An automatic weather station being installed on Butler Island, Antarctica
The belief that man is warming the Earth’s climate via greenhouse gas emissions is supported by evidence showing a modest increase in global temperatures over recent decades. But what is the scale of these increases, and are they in any way abnormal? To find out, we need an accurate record over a long period. This is where the University of East Anglia’s Climatic Research Unit (CRU) comes in, and why the “climategate” scandal over its leaked emails matters so much.
The CRU is one of the world’s leading climatic research bodies. Its scientists, along with the Met Office’s Hadley Centre, build and maintain the world’s temperature record. This may sound easy, especially as the figures they produce always seem precise: in 2007 the Intergovernmental Panel on Climate Change put warming over the past 100 years at exactly 0.74°C. But tracking such tiny temperature changes is tricky—even over the last century when we have had decent thermometer coverage. Do you measure highs, lows or averages? Should readings come from Siberia, Antarctica or Australia—and, if all of them, how do you weight the results for a global average? By land area? What about seas?
To get round this the CRU divides the world into geographical grids. It gathers data from meteorological offices all over the world, makes adjustments and obtains a temperature for each grid. Researchers then compare this result against a baseline of historical temperatures for that grid. The average of the differences across every grid reveals how much global temperatures today differ from historical temperatures.
But there are problems, even if you do this carefully. The first is urbanisation. Ideally, thermometers would be in the same place for long periods and unaffected by people. Yet they are often placed near airports, for example, where they are susceptible to an “urban heat island” (UHI) effect, as traffic increases or runways expand. This UHI effect has happened most in places where the record should be most trustworthy, like Britain and the US, where economic development has been strongest, so the surface temperature record has a manmade upward bias which needs to be adjusted for. The CRU’s adjustment methodology is not disclosed.
Even in the countryside, a thermometer on a farm could be exposed to more machinery today than a century ago. Comparisons of rural and urban thermometers have shown heat island effects of a few degrees celsius. Some studies claim that apparently rising temperatures are correlated with local economic activity rather than global warming. Other problems come when a thermometer is replaced, creating discontinuous data, so the series needs blending. This blending and averaging is far from perfect in places like Britain. But in Siberia and China the CRU has to take data without even observing the weather stations. (For 30 years there has been satellite data too, but this measures only at higher altitudes and has shown less warming than at ground level.)
This is why “sceptics” have, not unreasonably, asked to see the raw data. A scientist cannot say “I have discovered X” if he refuses to share the data to test his discovery. A 2005 email from CRU director Phil Jones to Australian sceptic Warwick Hughes highlighted this: “We have 25 or so years invested in the work. Why should I make the data available to you, when your aim is to try and find something wrong with it?” In another email he wrote that he had deleted “loads of emails” after receiving freedom of information requests, and had asked other scientists to do the same.
The good news is that now, post-climategate, the CRU and the Met Office have released data they had claimed was protected by copyright, or subject to confidentiality agreements. So the scandal has encouraged greater openness. But it has done less to solve a second problem, which arises when you try to deduce temperatures going back thousands of years. Palaeoclimatology (climate study of the history of the Earth) is harder than short term measurements, as there are no records. Temperature changes must be inferred through tree rings or ice cores. Yet some tree cores, for instance, suggest different histories from one side of the tree to the other, while their growth is changed by rainfall and CO2 as well as temperature. Another problem comes with the infamous “hockey stick” graph, devised by US climatologist Michael Mann and featured prominently in Al Gore’s film An Inconvenient Truth. This used tree ring data from Russia and the US to show temperatures gently falling for most of the last 1,000 years, then shooting up from the middle of the 20th century, like the end of a hockey stick. Yet the graph seemed to miss two crucial periods: the “medieval warm period” from the turn of the last millennium to the 15th century, and the “little ice age” at the start of the 17th century, when the Thames regularly froze. Mann claimed these periods were local in nature; sceptics, meanwhile, suspected that the techniques used to create graphs like the hockey stick had been designed to favour the idea that warming in the second half of the 20th century was unprecedented.
There is no conspiracy here. But the scientists involved in climate research for the past 30 years may have enjoyed their golden age too much. Research grants have flowed freely, although not, of course, as freely to scientists with contrary views. I am far from being a climate change denier. It seems perfectly likely that we are having, or will have, an effect on warming through the higher concentrations of greenhouse gases. But the evidence is not yet clear; there were, for example, periods of warming in the 19th century almost identical to the modest warming we seem to have experienced since 1975. We cannot rely on highly imperfect climate models as a basis for policy initiatives that cost billions and change how we live. An accurate and unbiased temperature record is critical.
Read what James Lovelock, Bjorn Lomborg, Ed Miliband and many other experts have to say about climate change in Prospect’s Copenhagen special