An important discussion is developing among climate and polar researchers around the central point of a landmark?Nature paper on Greenland conditions?during Earth?s last (very warm) interval between ice ages. The paper, in which a critically important Greenland ice core is analyzed by 133 authors from a host of research centers, concludes that the vast ice sheet?largely endured over a period of 6,000 years that was warmer than what is forecast for coming decades.
The graph above, with the Eemian at the left and current period at the right, conveys the key points. The contribution of Greenland melting to the 12-24 foot rise in sea levels at that time must have been relatively small, which means the contribution from ice loss in West Antarctica, the other main zone of concern, must have been larger. (The big question for coastal communities remains the rate of coastal retreat, not the total rise over many centuries.)
I touched on the Nature paper yesterday in?a roundup of ice research, but it deserves front-and-center attention, particularly given its relevance to recent narratives about Greenland?s meltdown being the key worry if you live on a coast.?See Chris Mooney?s new Mother Jones profile of Ohio State ice researcher Jason Box for the latest example: ?Why Greenland?s Melting Could Be the Biggest Climate Disaster of All.? The article is part of the coordinated ?Climate Desk? collaborative journalism project that includes a live event in Washington next week asking, ?Can Greenland be Saved??
The new research suggests we need to be more concerned about Antarctica.
I hope they discuss these new findings at the event next week. My answer to their question is below. (Given how the new coring study meshes with analysis that I covered?in 2008?hinting that Greenland?s ice sheet has a reduced, but robust, warm-climate condition, you might be able to guess.
But first here?s more on this ice-core study and the broader context, including some great input from the wise and deeply experienced climate and ice researcher Richard Alley of Penn State.?
To review, the concerns about recent trends on Greenland?s giant ice mass have been warranted, given signs of extensive surface melting and the possibility, explored here, that meltwater gushing to the ice sheet?s base through natural ?drain pipes? called moulins could accelerate the flow of ice to the sea. The stunning documentary ?Chasing Ice? conveys the drama and mystery in all that melting, and the impact of warmer sea water on the areas where Greenland?s glaciers meet the sea.
I saw these dynamics up close in 2004, and it?s a thrilling, chilling experience (video).
But the new work puts all of this in the context of long stretches of time, which is vital if one?s goal is to avoid overreaction and to focus attention where real risks lie. I encourage you to read pieces by?Michael Lemonick at Climate Central and?Quirin Schiermeier in Nature?s news section?for more on the basics of the study.
The lead researcher, Dorthe Dahl-Jensen of the Niels Bohr Institute at the University of Copenhagen, offers useful insights in?a news release from the university:
The new results?show that the climate in Greenland was around 8 degrees C. [14.4 degrees F.] warmer than today during the last interglacial period, the Eemian period, 130,000 to 115,000 thousand years ago.
?Even though the warm Eemian period was a period when the oceans were four to eight meters higher than today, the ice sheet in northwest Greenland was only a few hundred meters lower than the current level, which indicates that the contribution from the Greenland ice sheet was less than half the total sea-level rise during that period,? says Dorthe Dahl-Jensen, Professor at the Niels Bohr Institute, University of Copenhagen?.
I turned to Richard Alley, who?s become a vital touchstone for me on such research, for some insights. Here?s his comment, followed by my closing thoughts:
I have three immediate responses: Satisfaction in the great success of the collaboration, concern that this slightly increases worries about future sea-level rise from human-caused warming, but technical questions that may leave us more-or-less where we were before on the biggest picture.
Taken in turn:
Having watched colleagues go to the immense effort of learning what information is desired by policymakers and other citizens, assemble the logistical and scientific abilities to supply that information, and actually do it over a lot of years, and knowing just how many of their kids? soccer games and recitals some of the scientist-parents missed, I have to smile when the team succeeds so well.
