What is the long term sensitivity to increasing CO2? What, indeed, does long term sensitivity even mean? Jim Hansen and some colleagues (not including me) have a preprint available that claims that it is around 6ºC based on paleo-climate evidence. Since that is significantly larger than the ‘standard’ climate sensitivity we’ve often talked about, it’s worth looking at in more detail.
Paleoclimate
536 AD and all that
“during this year a most dread portent took place. For the sun gave forth its light without brightness… and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear.”
This quote from Procopius of Caesarea is matched by other sources from around the world pointing to something – often described as a ‘dry fog’ – and accompanied by a cold summer, crop failures and a host of other problems. There’s been a TV special, books and much newsprint speculating on its cause – volcanoes, comets and other catastrophes have been suggested. But this week there comes a new paper in GRL (Larsen et al, 2008) which may provide a definitive answer….
The debate is just beginning — on the Cretaceous!
Most of us who are involved in research related to climate change have been asked at one time or another to participate in public debates against skeptics of one sort or another. Some of us have even been cajoled into accepting. In the pre-YouTube days, I did one against the then-head of the American Petroleum institute at the U. of Chicago law school. Gavin did an infamous one against Crichton and company. People are always demanding that Al Gore debate somebody or other. Both Dave Archer and I have been asked to debate Dennis Avery (of “Unstoppable Global Warming” fame) on TV or radio more than once — and declined. It’s a no win situation. If you accept you give the appearance that these skeptics have something to say that’s actually worth debating about — and give their bogus ideas more publicity. If you decline there are all sorts of squawks that “X won’t debate!” or implications that scientists have declared “the debate” (whatever that is supposed to mean) prematurely closed when in fact it is “just beginning.”
Scientists tend to react badly to demands like this in part because the word “debate” is a rather poor description of the way disagreements get hashed out in science. John Ziman has a good discussion of the extent to which scientific questions are ‘debatable’ here (pdf). In a lawyerly debate, it is fair game for each side to pick and choose whatever argument has the most persuasive force with the audience, jury or judge, without any obligation to consider the force of counter-arguments except insofar as they affect one’s defense against the opponent. Science, in contrast, is a deliberative, cooperative, yet still competitive enterprise, where each side is duty bound to fairly consider all arguments and data that bear on the matter at hand. This is not to say that scientific disputes are necessarily dispassionate or orderly. Indeed, I’ve seen near-fistfights break out over things like the Snowball Earth and the interpretation of Neoproterozoic carbon isotope excursions.
The repeated challenges to debate are probably meant to imply that scientists — and their supporters, including Al Gore — are fixed in their ideas, unreceptive to the new and challenging, and unwilling to defend their ideas in public. This picture is hard to square with how scientists actually behave among themselves. It is not that scientists don’t debate, dispute, disagree about matters related to climate. All those things happen, but not on the subjects that skeptics like Inhofe or Fred Singer or Dennis Avery would like to debate (like whether global warming is mainly caused by CO2 or solar variability, or whether the IPCC warming forecasts represent a credible threat.). Those sorts of things are indeed considered settled science by serious climate scientists. Then, too, scientists are justifiably wary of being drawn into staged debates on such diffuse, ill-defined and largely meaningless topics as whether global warming counts as a “crisis.” In the war of the sound bites, the people who feel free to lie and distort can always win. David Mamet made this point eloquently in Bambi vs. Godzilla. A debate like that is not any kind of debate in the sense understood by scientists.
In fact scientists are probing theories and conceptions all the time, trying to break them. The best way to become famous is to overturn established wisdom, so scientists look hard all the time for opportunities to do this. The problem of Hothouse climate states like the Cretaceous and Eocene is a case in point.
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Past reconstructions: problems, pitfalls and progress
Many people hold the mistaken belief that reconstructions of past climate are the sole evidence for current and future climate change. They are not. However, they are very interesting and useful for all sorts of reasons: for modellers to test out theories of climate change, for geographers, archaeologists and historians to examine the impact of climate on past civilizations and ecosystems, and for everyone to get a sense of what climate is capable of doing, how fast it does it and why.
As a small part of that enterprise, the climate of the medieval period has received a very high (and sometimes disproportionate) profile in the public discourse – due in no small part to the mistaken notion that it is an important factor for the attribution of current climate change. Its existence as a period of generally warmer temperatures (at least in the Northern hemisphere) than the centuries that followed is generally accepted. But the timing, magnitude and spatial extent are much more uncertain. All previous multiproxy reconstructions indicate a Northern Hemisphere mean temperature less than current levels, though possibly on a par with the mid- 20th century. But there are only a few tenths of a degree in it, and so the description that it is likely to have been warmer now (rather than virtually certain) is used to express the level of uncertainty.
A confounding factor in discussions of this period is the unfortunate tendency of some authors to label any warm peak prior to the 15th Century as the ‘Medieval Warm Period’ in their record. This leads to vastly different periods being similarly labelled, often giving a misleading impression of coherence. For instance, in a recent paper it was defined as 1200-1425 CE well outside the ‘standard’ definition of 800-1200 CE espoused by Lamb.
Since a new ‘reconstruction’ of the last 2000 years from Craig Loehle is currently doing the rounds, we thought it might be timely to set out what the actual issues are in making such reconstructions (as opposed to the ones that are more often discussed), and how progress is being made despite the pitfalls.
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A phenomenological sequel
Does climate sensitivity depend on the cause of the change?
