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Repost: James Hansen and Sea Level Rise – the peer-reviewed version

Editor’s note:

Today we repost a blog entry by PLOS Ecology Community Editor Sasha Wright from August 2015. In it, she interprets and discusses the startling findings by the individual often called the world’s most well-known climate scientist: former NASA Director, James Hansen of Columbia University’s Earth Institute. With 18 co-authors representing far-flung disciplines, Hansen had just released the first draft of “Ice melt, sea level rise, and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 degrees C global warming could be dangerous.” Hansen chose to post this paper on the open access discussion site of the European science journal, Atmospheric Chemistry and Physics prior to its undergoing formal peer review. At the time, Hansen made clear that he’d posted this draft in order to receive constructive feedback from his scientific peers. What he got was more of a firestorm.

Initial reports on the final, peer-reviewed paper, published yesterday in the formal journal, Atmospheric Chemistry and Physics, are that its major findings have not fundamentally changed. In their abstract, Hansen et al describe the primary changes made as structural.

Our paper published in Atmospheric Chemistry and Physics Discussion was organized in the chronological order of our investigation. Here we reorganize the work to make the science easier to follow. First, we describe our climate simulations with specified growing freshwater sources in the North Atlantic and Southern oceans. Second, we analyze paleoclimate data for evidence of these processes and possible implications for the future. Third, we examine modern data for evidence that the simulated climate changes are already occurring.

Here is Sasha’s original August 10th blog post, written as part of PLOS Ecology’s coverage of the Ecological Society of America 2015 annual meeting. It was posted on PLOS Blogs as a preview for Jim Hansen’s appearance on the August 12th PLOS Science Wednesday, the weekly redditscience ‘Ask Me Anything’ hosted by PLOS — where the authors would take questions on the paper’s findings.

At the end of Sasha’s post, we’ve added a Q&A from that AMA; a question that will no doubt be asked again in response to Hansen’s latest publication. It addresses how the authors were able to correlate paleoclimate data with current day climate changes to make these dire predictions.

Your comments are invited on both the authors’ publication process, and the findings in their paper.

— Victoria Costello, PLOSBLOGS


All Eyes on the Oceans: James Hansen and Sea Level Rise

By Sasha Wright

On July 23, James Hansen and 16 co-authors posted a discussion paper on an open-review website about sea level rise and climate change. The article has garnered massive attention around the internet and scientific communities — both for its content and for the unconventional manner in which it waspublished.

The authors bring special attention to a particular aspect of global climate change that often isn’t discussed. Namely, global sea level rise. While many of us discuss the catastrophic impacts of droughtsflooding, and dwindling food supply related to climate change, the authors emphasize that up to 5 meters of sea level rise may happen over the course of 50 years, carrying with it the “economic and social cost of losing functionally all coastal cities…” Seeing as the more conservative IPCC report puts this estimate closer to 1 meter by the year 2100, this news is making an impact.

Understanding the James Hansen sea level story requires a little bit of background. First, the importance of climate models. Climate models take known “forcings” — potential drivers of climate change such as the natural elliptical shape of the earth’s orbit, man-made greenhouse gas emissions, and surface albedo — to understand the relationship between what we do here on earth now and how it affects our climate. We can then use those relationships to project how our activities in the future will affect our future climate. Simple enough, but there are shortcomings. The predictions rely on our comprehensive understanding of the climate system. But, what about the areas of research that are less well understood? Or even mechanisms that may affect the climate that we haven’t yet identified?

Hansen and colleagues emphasize a parallel approach. Their approach points to known limitations in ice sheet models and suggests that sea levels may rise more severely and rapidly than current climate models predict.

The authors look very carefully at a recent period in the earth’s history when the climate was similar to what it is now — the Eemian period (~115,000 years ago). During this time, the earth’s temperature was likely less than 1 degree C greater than it is now and atmospheric CO2 levels were ~270 ppm (compared with 400 ppm today). This period also corresponds with notably rapid changes in sea level elevation: up to 1m/century. But why?

1-uNzVj4l6sHiTujbxUojxYAThe authors point to a less well examined feedback loop that could accelerate sea ice melt well beyond our current predictions from models. While the science of modeling ocean currents and interactions between ocean currents and global climate is complicated, the pattern they point to is this: increased sea ice melt can act as an insulator to warmer deep waters and shut down ocean circulation. If deeper ocean waters cannot mix with cooler surface waters this may further accelerate ice melt.

When they add these sea ice — ocean current feedbacks into course climate models, and combine them with the massive anthropogenic climate forcings that are abundant today (but were absent during the Eemian), the result is catastrophic.

