The Last Millennium

Evidence for climate variability and change during the period of roughly the past millennium is reviewed. Insights are available from both empirical reconstructions based on natural archives ('proxy' records) of climate and from the results of simulations using theoretical models of the climate system. Both empirical and model-based approaches provide a fairly consistent picture of past changes. Significant regional climate changes are established as having taken place in past centuries, including periods of substantial cooling and warming in certain regions (e.g., Europe) and notable periods of drought and wetness in other regions (e.g., equatorial Africa and the desert Southwest of the United States). Some of these changes may be associated with the response of the climate to natural changes in radiative forcing of the climate, variations in solar output over time, and the effects of atmospheric aerosols due to past explosive volcanic eruptions. At hemispheric or global scales, the late twentieth-century warming appears to be without precedent in at least the past 1000 years. Model simulation studies suggest that this warming is due, in large part, to human or 'anthropogenic' climate impacts, particularly the increased concentration of greenhouse gasses in the atmosphere due to fossil fuel burning.