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On a Weakening of the Walker Circulation

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by Ray Pierrehumbert and Rasmus Benestad

Second article of our 3-part series on atmospheric circulation and global warming

In Part I we outlined some general features of the tropical circulation, and discussed ways in which increases in anthropogenic greenhouse gases might affect El Niño. Now we take up the question of how global warming might affect the quasi-steady east-west overturning circulation known as the Walker Circulation. The Walker circulation affects convection and precipitation patterns, the easterly Trade Winds, oceanic upwelling and ocean biological productivity; hence, changes in this circulation can have far-reaching consequences. It also provides the background state against which El Niño events take place, and so changes in the Walker circulation should form an intrinsic part of thinking about how global warming will affect El Niño. In a paper that recently appeared in Nature, Vecchi, Soden, Wittenberg, Held, Leetmaa and Harrison present intriguing new results which suggest that there has already been a weakening of the Walker circulation in the past century, and that the observed changes are consistent with those expected as a response to increases in anthropogenic greenhouse gases. The discussion in Vecchi et al. also raises some very interesting issues regarding the way the hydrological cycle might change in a warming world.

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How Red are my Proxies?

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Guest commentary by David Ritson

Realclimate recently gave a detailed review of the issues surrounding the Von Storch et al. (2004) Science article that purported to show that the paleo-reconstructions of Mann et al. were invalid. Part of the review centered on a comment of Wahl, Amman and myself and the response to it by Von Storch et al that appeared on April 27 in Science. The response admitted that our critique of their original results was correct but then opened up a new area of debate. As in their original 2004 article von Storch et al had used a coupled climate model (AOGCM) to simulate the temperatures of the last two thousand years. They had then generated pseudo-proxies by adding noise at selected spatial locations to the AOGCM generated temperature histories. The added noise was purportedly designed to represent non-climatic effects such as disease or insect infestation. This simulated ‘noisy’ world then can be used as a test-bed for the reconstruction methodology. A given analysis procedure is validated if it successfully recovers the original AOGCM noise free results and could be rejected if it fails to recover the original results. Of course such testing only makes sense if the simulated test world has characteristics similar to the real-world. [Read more…] about How Red are my Proxies?

Thank you for emitting

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A recent movie, ‘Thank You for Smoking‘, amusingly highlighted the lengths that PR reps for the tobacco companies would go to distort the public discourse on the health effects of smoking. Lest you thought that was of merely historical relevance, we would like to draw your attention to two of the funniest videos around. Lifting a page straight out of the Nick Naylor playbook, the CEI (an industry-funded lobby group) has launched a new ad campaign that is supposed to counteract all those pesky scientific facts about global warming.

The first ad (both available here) deserves to become a classic of the genre. It contains the immortal lines ‘CO2: they call it pollution, we call it Life!’ – it is beyond parody and without content – and so you should definitely see it. The second ad has a little more substance – but is as misleading as you might expect.
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El Niño and Global Warming

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By Rasmus Benestad & Raymond Pierrehumbert

This is the first part of a planned mini-series of 3 posts on tropical climate, circulation, and oceanic response in conjunction with a global warming. Climate change related to a global warming is more than just temperature and precipitation -massive atmospheric circulations change too, and these changes can have consequences.

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IPCC draft: No comment.

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As everyone has now realised, the second-order draft of the new IPCC report has become very widely available and many of the contributors to this site, commenters and readers will have seen copies. Part of the strength of the IPCC process are the multiple stages of review – the report is already significantly improved (in clarity and scientific basis) from the first round of reviews, and one can anticipate further improvements from the ongoing round as well. Thus no statements from this draft report can be considered ‘official’. While most of the contents of the report will come as no surprise to frequent visitors here, we have decided that we are not going to discuss the report until it is finalised and released (sometime in February 2007). At that time, we’ll go chapter by chapter hopefully pulling out the interesting bits, but until then, we feel it’s more appropriate to respect the ‘Do not cite or quote’ injunctions that can be found on every page. We trust that our commenters will likewise respect the process. Patience, people, patience!

Addendum to “A Mistake with Repercussions”

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1. What are “pseudo-proxies” and why are they useful?

