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Arctic and Antarctic

Arctic sea ice watch

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A few people have already remarked on some pretty surprising numbers in Arctic sea ice extent this year (the New York Times has also noticed). The minimum extent is usually in early to mid September, but this year, conditions by Aug 9 had already beaten all previous record minima. Given that there is at least a few more weeks of melting to go, it looks like the record set in 2005 will be unequivocally surpassed. It could be interesting to follow especially in light of model predictions discussed previously.

There are a number of places to go to get Arctic sea ice information. Cryosphere Today has good anomaly plots. The Naval Sea ice center has a few different algorithms (different ways of processing the data) that give some sense of the observational uncertainty, and the National Snow and Ice Data Center give monthly updates. All of them show pretty much the same thing.

Just to give a sense of how dramatic the changes have been over the last 28 years, the figures below show the minimum ice extent in September 1979, and the situation today (Aug 9, 2007).

Sep 05 1979Aug 09 2007

The reduction is around 1.2 million square km of ice, a little bit larger than the size of California and Texas combined.

Update: As noted by Andy Revkin below, some of the discussion is about ice extent and some is about ice area. The Cryosphere Today numbers are for area. The difference is whether you count ‘leads’ (the small amounts of water between ice floes) as being ice or water – for the area calculation they are not included with the ice, for the extent calculation they are.

Update: From the comments: NSIDC will now be tracking this on a weekly basis.

Making sense of Greenland’s ice

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A widely publicised paper in Science last week discussed the recovery ancient DNA from the base of the Dye-3 ice core (in southern Greenland). This was an impressive technical feat and the DNA recovered may well be the oldest pure DNA ever, dating back maybe half a million years. However much of the press coverage of this paper dwelt not on the positive aspects of the study but on its supposed implications for the stability of the Greenland ice sheet and future sea level rise, something that was not greatly discussed in the paper at all. So why was this?
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The lag between temperature and CO2. (Gore’s got it right.)

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Translations: (Italian) (Español)

When I give talks about climate change, the question that comes up most frequently is this: “Doesn’t the relationship between CO2 and temperature in the ice core record show that temperature drives CO2, not the other way round?”

On the face of it, it sounds like a reasonable question. It is no surprise that it comes up because it is one of the most popular claims made by the global warming deniers. It got a particularly high profile airing a couple of weeks ago, when congressman Joe Barton brought it up to try to discredit Al Gore’s congressional testimony. Barton said:

    In your movie, you display a timeline of temperature and compared to CO2 levels over a 600,000-year period as reconstructed from ice core samples. You indicate that this is conclusive proof of the link of increased CO2 emissions and global warming. A closer examination of these facts reveals something entirely different. I have an article from Science magazine which I will put into the record at the appropriate time that explains that historically, a rise in CO2 concentrations did not precede a rise in temperatures, but actually lagged temperature by 200 to 1,000 years. CO2 levels went up after the temperature rose. The temperature appears to drive CO2, not vice versa. On this point, Mr. Vice President, you’re not just off a little. You’re totally wrong.

Of course, those who’ve been paying attention will recognize that Gore is not wrong at all. This subject has been very well addressed in numerous places. Indeed, guest contributor Jeff Severinghaus addressed this in one of our very first RealClimate posts, way back in 2004. Still, the question does keep coming up, and Jeff recently received a letter asking about this. His exchange with the letter writer is reproduced in full at the end of this post. Below is my own take on the subject.

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The IPCC sea level numbers

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Translations: (Português)

The sea level rise numbers published in the new IPCC report (the Fourth Assessment Report, AR4) have already caused considerable confusion. Many media articles and weblogs suggested there is good news on the sea level issue, with future sea level rise expected to be a lot less compared to the previous IPCC report (the Third Assessment Report, TAR). Some articles reported that IPCC had reduced its sea level projection from 88 cm to 59 cm (35 inches to 23 inches) , some even said it was reduced from 88 cm to 43 cm (17 inches), and there were several other versions as well (see “Broad Irony”). These statements are not correct and the new range up to 59 cm is not the full story. Here I will try to clarify what IPCC actually said and how these numbers were derived. (But if you want to skip the details, you can go straight to the critique or the bottom line).
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What triggers ice ages?

