Before we embark on drastic plans to combat climate change, we must be sure of the factsby Roddy Campbell / February 24, 2010 / Leave a comment
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…