There is a new critique of IPCC climate projections doing the rounds of the blogosphere from two ‘scientific forecasters’, Kesten Green and Scott Armstrong, who claim that since the IPCC projections are not ‘scientific forecasts’ they must perforce be wrong and that a naive model of no change in future is likely to be more accurate that any IPCC conclusion. This ignores the fact that IPCC projections have already proved themselves better than such a naive model, but their critique is novel enough to be worth a mention.
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Climate modelling
Why global climate models do not give a realistic description of the local climate
) Global climate
Global climate statistics, such as the global mean temperature, provide good indicators as to how our global climate varies (e.g. see here). However, most people are not directly affected by global climate statistics. They care about the local climate; the temperature, rainfall and wind where they are. When you look at the impacts of a climate change or specific adaptations to a climate change, you often need to know how a global warming will affect the local climate.
Yet, whereas the global climate models (GCMs) tend to describe the global climate statistics reasonably well, they do not provide a representative description of the local climate. Regional climate models (RCMs) do a better job at representing climate on a smaller scale, but their spatial resolution is still fairly coarse compared to how the local climate may vary spatially in regions with complex terrain. This fact is not a general flaw of climate models, but just the climate models’ limitation. I will try to explain why this is below.
Hansen’s 1988 projections
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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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
) 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?
) () (
) 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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