At Jim Hansen’s now famous congressional testimony given in the hot summer of 1988, he showed GISS model projections of continued global warming assuming further increases in human produced greenhouse gases. This was one of the earliest transient climate model experiments and so rightly gets a fair bit of attention when the reliability of model projections are discussed. There have however been an awful lot of mis-statements over the years – some based on pure dishonesty, some based on simple confusion. Hansen himself (and, for full disclosure, my boss), revisited those simulations in a paper last year, where he showed a rather impressive match between the recently observed data and the model projections. But how impressive is this really? and what can be concluded from the subsequent years of observations?
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Climate modelling
Learning from a simple model
A lot of what gets discussed here in relation to the greenhouse effect is relatively simple, and yet can be confusing to the lay reader. A useful way of demonstrating that simplicity is to use a stripped down mathematical model that is complex enough to include some interesting physics, but simple enough so that you can just write down the answer. This is the staple of most textbooks on the subject, but there are questions that arise in discussions here that don’t ever get addressed in most textbooks. Yet simple models can be useful there too.
I’ll try and cover a few ‘greenhouse’ issues that come up in multiple contexts in the climate debate. Why does ‘radiative forcing’ work as method for comparing different physical impacts on the climate, and why you can’t calculate climate sensitivity just by looking at the surface energy budget. There will be mathematics, but hopefully it won’t be too painful.
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Swindled: Carl Wunsch responds
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) The following letter from Carl Wunsch is intended to clarify his views on global warming in general, and the The Great Global Warming Swindle which misrepresented them.
Partial Response to the London Channel 4 Film “The Global Warming Swindle”
Carl Wunsch 11 March 2007
I believe that climate change is real, a major threat, and almost surely has a major human-induced component. But I have tried to stay out of the `climate wars’ because all nuance tends to be lost, and the distinction between what we know firmly, as scientists, and what we suspect is happening, is so difficult to maintain in the presence of rhetorical excess. In the long run, our credibility as scientists rests on being very careful of, and protective of, our authority and expertise.
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What triggers ice ages?
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) () 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.
Stern Science
) () () Halldór Björnsson, William Connolley and Gavin Schmidt
Late last year, the UK Treasury’s Stern Review of the Economics of Climate Change was released to rapturous reception from all sides of the UK political spectrum (i.e. left and right). Since then it has been subject to significant criticism and debate (for a good listing see Rabbett Run). Much of that discussion has revolved around the economic (and ethical) issues associated with ‘discounting’ (how you weight welfare in the future against welfare today) – particularly Nordhaus’s review. We are not qualified to address those issues, and so have not previously commented.
However, as exemplified by interviews on a recent Radio 4 program (including with our own William Connolley), some questions have involved the science that underlies the economics. We will try and address those.
The Physics of Climate Modelling
) () () This is just a pointer to a ‘Quick Study’ guide on The physics of climate modelling that appears in Physics Today this month, and to welcome anyone following through from that magazine. Feel free to post comments or questions about the article here and I’ll try and answer as many as I can.
On Mid-latitude Storms
Statements often appear in the media about suggesting that more extreme mid-latitude storms will result from global warming. For instance, western Norway was recently battered by an unusually strong storm which triggered many such speculations. But scientific papers on how global warming may affect the mid-latitude storms give a more mixed picture. In a recent paper by Bengtsson & Hodges (2006), simulations with the ECHAM5 Global Climate Model (GCM) were analysed, but they found no increase in the number of mid-latitude storms world-wide. Another study by Leckebusch et al. (2006) showed that the projection of storm characteristics was model-dependent. (Note that the dynamics of tropical and mid-latitude (often called ‘extra-tropical’) storms involve different processes, and tropical storms have been discussed in previous posts here on RC: here, here, here, and here).
The factors that control this are often confounding and so make this a tricky prediction. Simple arguments based on the expected ‘polar amplification‘ and the fact that the surface temperature gradient between the tropics and the poles will likely decrease would reduce the scope for ‘baroclinic instability’ (the main generator of mid-latitudes storms). However, there are also increases in the upper troposphere/lower stratospheric gradients (due to the stratosphere cooling and the troposphere warming) and that has been shown to lead to increases in wind speeds at the surface. And finally, although latent heat release (from condensing water vapour) is not a fundamental driver of mid-latitude storms, it does play a role and that is likely to increase the intensity of the storms since there is generally more water vapour available in warmer world. It should also be clear that for any one locality, a shift in the storm tracks (associated with phenomena like the NAO or the sea ice edge) will often be more of an issue than the overall change in storm statistics.
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Historical climatology in Greenland
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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Broadly Misleading
Just when we were beginning to think the media had finally learned to tell a hawk from a handsaw when covering global warming (at least when the wind blows southerly), along comes this article ‘In Ancient Fossils, Seeds of a New Debate on Warming’ by the New York Times’ William Broad. This article is far from the standard of excellence in reporting we have come to expect from the Times. We sincerely hope it’s an aberration, and not indicative of the best Mr. Broad has to offer.
Broad’s article deals with the implications of research on climate change over the broad sweep of the Phanerozoic — the past half billion years of Earth history during which fossil animals and plants are found. The past two million years (the Pleistocene and Holocene) are a subdivision of the Phanerozoic, but the focus of the article is on the earlier part of the era. Evidently, what prompts this article is the amount of attention being given to paleoclimate data in the forthcoming AR4 report of the IPCC. The article manages to give the impression that the implications of deep-time paleoclimate haven’t previously been taken into account in thinking about the mechanisms of climate change, whereas in fact this has been a central preoccupation of the field for decades. It’s not even true that this is the first time the IPCC report has made use of paleoclimate data; references to past climates can be found many places in the Third Assessment Report. What is new is that paleoclimate finally gets a chapter of its own (but one that, understandably, concentrates more on the well-documented Pleistocene than on deep time). The worst fault of the article, though, is that it leaves the reader with the impression that there is something in the deep time Phanerozoic climate record that fundamentally challenges the physics linking planetary temperature to CO2. This is utterly false, and deeply misleading. The Phanerozoic does pose puzzles, and there’s something going on there we plainly don’t understand. However, the shortcomings of understanding are not of a nature as to seriously challenge the CO2.-climate connection as it plays out at present and in the next few centuries.
Tropical SSTs: Natural variations or Global warming?
by Michael Mann and Gavin Schmidt
Roughly a year ago, we summarized the state of play in the ongoing scientific debate over the role of anthropogenic climate change in the observed trends in hurricane activity. This debate (as carefully outlined by Curry et al recently) revolves around a number of elements – whether the hurricane (or tropical cyclone) data show any significant variations, what those variations are linked to, and whether our understanding of the physics of tropical storms is sufficient to explain those links.
Several recent studies such as Emanuel (2005 — previously discussed here) and Hoyos et al (2006 — previously discussed here) have emphasized the role of increasing tropical sea surface temperatures (SSTs) on recent increases in hurricane intensities, both globally and for the Atlantic. The publication this week of a comprehensive paper by Santer et al provides an opportunity to assess the key middle question – to what can we attribute the relevant changes in tropical SSTs? And in particular, what can we say about Atlantic SSTs where we have the best data? [Read more…] about Tropical SSTs: Natural variations or Global warming?