mm / 50 years = 12 mm/yr

Rate of sea level rise between 2065 and 2080: 1100 mm / 15 years = 73.3 mm/yr

Rate of sea level rise between 2080 and 2096: 3300 mm / 16 years = 206 mm/yr

That sounds bad

Yeah. But if melting doubling time is every 10 years (which looks like it’s happening), and land ice continues to melt at faster rates over time (which looks like it’s happening), that might be the 21st century we are facing. But climate models are notoriously fickle things; some aspects of Earth’s climate are relatively easy to model, while others aren’t. Sea level rise in particular is entirely dependent on a lot of factors, including figuring out how quickly freshwater input into the oceans is occurring. If freshwater input is exponentially increasing on a decadal timespan, then probably so will sea level rise, and most published forecasts of sea level rise do not forecast 5 meters of change before the end of this century.

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And I’m not even going into the possible effect that this might have on storms, particularly those in the North Atlantic. Let’s just say that a larger-than-current latitudinal gradient in temperatures would probably lead to more interesting storms.

Where to go from here

As this paper isn’t even peer-reviewed yet, that needs to happen. More specialists need to look at this paper to figure out if it’s chasing reasonable leads or if it went in wrong directions.

As a non-climatologist, I think this paper does a good job of presenting what evidence there is that very rapid sea level rise is observed in the paleoclimatic record, making a reasonable assumption on what could make that rapid change occur, back then and today, and trying to forecast what that means. I’m not ... I agree that modern sea level rise is a non-linear process, but I also think that 5 meters within a century is outside of known bounds.

This paper, like this one from Hansen et al. in 2013, or another one from Hansen et al in 2013, argues, strongly, that even 2°C of global warming leads to unsettling effects, and that humans should start carbon withdrawal as fast as possible in order to avoid as much future warming as is possible. Whether or not humans in charge of anything will respond is in their hands; Hansen et al. 2015 deliver an interpretation of evidence, and their own slight bit of editorializing...

It is also clear that continued high emissions are likely to lock-in continued global energy imbalance, ocean warming, ice sheet disintegration, and large sea level rise, which young people and future generations would not be able to avoid. Given the inertia of the climate and energy systems, and the grave threat posed by continued high emissions, the matter is urgent and calls for emergency cooperation among nations.