Lessons for a New Millennium

The climate changes believed to have occurred before this century have remained enigmatic, hampering our understanding of today's climatic changes. Reseachers trying to assess the human influence on the warming of the past century have commonly used complex ocean atmosphere models, but these models have their drawbacks. In his Perspective, Mann discusses a research article by Crowley, who has used a simple model driven by the most important natural and human-induced climate forcings to simulate climatic change over the last millennium. Comparing the model results with observations, Crowley makes what may be the most compelling case to date for the assertion that anthropogenic greenhouse gas increases are behind the dramatic continued warming of the globe.