This project aims to provide a state-of-the-art quantitative description of the variability and causes of variability of Northern Hemisphere climate, over the last five centuries. It will investigate the behaviour of important regional systems as well as hemispherically-integrated changes, and quantify the changing influences of natural and anthropogenic climate forcings, using an integrated study of palaeoclimate proxies and carefully prescribed general circulation model (GCM) experiments. Another major objective is to establish the reliability of GCM simulations of natural climate variability, and hence gain additional insights into the uncertainty of model-based anthropogenic climate change detection studies. This will provide a more secure basis from which to assess the likelihood of future abrupt and unusual climate changes. We will also undertake this assessment.