Fresh doubt has been cast on evidence for global warming following the discovery that a key method of measuring temperature change has exaggerated the warming rate by almost 40 percent. Studies of temperature records dating back more than a century have seemed to indicate a rise in global temperature of around 0.5C, with much of it occurring since the late 1970s. This has led many scientists to conclude global warming is under way, with the finger of blame usually pointed at man-made emissions of such greenhouse gases as carbon dioxide.Now an international team of scientists, including researchers from the Met Office in Bracknell, Berkshire, United Kingdom, has found serious discrepancies in the temperature measurements, suggesting that the amount of global warming is much less than previously believed.Measuring water, not airThe concern focuses on the temperature of the atmosphere over the oceans, which cover almost three-quarters of the Earth's surface. While scientists use standard weather station instruments to detect warming on land, they have been forced to rely on the crews of ships to make measurements over the vast ocean regions.Crews have taken the temperature by dipping buckets into the sea or using water flowing into the engine intakes. Scientists have assumed there is a simple link between the temperature of seawater and that of the air above it.However, after analyzing years of data from scientific buoys in the Pacific that measure sea and air temperatures simultaneously, the team has found no evidence of a simple link. Instead, the seawater measurements have exaggerated the amount of global warming over the seas, with the real temperature having risen less than half as fast during the 1970s than the standard measurements suggest.Reporting their findings in the influential journal Geophysical Research Letters, the scientists say the exact cause of the discrepancy is not known. One possibility is that the atmosphere responded faster than the sea to cooling events such as volcanic eruptions.A big cutThe findings have major implications for the climate change debate because sea temperature measurements are a key part of global warming calculations. According to the team, replacing the standard seawater data with the appropriate air data produces a big cut in the overall global warming rate during the last 20 years, from around 0.18C per decade to 0.13C.This suggests that the widely quoted global warming figure used to persuade governments to take action on greenhouse gas emissions exaggerates the true warming rate by almost 40 percent. The team is now calling for climate experts to switch from seawater data to sea-air temperature measurements.One member of the team, David Parker, of the Hadley Centre for Climate Prediction and Research at the Met Office, said the discovery of the discrepancy "shows we don't understand everything, and that we need better observations--all branches of science are like that." Yet according to Parker, the new results do not undermine the case for global warming: "It is raising questions about the interpretation of the sea-surface data."Even so, the findings will be seized on by skeptics as more evidence that scientists have little idea about the current rate of global warming, let alone its future rate. Climate experts are still trying to explain why satellites measuring the temperature of the Earth have detected little sign of global warming, despite taking measurements during supposedly the warmest period on record.Some researchers suspect the fault may again lie with the ground-based temperature measurements. They say many of the data come from stations surrounded by growing urban sprawl, whose warmth could give a misleading figure. A study of data taken around Vienna, Austria, between 1951 and 1996 found that the air temperature rose by anything from zero to 0.6C, depending on precisely where the measurements were made.Robert Matthews is a staff reporter for the UK Telegraph, with whose permission this article is reprinted.