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Translations: (Português) (Türkçe) (Français)

by Rasmus Benestad, with contributions from Caspar & Eric

In a recent article in Climatic Change, D.G. Martinson and W.C. Pitman III discuss a new hypothesis explaining how the climate could change abruptly between ice ages and inter-glacial (warm) periods. They argue that the changes in Earth’s orbit around the Sun in isolation is not sufficient to explain the estimated high rate of change, and that there must be an amplifying feedback process kicking in. The necessity for a feedback is not new, as the Swedish Nobel Prize winner (Chemistry), Svante Arrhenius, suggested already in 1896 that CO2 could act as an amplification mechanism. In addition, there is the albedo feedback, where the amount of solar radiation that is reflected back into space, scales with the area of the ice- and snow-cover. And are clouds as well as other aspects playing a role.

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Arctic Sea Ice decline in the 21st Century

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Translations: (Français)

Guest Commentary by Cecilia Bitz, University of Washington

Last month a paper I co-authored received considerable media attention. Headlines read “Experts warn North Pole will be ‘ice free’ by 2040”, “The Big Melt: Loss of Sea Ice Snowballs“, and “Arctic Clear for Summer Sailing by 2040: Models Predict Rapid Decline of Sea Ice”. The story also reached NPR, BBC, CBC, the Discovery channel, and Fox News, among others. Dr. Marika Holland, the first author of the paper, was inundated with media attention. About a dozen journalists contacted me too. I was impressed by the questions they posed — questions that probably reflect what the public most wants to know. However, after giving lengthy interviews, I would read the resulting article and see my explanations boiled down to a few lines. In this essay, I’d like to explain the science in the paper and give my answers to the most often asked questions.

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Not just ice albedo

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Translations: (Français)

A recent paper by Francis & Hunter provides an interesting discussion about reasons for the recent decline in the Arctic sea-ice extent, based on new satellite observations. One common proposition about sea ice is that it involves a positive feed-back because the ice affects the planetary albedo (how the planet reflects the sunlight back to space before the energy enters the ‘climate system’). Yet, there is more to the story, as the ice acts more-or-less like an insulating lid on top of the sea. There are subtle effects such as the planet losing more heat from the open sea than from ice-covered region (some of this heat is absorbed by the atmosphere, but climates over ice-covered regions are of more continental winter character: dry and cold). The oceanic heat loss depends of course on the sea surface temperature (SST). Open water also is a source of humidity, as opposed to sea-ice (because its cold, not because its dry), but the atmospheric humidity is also influenced by the moisture transport associated with the wind (moisture advection). Francis & Hunter found a positive correlation between lack of ice and the downward long-wave radiation, something they attributed primarily to cloudiness. Hence, clouds play a role, both in terms of influencing the albedo as well as trapping out-going heat. Francis & Hunter suggest that the changes in the long-wave radiation is stronger than the clouds’ modulation of the direct sunlight.
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Historical climatology in Greenland

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Gavin Schmidt & Michael Mann

Extending the instrumental record of climate beyond the late 19th Century when many of the national weather centers were first started is an important, difficult and undervalued task. It often is more akin to historical detective work than to climatology and can involve long searches in dusty archives, the ability to read archaic scripts and handwriting, and even Latin translations (for instance, when going through the archives of the Paris Observatory) (sounds like a recent bestseller, only less lucrative, no?).
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Con Allègre, ma non troppo

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Guest Commentary by Georg Hoffmann (LSCE)

Climate change denial is not necessarily a speciality of Washington DC think tanks – sometimes it can also be found in old Europe. Right now there is a little media storm passing by in France evoked by an article from Claude Allègre in L’Express. Who is Claude Allègre? He is one of the most decorated french geophysicists specializing in geochemistry and the use of paleomagnetism. Being a longtime friend of the former prime minister, Lionel Jospin, he even became Minister of Education and Research in the former Socialist government. He still plays an active role within the Socialist party and though he has never published anything directly related to anthropogenic climate change, one would assume that he has some understanding of the scientific matter. But this assumption would be wrong.

