- 20 March 2007
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- Alan Thorpe
Few areas of science have implications as momentous as those of climate change. Much is riding not only on ensuring that the science is as accurate as possible but also on getting the political and social response right. Given the high stakes, it is hardly surprising that scientists' methods and conclusions are coming under considerable scrutiny. This is as it should be. After all, scepticism is fundamental to the scientific method.
Scepticism is one thing; cynicism and conspiracy-theorising are quite another. These are the hallmarks of a recent attempt to discredit the widely accepted theory that human-made carbon dioxide emissions are causing global warming. A loose affiliation of scientists and writers is pushing the alternative idea that fluctuations in solar activity provide a better explanation for the rise and fall in the temperature of Earth's atmosphere over the past few centuries.
Their basic argument goes something like this. When the cosmic rays that constantly bombard Earth from outer space hit water vapour rising from the oceans, they cause clouds to form in the atmosphere which shield the planet from solar radiation and cause it to cool. The sun's magnetic field dampens the effect of cosmic rays, so reducing cloud cover and causing Earth to heat up. Thus an active sun makes for a warmer planet - a correlation these scientists claim is borne out by the records.
Readers in the UK may have seen the most recent incarnation of this theory in the Channel 4 television programme The Great Global Warming Swindle, broadcast last week. The programme questioned not only the mainstream of global warming science but also the integrity of the researchers involved in it. As I am the head of the major funder of climate science in the UK, the Natural Environment Research Council (NERC), such accusations of bias, lying and prejudice were bound to catch my attention.
First, let's deal with the main thesis: that the presence or absence of cosmic rays in Earth's atmosphere is a better explanation for temperature variation than the concentration of CO2 and other gases. This is not a new assertion and it is patently wrong: there is no credible evidence that cosmic rays play a significant role. The climate system is complex and it is likely that many factors affect it, cosmic rays among them. But to claim they are a major influence is disingenuous. There is far greater evidence suggesting CO2 is the major cause of warming.
"To claim that cosmic rays are a major influence is disingenuous
Another claim made by the sceptics relates to the observation that in the long-term history of Earth's climate, variations in atmospheric concentrations of CO2 have lagged behind variations in the temperature of the atmosphere. Therefore, they say, the theory that human-produced greenhouse gases are the cause of current warming must be wrong.
Not so. True, the historical rhythm of major ice ages and interglacial periods is set by Earth's orbital variations, known as Milankovitch cycles, not by levels of greenhouse gases. However, these cycles in turn trigger feedback effects - such as increases or decreases in levels of CO2 in the atmosphere - which amplify the change in temperature.
There is no question that the more CO2 there is in the atmosphere, the warmer the planet becomes. It is not the only mechanism for warming, but it is a prominent one. We are adding CO2 and other greenhouse gases to the atmosphere in a way that has never happened before. The physics of how these gases cause warming by trapping the sun's radiation within the lower atmosphere - the greenhouse effect - is well established and it is no surprise that temperatures have been rising over the past 40 years. What's more, from the comprehensive models that climate scientists have built up, it is clear that only human-made greenhouse gases can explain this warming. Other factors, such as solar variations, have been found to be insignificant in comparison.
This debate is not just about science. Implicit in the sceptics' message is the suggestion that scientists are lying about the role of CO2 in climate change. The impression given is that this is a conspiracy; that climate scientists are deliberately trying to mislead the public, either to affect policy because of their private political motivations or to be more successful in attracting research funding.
Again, this is not backed up by any evidence. In my experience the climate science community operates at the highest ethical level and sticks to the scientific evidence.
The problem with debating the science of something like climate change is that it is hard for the public to assess the arguments across the whole spectrum of scientific opinion. It is partly in recognition of this that the Intergovernmental Panel on Climate Change periodically publishes its scientific assessments that draw together the full body of knowledge on the subject. That is not a political process. It is a scientific one. Let scepticism reign, but let's not play games with the evidence.