The authors predict a period in the next century when ice melt may cause more than 5 meters of increase in sea levels over the course of as little as 50 years — much greater than the 18–59 cm rise by 2100 predicted by the most recent IPCC report.

Further, these changes may be possible with less than a degree of further climate warming. When converted back to carbon emissions, the authors urge a return to the 350 ppm atmospheric carbon dioxide concentrations that we passed several years ago.

Prior work by Hansen and colleagues, published in the journal PLOS One in 2013 and contained in the  PLOS Responding to Climate Change Collection, emphasized that earth’s energy imbalance (energy inputs — energy outputs associated with the greenhouse effect) is large enough that we will experience further warming of the climate even if atmospheric CO2 is maintained at current levels (400 ppm) — instead atmospheric CO2 levels must be reduced. Without such carbon reductions the authors indicate that the ice melt feedbacks will be unpredictable and will likely have massive effects on coastal areas.

If the authors are correct in their predictions, the consequences for planet earth and human societies will be dire. In fact, many have argued that the consequences of climate change are already happening. Saltwater in coastal water supplies leads to water contamination and the need for expensive desalinization plantsLoss of arable coastal lands is worsening the already existing issues of food security and feeding a global population of 7 billion. Warming waters have led to massive mortality events in the Mediterranean, loss of polar bear habitat in the arctic, and the possible extinction of 2–10 freshwater shrimp species. Consequent losses of biodiversityworldwide will likely make the global system even less stable to perturbation in the future.

On a personal level, this type of sea level rise would devastate every city I’ve ever called home. From Seattle to Miami to Brooklyn, NY. The economic losses would be enormous. If lack of political action on greenhouse gas emissions is simply a reflection of a lack of understanding of risk, Hansen’s alarm bells may do the trick. Unfortunately, the psychological research indicates that the real barrier to climate change mitigation may be more closely related to our limited cognitive abilities, branding, and preexisting cultural identities.

Sasha Wright, PhD is a Plant Biologist and Theoretical Ecologist, a former NSF Graduate Research Fellow, and a Professor of urban ecology at FIT and Bard College. She has published in numerous peer reviewed journals and written for Popular Science @sashajwright





Post updated on March 23, 2016 with an excerpt from the August 12, 2015 PLOS Science Wednesday AMA with Jim Hansen & Colleagues — on redditscience.




Two of the past 5 (normal/natural) interglacials, when CO2 was less than 300 pm, show geological evidence of sea levels well over 5 m (16 feet). So what are the potential consequences of 400, 500, 700 ppm as predicted? Can you please expand on this data.


Paul Hearty (paleoecologist and professor at University of North Carolina at Wilmington, NC Dept. of Environmental Studies, who describes his research focus as “I study the geology of sea-level changes”)

There are leads and lags (in time) throughout the atmosphere, hydrosphere, and cryosphere. In order to understand the timing of these leads and lags in the geologic record, we would have to “slice time” (date fossil deposits) in much thinner layers. With current dating technology, at the time of the last interglacial (Eemian; MIS 5e) we are able to see 1000s and under ideal circumstances, 100s of year time slices. What we see in the rock record is the end product (i.e., sea level rise and storms) of global conditions that existed when CO2 was at less than 300 ppm, as revealed in ice cores, and temperatures were marginally warmer than present.

Jim Hansen: in fact, those natural climate variations are our best source of information about how sensitive the climate system is to changes of atmospheric composition. Those natural changes over millions of years are due mainly to changes of atmospheric CO2, as the balance between the volcanic source of CO2 (associated with “continental drift”) and the weathering sink changes. The natural changes on millennial time scales are due mainly to how much carbon is stored in the deep ocean, as we discuss in our ACPD paper. The problem is that the human-made CO2 source, fossil fuel burning, totally overwhelms the rate of natural changes – humans are now in charge of the carbon cycle, unfortunately. If we do not slack off on that very soon, by rapidly reducing fossil fuel emissions, young people will inherit a climate system that is out of their control.

August 12th PLOS Science Wednesday with Jim Hansen can be read in its entirety here.
Here is a youtube video primer on the final peer-reviewed research paper released by Jim Hansen yesterday.



  1. Given the relatively cavalier attitude of institutions and voters in the Western world (not even considering the almost unstoppable effect of the developing world) to this likely outcome it’s hard to see a solution other than fleeing inland and hoping for the best as a long term strategy. I don’t know how many reports such as this will be needed before the world opens its eyes and ears – and it will probably be too late.

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