Yes, Deniers, Nature Reports Global Warming Was Preceded By Increasing CO2 Levels During Last Deglaciation

JR: The fully study in the journal Nature, “Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation” is here (subs. req’d).

Credit: flickr/Rita Willaert

by Michael D. Lemonick, via Climate Central

Climate scientists have long argued that ancient air trapped in Antarctic ice is the smoking gun that links carbon dioxide to global warming. Over the past 800,000 years or so the planet has gone through a series of ice ages interspersed with relatively warm periods (during which glaciers retreat back toward the poles) — and inevitably, these warm interludes happen when there’s more CO2 in the atmosphere.

The only tricky part of this argument is that the smoke seems to come before the gunshot. It’s most apparent in the most recent warming period, which began about 19,000 years ago: the temperature seems to begin rising before CO2 concentrations increase. Climate skeptics have argued that since effects don’t come before causes, the whole theory falls apart.

In fact, it’s not much of an argument, since even little bit of warming would release extra carbon dioxide into the air, leading to a feedback loop, causing even more warming. But whatever feeble merit the skeptic argument might have had, a new study just published in Nature — one of two climate studies from that prestigious journal that we’re reporting on — pretty much demolishes it. It’s the most comprehensive analysis ever done of carbon dioxide and temperature at the end of the last ice age, and it shows quite clearly that in most of the world, the thermometer began to shoot up only after the atmosphere was spiked with carbon dioxide. “I think,” said Jeremy Shakun, a Harvard postdoctoral fellow and the lead author of the study, at a press conference, “this ends the skeptic argument.“

Shakun’s confidence is based on the comprehensiveness of the research. Most of the evidence for an ancient CO2-warming link comes from cores drilled out of Antarctica’s 2-mile-thick blanket of ice. Air bubbles from different levels show how much of the heat-trapping gas the atmosphere held at different times, and the chemistry of the ice trapping the bubbles shows what the temperature was.

The problem, Shakun said, is that “these cores tell you only about temperatures in the Antarctic.” Just as you’d never infer global temperatures today from just a couple of sites, it’s not really reliable to look only to ice at the South Pole for global temperatures back then. So Shakun and his co-authors gathered no fewer than 80 different records of ancient temperatures, including lake sediments (different types of pollen at different depths point to what growing conditions were like) or sea-bottom cores (the shells of marine plankton, whose chemistry depends sensitively on ocean temperatures). It was, writes the British Antarctic Survey’s Eric Wolff in an accompanying Nature commentary, “. . . a major achievement: the difficulties of synchronizing the records and of ensuring that they are sufficiently representative of the whole planet, are considerable.”

What they found was that in Antarctica, there was indeed a bit of warming that preceded the rise in atmospheric carbon dioxide — but just a little, and only by a couple of hundred years. In the rest of the world, Shakun said, “global temperature clearly lags the CO2 buildup.” Cause, in short, really did come before effect.

The sequence of events as the authors see it is this: Around 20,000 years ago, changes in Earth’s tilt and orbit around the Sun brought a little more sunlight than average to the Northern Hemisphere, where massive glaciers covered much of North America and Europe. The glaciers began to melt, dumping fresh water into the North Atlantic. Since freshwater is less dense than salt water, this slowed something called the Atlantic Meridional Overturning Circulation (AMOC) — a current that includes the Gulf Stream, and which funnels heat from the southern hemisphere to the Northern (without the AMOC, Paris, which is at about the same latitude as Fargo, North Dakota, would be drastically chillier in winter).

Since the Southern Hemisphere was no longer shipping its heat northward, Antarctica began to warm, releasing a burst of carbon dioxide into the atmosphere — somehow. “Where it’s coming from,” Shakun said, “is a big question in paleoclimate.” One mechanism is simply that warm water can’t hold as much of the dissolved gas as colder water, so any heating of the ocean would have released some, just as a warm Coke loses its fizz faster than a cold one.

Beyond that, sea ice, which would have previously formed a lid on this CO2, would have melted back near Antarctica, letting more escape. “People also think,” Shakun said, “that there would have been lots of extra carbon stored in the depths of the southern ocean. Warming could have shifted the prevailing winds, pulling up this deeper carbon and helping it degas.”

It’s a persuasive story, albeit a bit complicated, but it does leave a couple of unanswered question. One is whether water dumped into the North Atlantic by melting glaciers would have affected the AMOC by just the right amount. There’s good evidence of the melting, say the authors, both in evidence left by the glaciers themselves and in a rise in sea level that happened at the same time, but the changes to the AMOC are assumed, not proven. (We wrote about another glacier-related rise in sea level last week).

Another is that while the level of sunshine did increase in the north, it started from a relatively low point. Still another, Wolff says, is that a period of warming in the Northern Hemisphere some 60,000 years ago did not lead to an overall global temperature rise — so what’s the difference between that episode and the more recent one?

