Climate change: is the science really settled? - Today, the IPCC releases its latest report on the science of climate change. At least, that's what the headlines will say. In fact, this is the first stage of a staggered publication of the organisation's Fourth Assessment Report (FAR), which supersedes the third report, published in 2001. But this first tranche is not, as might be expected, the full weighty document, in which all available evidence is evaluated and carefully summarised. Instead this is the so-called Summary for Policymakers (SPM), a short (only 14 pages in draft) summary of the key points. This is the text which nearly everyone will be quoting from over the coming months and years.
Not only is it an unusual step to publish the summary of a document which has not yet been finalised and released into the public domain, but the summary itself is not necessarily quite what it seems. Rather than simply being an attempt to summarise the main points from the much longer report, the SPM is a political document, agreed line by line by the governments of the countries which are members of the IPCC. Only the release of the complete chapter will enable those with sufficient staying power and understanding of the science to compare this with today's document, but the experience from the Third Assessment report was that there were clear messages coming from the SPM which did not necessarily represent a balanced view of the science. In other words, there was spin.
Interestingly, after a ramping up of concerns as the previous three assessment reports were published, the TAR tones down some of the more extreme projections which have been headlined in the past. The report seems set to say that, if carbon dioxide levels reach (and are constrained to) 550ppm (effectively a doubling of the reported pre-industrial average of 280ppm) the ultimate average temperature rise is likely to be 2-4.5 degrees C, which is a narrower range with a reduced upper limit. By the last decade of the century, projected temperature rise is in the range 1.7-4 degrees C compared with the 1980s, for a range of emissions scenarios. Sea level rise is projected as 28 to 43 centimetres over the century, with two-thirds of that being due to thermal expansion. These figures are lower than previously suggested.
However, the headline news will not be these projections or the fact that they have been moderated, but the fact that the IPCC now says that anthropogenic greenhouse gas emissions are 'very likely' to be the primary driver of recent climate change. 'Very likely' is defined as between 90 and 95% certain. But this 'certainty', which will be used constantly to discredit any dissenters, is based on the unproven assumption that the climate scientists and modellers have such a good understanding of natural climate processes that additional greenhouse gas emissions are the only possible cause of rising temperatures in recent years.
At present, we regard the link as a plausible but unproven hypothesis. While modellers claim to be able to reproduce the very variable trends of the twentieth century (by including allowances for aerosols, for example), this smacks of tinkering to get the right answer rather than a way of improving the realism and reliability of the models. Only time will tell whether the projections are anywhere near right but, in the meantime, we should remember that there has been no upward temperature trend in the last eight years.
We are quite prepared to have our scepticism proved wrong if new and convincing evidence emerges. We could be wrong. It will be progress indeed when the IPCC and scientific establishment says the same.
We are pleased to say that the Frazer Institute, a Canadian think tank, will be launching its own Independent Summary for Policymakers in London on 5th February. We hope that this will help to foster debate. (Scientific Allaince)
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