In the French weekly journal l’Express he exposed his “sceptical” views in an article entitled “The snows of Kilimanjaro”. In the short editorial, he somehow became lost when following Ernest Hemingway to East Africa. Allègre mentions two scientific examples to demonstrate that there is something fundamentally wrong in the IPCC statements on the reality of climate change. First, he commented on the disappearing glaciers of the Kilimanjaro, sometimes treated as the “Panda” of anthropogenic climate change. Citing a “Nature” study (which was in fact published in Science) by Pierre Sepulchre and colleagues from my laboratory, he claimed that this modelling study demonstrated that Kilimanjaro’s glaciers are controlled by tectonic activity. In fact, the article describes the impact of tectonics of the East African Highlands on Indian ocean moisture transport —- on a time scale of millions of years! This confuses glacier variability over the last ~100 years with rainfall trends extending back to the time of the early hominids (such as Lucy).

In fact, there are good reasons to believe that the situation on the Kilimanjaro is a bit more complicated than a simple “atmosphere gets warmer/ glaciers are melting” equation (for instance, see this previous post on tropical glacier retreat). Furthermore, the real link to climate change does not come from the retreat of one single tropical glacier, but from the fact that, to my knowledge, all studied tropical glaciers have retreated over the 20th century, and the retreat rates have generally increased in recent decades.

Allègre’s misunderstanding was immediately followed by another one. Citing a recent study on relatively stable Antarctic snowfall over the last 30 years (Monaghan et al, 2006, discussed here) , he highlighted what he thought was a clear contradiction to future climate simulations of global circulation models (melting of the Antarctic ice sheet). However, that’s not what they predict. All models predict a comparably stable Antarctic ice sheet for the 21th century in which comparably moderate temperature changes in Antarctica are compensated by slight increase in snowfall. The Monaghan et al study does not contradict these model scenarios.

The French climate research community was of course not very pleased about this short sequence of misrepresentations and personal attacks (“les Cassandres”) and corrected Allègre in an open letter published here on the website of the Institute Pierre Simon Laplace (which includes links to the ongoing back and forth, for those that speak French).

Curiously enough, twenty years ago Allègre wrote in “Clés pour la géologie”, (éd. Belin/France Culture):

“En brûlant des combustibles fossiles, l’homme a augmenté le taux de gaz carbonique dans l’atmosphère, ce qui fait, par exemple, que depuis un siècle la température moyenne du globe a augmenté d’un demi-degré.”

(Translation)
“By burning fossil fuels man enhanced the concentration of carbon dioxide in the atmosphere which has raised the global mean temperature by half a degree in the last century”.

But at that time he used this argument against the anti-nuclear energy movement. It might be that there is simply a bit too much politics in Allègre’s life…

Is Antarctic climate changing?

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Is the Antarctic ice sheet getting bigger or smaller? Is it warming or cooling?

As we’ve reported in earlier posts (here and here), getting accurate answers to these questions is non-trivial, because the available instrumental data remain sparse and generally date back only a few decades, at best. While modern satellite-based techniques such as laser altimetery and gravity anomaly measurements provide important information on very recent changes, to get at the longer term we must rely on less direct methods. In the last 5 years or so, an effort has been under way, much of it under the banner “International Trans Antarctic Scientific Expedition” (ITASE), to do this by collecting many dozens of ice cores from across the Antarctic continent. Two papers out this month represent the first major compilations of results from these efforts. The first, in Science on August 11th, provides a new estimates of Antarctic snowfall changes over the last 50 years. The second, in Geophysical Research Letters (August 30th) provides the first statistical reconstruction of Antarctic temperature change, extending about 200 years into the past.
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