Our only information from before the “instrumental period” (the period from which we have systematic measurements with thermometers, starting around 1850) comes from proxy records of climate (like tree rings, ice cores, corals, sediments, pollen etc.). Therefore it is important to know what the available kind and distribution of proxy records can tell us about quantities that we care about (like changes in the average temperature of the northern hemisphere). A typical question is: what accuracy for the northern hemisphere temperature can one expect, given the available number and spatial distribution of proxies? How much uncertainty arises from the non-climatic ‘noise’ in these records? How do the different methods for combining the proxies compare? And so on…

If there was sufficient length of good instrumental data, then we would be able to answer these questions simply by comparing measurements with proxy records. But the instrumental record is short – after all this is the prime reason why we have to rely on proxies.
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A Mistake with Repercussions

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Today, Science published an important comment pointing out that there were serious errors in a climate research article that it published in October 2004. The article concerned (Von Storch et al. 2004) was no ordinary paper: it has gone through a most unusual career. Not only did it make many newspaper headlines [New Research Questions Uniqueness of Recent Warming, Past Climate Change Questioned etc.] when it first appeared, it also was raised in the US Senate as a reason for the US not to join the global climate protection efforts. It furthermore formed a part of the basis for the highly controversial enquiry by a Congressional committee into the work of scientists, which elicited sharp protests last year by the AAAS, the National Academy, the EGU and other organisations. It now turns out that the main results of the paper were simply wrong.
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Gray and Muddy Thinking about Global Warming

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Anybody who has followed press reporting on global warming, and particularly on its effects on hurricanes, has surely encountered various contrarian pronouncements by William Gray, of Colorado State University. A meeting paper that Gray provided in advance of the 2006 27th Conference on Hurricanes and Tropical Meteorology (taking place this week in Monterey California, and covered here by CNN), provides an illuminating window into Gray’s thinking on the subject. Our discussion is not a point-by-point rebuttal of Gray’s claims; there is far more wrong with the paper than we have the patience to detail. Gray will have plenty of opportunities to hear more about the work’s shortcomings if it is ever subjected to the rigors of peer review. Here we will only highlight a few key points which illustrate the fundamental misconceptions on the physics of climate that underlie most of Gray’s pronouncements on climate change and its causes.

Gray’s paper begins with a quote from Senator Inhofe calling global warming a hoax perpetrated on the American people, and ends with a quote by a representive of the Society of Petroleum Geologists stating that Crichton’s State of Fear has "the absolute ring of truth." It is the gaping flaws in the scientific argument sandwiched between these two statements that are our major concern. [Read more…] about Gray and Muddy Thinking about Global Warming

Global Dimming and climate models

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Guest posting from Beate Liepert (LDEO)

On April 18th PBS will air the NOVA documentary “Dimming the Sun” which stirred up lively discussions among scientists and non-scientists when originally shown by BBC in the UK (under the name ‘Global Dimming’ – see our previous posts). [The NOVA version has been thoroughly re-edited and some of the more controversial claims have apparently been excised or better put into context [and we look forward to seeing it! – Ed.].

Global dimming is the phenomena of an observed reduction (about 1-2% per decade since ~1960) of sunlight reaching the surface of the Earth caused by air pollution (aerosols – small particles) and cloud changes. Some of this solar energy is reflected back out to space and this cooling effect is believed to have counteracted part of the greenhouse gas warming. The original version of the film focused mainly on the observational recognition of global dimming, but one aspect did not receive much attention in the film – namely the oft-claimed lack of global dimming in climate models. This led some to assume that climate modelers were ignoring air pollution other than greenhouse gases emissions from fossil fuel burning. Another implication was that climate models are not capable of adequately simulating the transfer of sunlight through the atmosphere and the role of clouds, sunlight extinction of aerosols and aerosol effects on clouds etc, and therefore model projections should not be trusted. The NOVA version will address this issue more prominently by adding an interview with Jim Hansen from NASA Goddard Institute for Space Studies. Along this line, I’d like to elaborate on aerosols in climate models in more detail. [Read more…] about Global Dimming and climate models

Lindzen: point by point

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Daniel Kirk-Davidoff (U. Maryland and one-time Lindzen co-author) provided a more detailed rebuttal of Lindzen’s argument in the comments to our previous post. It deserves to be more widely seen, so here it is again.

Here’s an effort at a point by point rebuttal. I would say that the central flaw in the op-ed is a logical one: if you’re trying to stifle dissent, then you want less funding for climate research, not more. If you’re trying to stop global warming, then you want more money for carbon sequestration research, and you don’t care how much is spent on climate research. On the other hand if you just love climate research as a really interesting intellectual pursuit, that’s when you’ve got an interest in shedding doubt on the reigning view that CO2-induced climate change is a serious policy program, requiring action. Twenty-five years ago, when global warming wasn’t a big public worry, one might expect climate change researchers to hype the problem. In 2006, when public opinion mostly accepts that there’s a problem, scientists who want research money should be emphasizing uncertainty.
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