Detection of Anthropogenic Climate Change

The Intergovernmental Panel on Climate Change concluded in 2001 report that "most of the warming observed over the last 50 years is likely due.to the increase in greenhouse gases". The evidence leading to this conclusion will be briefly reviewed, including recent results further addressing uncertainties. For example, reconstructions of hemispheric scale surface temperature over the last millennium can help to address uncertainties in estimates of natural (internal and externally influenced) climate variability. The comparison between simulations with a simple climate model and these data suggest that the probability density function for the expected temperature increase due to CO2 doubling can be estimated based on comparisons between simulations with a simple climate model and this long temperature record. While detection of climate change in large scale temperature indicators is an important exercise in model validation, society is more concerned about changes in variables that affect it more directly, such as changes in rainfall and climate extremes. The study of changes in extreme events raises interesting statistical questions, such as the relationship between point observations and area averaged rainfall data. The outlook for detection of changes in temperature and precipitation extremes is discussed by studying changes from simulations with two different atmosphere-ocean general circulation models.