An intense lobbying campaign by climate change 'sceptics' has been struck a huge blow by the publication this week of a new paper by the Met Office.
Over the past few months, 'sceptics' in the United States and UK have been attempting to convince journalists and politicians that scientists have revised significantly downwards their estimate of the value of equilibrium climate sensitivity, which describes the global average surface warming over the long term following a doubling of the atmospheric concentration of carbon dioxide.
They have been selectively highlighting studies which have yielded low estimates of climate sensitivity based on extrapolations from the record of global average temperature over the past few decades. On this basis, they claim that previous projections of future global warming have been "alarmist".
In particular, the blogs Watts Up With That? and Bishop Hill, which rally the tiny 'sceptic' communities in the United States and UK, have hyped up the work of Nic Lewis, a semi-retired finance professional, who last year persuaded the Journal of Climate to publish his paper estimating that the value of climate sensitivity lies between 1.2°C and 2.2°C.
Lewis then teamed up with some established climate scientists, including authors on the forthcoming Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) which is due for publication in September 2013. They produced a two-page scientific paper, published in the journal Nature Geoscience in May, which offered a new climate sensitivity estimate of between 0.9°C and 5.0°C, based on an extrapolation of their analysis for the period from 1970 to 2009.
However, the authors also drew attention to their estimated range, based on the 10-year period between 2000 and 2009 only, of 1.2°C to 3.9°C, suggesting that "it is arguably the most reliable".
These results differ somewhat from the climate sensitivity range of 2.0°C to 4.5°C that was put forward by the IPCC in its Fourth Assessment Report in 2007.
But the IPCC also pointed out that because the equilibrium climate sensitivity cannot be measured directly, its value should also be inferred using two other methods: studies of past climate change in the geological history of the Earth, and complex calculations based on fundamental physics used by climate models.
However, 'sceptics' have downplayed these many other methods because they tend to produce results that are in line with the IPCC. For instance, a major paper last year outlined the findings of a number of studies of prehistoric climates and concluded that climate sensitivity lies between 2.2°C and 4.8°C.
The 'sceptic' campaign has enjoyed a number of minor successes by gaining coverage in the mainstream media, usually through the opinion columns in newspapers such as The Wall Street Journal.
However, an article in the 20 July edition of The Economist suggests that the 'sceptics' may also have achieved a minor victory in an unexpected way, by influencing the IPCC. The article claims a leak of the final draft of the new IPCC report, which is currently being reviewed by governments, indicates that climate sensitivity lies in the range between 1.5°C and 4.5°C.
This represents a change from the last IPCC draft that was distributed for expert review late last year, and leaked by a 'sceptic', in which climate sensitivity was estimated to be between 2.0°C and 4.5°C, unchanged from the Fourth Assessment Report.
But the new Met Office paper this week casts serious doubt on the low estimates of climate sensitivity. It points out that a doubling of atmospheric levels of carbon dioxide directly causes a 1°C rise in global average temperature. However, warming the atmosphere causes it to hold more water vapour, which itself is a powerful greenhouse gas. This positive feedback increases the warming by a further 1°C.
There are other feedbacks as well, but the Met Office points out that when they are all taken into account, they result in a climate sensitivity of more than 2°C. Hence estimates of less than 2°C need to assume an unexpectedly strong negative feedback. 'Sceptics' argue that clouds reduce warming, but the latest research indicates that they are unlikely to justify a huge reduction in climate sensitivity below 2°C.
Even if climate sensitivity is only 1.5°C, it does not mean that there are no risks from continued greenhouse gas emissions. Carbon dioxide levels are already about 40 per cent higher than before the industrial revolution, and at current rates of global emissions, they will be much more than double the pre-industrial baseline by the end of the century. That would mean a very significant probability of global warming of more than 2°C, which is the threshold that governments have agreed should not be breached in order to avoid dangerous climate change. It is clear that the 'sceptic' campaign on climate sensitivity is merely an attempt to distract from the real task of managing the risks of climate change.
Bob Ward is policy and communications director at the Grantham Research Institute on Climate Change and the Environment at London School of Economics and Political Science.