Climate change: The past as guide to the future

Earth's climate is getting warmer — why? The difficulty for climatologists is discriminating between human-induced changes, principally through the release of 'greenhouse gases' such as carbon dioxide, and natural climate fluctuations. A new approach to this issue employs a variety of indicators to provide annual temperature anomaly patterns over much of the world back to the year AD 1400. For all the uncertainties of such a reconstruction, when these results are compared with external influences on climate it does seem that the main effect on global temperature in the twentieth century has been from increases in greenhouse gases.