It's a tough job to excavate trustworthy records about past temperatures from the palaeoclimate archives. The application of a fresh approach, in the form of wavelet analysis of the data, is a step forward. Records of temperature during the past two millennia provide clues to the natural variation we might expect in the future. They also support attempts to partition recent warming into natural and anthropogenic components, and to measure the sensitivity of climate to greenhouse gases in the atmosphere. Such long records come only from natural archives, for example tree rings, ice cores and other 'proxy' evidence, and interpreting them has generated spirited debate. The crux of the issue is how much warmer or colder the average temperature has beenin several century-long intervals of the past 2,000 years, the two most intensively studied intervals being the Medieval Warm Period (about AD 1200-1400) and the Little Ice Age (AD 1600-1850). Could we be in for one of these natural swings in the future? If so, how large would it be?
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