All good questions, but Shakun and his collaborators are convinced their analysis will stand up, even if all the i’s haven’t been dotted or the t’s crossed. “As a diligent scientist,” he said, “I never say never, but I think that while our analysis will get better as we go from 80 records to 800, it’s pretty unlikely that things will change significantly.”

Michael Lemonick is a Senior Science Writer with Climate Central. Mr. Lemonick covered science and the environment for TIME magazine for nearly 21 years, where he wrote more than 50 cover stories, and has also written for Discover, Scientific American, Wired, New Scientist and The Washington Post. This post was originally published at Climate Central and was re-printed with permission.

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12 Responses to Yes, Deniers, Nature Reports Global Warming Was Preceded By Increasing CO2 Levels During Last Deglaciation

  1. Steve Bloom says:

    Joe, I don’t find the spin put on this (excellent) research by the authors, coverage by Lemonick and others, and by your own headline, to be at all helpful.

    The fact was and remains, as a matter of basic physics (easy to demonstrate using a carbonated beverage and a refrigerator), that the initial warming preceded the CO2 rise. That the latter was responsible for the bulk of the warming has also been clear. Now it turns out that the CO2 feedback proceeded quickly enough that it was even responsible for the bulk of the early *global* signal (via a large enough amount of it being forced into the atmosphere due to the *local* orbitally-driven temperature increase), making your headline almost literally true (see below for the caveat), but basically an exercise in sophistry since it implies something other than what it says. Delete the *global* from it and it becomes precisely false.

    (Really, it’s technically false even with the modifier. I haven’t read the paper itself yet, but the supplement neatly summarizes the actual relationships (p. 30): “The results indicate Antarctic temperature led CO2 by a small amount throughout the deglaciation (Figure S25a). The global temperature stack, on the other hand, was synchronous with or lagged CO2, except at the onset of deglaciation when it led (Figure S25b).” So there we are.)

    How to do better? Well, maybe “New research shows that CO2 had a global effect far more quickly than previously thought.” OK, that needs an editor, but I think it’s enough to make the point. Denialists are of course just as wrong, but in a slightly different way.

    Reference could also be made to Hansen’s 1980s prediction about this, before meaningful data was available.

    The statements about this new paper putting paid to the accompanying denialist trope are ludicrous. I predict that the strange twisting of the results will if anything just make things worse. Even to members of the public with little climate science knowledge, it will be easy to paint as a dodge. Sadly, for many people, the process by which the initial forcing and following feedbacks drive deglaciations is hard enough to grasp that it sounds like a dodge to begin with, so the denial machine has fertile ground to plow.

    Speaking of grasping the science, it appears to me that Lemonick’s own understanding of this stuff is less than complete. Fiona Harvey’s Guardian article struck me the same way. Perhaps editors are partly or wholly to blame for the confusion, but it’s not at all good to see.

  2. Sadly this won’t end the deniers arguments, since those arguments were never based on anything like logic, principles, evicence or facts in the first place.

  3. I don’t think it’s important what the skeptics think. They will doubt the science regardless. They don’t even believe CO2 causes warming. If humans created more sunlight, they’d claim sunlight didn’t cause warming.

  4. Steve Bloom says:

    Well, first of all we should be clear that such people are not “skeptics” in any sense of the word. If the denialist shoe fits, they should be made to wear it.

    And of course you’re quite right about them. My concern is about merely uninformed people.

  5. Joe Romm says:

    I think you are overthinking this. The headline comes from the Nature article itself.

  6. Lisa Boucher says:

    Many thanks to Joe Romm for sharing this.

    By the way, this French web site has the whole paper.  Our tax dollars help finance such research, so I think PDF copies of published articles should be free anyway.

  7. Lazarus says:

    I won’t end them but they appear to be struggling to have a mature answer for this;

  8. Steve Bloom says:

    I know you quoted the article title, Joe. From that and the press release, it’s entirely clear that Lemonick and others went with the authors’ spin. But that spin was a mistake IMO.

    On the plus side, it’s been a busy time for new results (most of them very unfortunate), which will tend to limit any damage to the extent I’m right. Perhaps it’s also the case that the necessity of focusing on the details of CO2’s deglacial role in order to attack the spin will make doing so seem counter-productive.

    Re spin generally, I am completely in favor of scientists imparting it in this manner, and it’s actually gratifying to see journalists going with it. I just think that in this instance a different, internally consistent argument with the same upshot could have been made.

  9. Steve Bloom says:

    Thanks for spotting that, Lisa.

  10. Mulga Mumblebrain says:

    I must say that I’m with you on this. The original denialist canard was a typical misrepresentation and confusion, reliant on the denialist rabble being unaware of the Milankovitch cycles that initiated deglaciation (and glaciation), and ideologically uninterested in the fact that rising greenhouse gas levels then furthered the warming. Your suggestion for a more apt heading seems pretty appropriate, to me at least.

  11. Mulga Mumblebrain says:

    Mature is not in their nature.

  12. Mulga Mumblebrain says:

    I worry about the uni-informed, with the denialist hydra as their sole source of information.