Can a response to a forcing wait and then bounce up after a period of inertness?
Does the existence of an 11-year time-scale prove the existence of solar forcing?
Why does the amplitude of the secular response drop when a long-term trend is added?
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Younger Dry-as dust?
The Younger Dryas is so called because it corresponds, in the pollen record from Europe, to the latest (i.e. youngest) appearance of the Dryas octopetala pollen, an alpine flower in regions that are now far from alpine. It marks a clear period towards the end of the last ice age when the warming trend of the deglaciation in Europe particularly was interrupted for a period of about 1300 years before it got going again. There were clear glacier advances during this time and the moraines can be seen very clearly all around Europe and Scandinavia.
The clues to what caused this remarkable, if temporary, turnaround have always lain in assessing its spatial extent, the exact timing and correspondence with other events. Two recent papers have shed some welcome and potentially controversial light on the subject.
Perspectives from China
I spent the last three weeks in China partly for a conference, partly for a vacation, and partly for a rest. In catching up over the last couple of days, I notice that the break has given me a slightly different perspective on a couple of issues that are relevant here.
First off, the conference I attended was on paleoceanography and there were was a lot of great new science presented, particularly concerning the Paleocene-Eocene Thermal Maximum (around 55 million years ago), and on past changes to tropical rainfall patterns (see this week’s Nature) – two issues where there is a lot of relevance for climate change and its impacts today. I’ll discuss the new data in separate posts over the next few weeks, but for now I’ll just mention a topic that came up repeatedly in conversations over the week – that was how to improve the flow of information from the paleo community to the wider climate community, as represented by the IPCC for instance.
There was a palpable sense that insights from paleo-climate (in this case referring mainly to the ocean sediment record rather than ice cores or records from the last millennium) were not being given their due, and in fact were frequently being misused. In a panel discussion (hosted by Stefan), people lamented the lack of ‘synthesis’ that would be useful for the outside community, while others stressed (correctly) that synthesis is hard and frankly not well regarded within the community or their funders. I think this is a general problem; many of the incentives for success within an academic field – the push for novel techniques, the ownership of specific slices of data, the desire to emulate the paths to success of the previous generation – actually discourage work across the field that pulls together disparate sources of information.
In the paleo-oceanography case, this exhibits itself in the overwhelming focus on downcore records (the patterns of change at a single point through time) and the relative lack of integrated products that either show spatial patterns of change at a single time, or that try to extract common elements from multiple events in the past. There are of course numerous exceptions – the MARGO project that compiled records from the peak of the last ice age, or the work of PMIP for the mid-Holocene – but their visibility makes their uniqueness all the more obvious. There were no ideas presented that would fix this overnight, but the discussions showed that the community realises that there is a problem – even if the solutions are elusive.
My second thought on China came from travelling through some of the most polluted cites in the world. Aerosol haze that appeared continuous from Beijing to Hong Kong is such an obvious sign of human industrial activity that it simply takes your breath away (literally). In places, even on a clear day, you cannot see the sun – even if there is no cloud in the sky. Only in the mountains or in deeply rural parts of the country was blue sky in evidence. This is clearly an unsustainable situation (even if you are only thinking about the human health impacts) and it points the way, I think, to how China can be engaged on the climate change front. If reducing aerosol emissions can be done at the same time that greenhouse gases can be cut, the Chinese will likely jump at the chance. As an aside, I noticed that Compact Florescent Light bulbs were being used almost everywhere you looked, and that the majority of Shanghai’s motorbikes and scooters were electric rather than gasoline powered. These efforts clearly help, but they are just as clearly not sufficient on their own.
Finally, the limited access to the Internet that one gets in China (through a combination of having better things to do with one’s time and the sometimes capricious nature of what gets through the Great Firewall) allowed me to take a bit of break from the constant back and forth on the climate blogs. In getting back into it, one appreciates just how much time is wasted dealing with the most ridiculous of issues (Hansen’s imagined endorsement of a paper he didn’t write thirty six years ago, the debunking of papers that even E&E won’t publish, and the non-impact of the current fad for amateur photography) at the expense of anything substantive. In effect, if possibly not in intention, this wastes a huge amount of people’s time and diverts attention from more significant issues (at least in the various sections of the blogosphere). Serious climate bloggers might all benefit from not getting too caught up in it, and keeping an closer eye on the bigger picture. We will continue to try and do so here.
Making sense of Greenland’s ice
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 weirdest millennium
Much research effort over the past years has gone into reconstructing the temperature history of the last millennium and beyond. The new IPCC report compiles a dozen reconstructions for the temperature of the Northern Hemisphere (including of course the original “hockey stick” reconstruction, despite opposite claims by the Wall Street Journal). Lack of data does not permit robust reconstructions for the Southern Hemisphere. Without exception, the reconstructions show that Northern Hemisphere temperatures are now higher than at any time during the past 1,000 years (Figure 1), confirming and strengthening the conclusions drawn in the previous IPCC report of 2001.
Fig. 1: Figure 6.10 (panel b) from the paleoclimate chapter of the current IPCC report (see there for details).
“Climate sceptics” do not like this and keep coming up with their own temperature histories. One of the weirdest has been circulated for years by German high-school teacher E.G. Beck (notorious for his equally weird CO2 curve). This history shows a medieval warm phase that is warmer than current climate by more than 1 ºC (see Figure 2). So how did Beck get this curve?
The lag between temperature and CO2. (Gore’s got it right.)
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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