As to the big picture, there is strong evidence from the history of sea level on coasts from the Eemian that both Greenland and Antarctic ice sheets shrank notably, contributing to a globally averaged sea-level rise of very roughly 20 feet. This occurred primarily in response to a rearrangement of where sunshine reached the planet and when during the year, with more summer sunshine in the north but very little total change. And, some uncertainty has remained on the exact balance between Greenland and Antarctic contributions. The new paper suggests that the contribution from Greenland was on the low end of the prior estimates, but has little effect on the estimated total sea-level change, which points to a larger Antarctic source than the previous best estimate.
In my opinion (and I believe the opinions of many colleagues), we have greater understanding of Greenland?s ice than Antarctica?s, and we have greater confidence that Greenland will be ?well-behaved? ? we will more easily project changes in Greenland?s ice, with greater confidence that changes begun now will take centuries or longer to be mostly completed.
By shifting more of the sea-level rise into the less-understood ice, and thus into the ice with greater chance of doing something rapidly, I believe the new paper at least slightly increases the concerns for coastal planners, even if the chance of a rapid change from Antarctic ice remains small.
As to the technical parts, as described in many sources, we have lots of paleothermometers for the central Greenland ice cores over the last 100,000 years, providing multiple validation and high confidence that temperatures have been estimated accurately. The very changes in the ice sheet that are of greatest interest here also make the effort quite difficult. The melting of the Eemian interferes with gas-based paleothermometry, and with the total-gas technique that provides constraints on changes in surface elevation.
A U.S. government CCSP report on Arctic paleoclimates a few years ago (to which I contributed) [link] estimated changes in temperature and ice volume for this interval. The new estimates overlap with the older ones. Were I working on that report now, I would recommend expanding the uncertainties a little to include the new results. However, considering that ice shrinkage on Greenland has a feedback effect (exposing rocks allows more sun to be absorbed, causing more warming), considering the evidence of Eemian warmth from marine records around Greenland, considering climate model runs for that time, considering other studies of Greenland, and recalling the notable uncertainties associated with untangling the changes in total gas and in the ice sheet itself, I suspect that the estimates in that CCSP report will stand up pretty well, with the new work primarily confirming the prior understanding of climate changes and ice-sheet and sea-level response in the Eemian.
If anyone is thinking that this paper means we can crank up the temperature without worrying about sea level, they should seriously re-think. Overall, a great and successful scientific effort leaves us with the knowledge that warming does tend to melt ice, and that contributes to sea-level rise.
In a followup note to him, I said:
Beautifully articulated. but I do think [the new work] closes the case that Greenland, despite all of its drama (moulins, for example) ? drama that focused my attention for a few years too ? is a sideshow in the sea level question.
That?s not how it?s been cast. There?s been talk of regional geo-engineering to ?save? the ice sheet. The dramatic surface melting, while important to track and understand (as is being done by Jason Box and others) has little policy significance.
Alley replied:
I do think it has been clear for a while that interactions with the ocean provide the greatest potential for surprises and rapid changes, and that Greenland?s ice sheet would mostly pull out of the ocean before it lost most of its mass. The discussion in the attached, as well as in Ian Joughin?s and my [West Antarctic Ice Sheet] review in 2011, were pointing in that direction. The lack of huge danger from the lake drainages probably was argued (possibly for the first time) by Byron Parizek and I in Quaternary Science Reviews in 2004. There are dynamics issues, but the biggest ones go away once shrinkage pulls the ice out of the ocean. Then, a serious focused research effort should be able to produce (and indeed, is producing) quantified projections with useful uncertainties that can be narrowed by continuing effort on the established research path. We are still thinking about one or two interesting and possibly surprising things, but Greenland looks like it is mostly the known-unknown ice sheet.
Considering what I?ve learned in the nine years since I got to visit the frigid summit and eroding edges of Greenland?s amazing ice sheet, in a final note to Alley here?s how I described my reply to the Climate Desk?s ?Can We Save Greenland?? question:
My answer would be that Greenland doesn?t need ?saving,? Antarctica needs much more observation and analysis, and ? separately ? the long effort to slow greenhouse gas emissions needs to continue.
chuck elisabeth hasselbeck fran drescher scarlett o hara pat sajak vanna white michael robinson
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