The recent Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) reported that there has been an increase of about 0.6°C in the global-mean temperature over the last century, with 1998 being the warmest year in the last 100 years. The IPCC Assessment concluded that most of the observed warming over the last 50 years was likely to have been due to anthropogenic factors; increasing greenhouse gases and sulphate aerosols in the atmosphere. All studies that have successfully detected an anthropogenic influence on recent climate change have considered global scale variations. It is harder to identify the effects of anthropogenic climate change on regional scales than on global scales because of the greater magnitude of the internal climate variability relative to forced climate change signals.

The results from a regional climate change detection and attribution study over North America will be described. Observed climate variability and change in the North American region will be compared with climate model simulations of natural and anthropogenically-forced climate variations over the 20th century. A number of simple indices of climate variability and change are defined, including the area-mean temperature over land, the land-ocean temperature contrast, the meridional temperature gradient, and the mean diurnal cycle and annual cycle of temperature over land. These indices are all expected to show a common response to increasing greenhouse gases but are reasonably independent for natural climate variations.

The simulated interannual variability and correlation structure of the indices, averaged over the North American region, compares reasonably well with the observed indices. Most models appear to have slightly too much interannual variability of temperature over land. The observed trends over the last 50 years in all the indices over North America are consistent with simulated trends in model experiments that include increasing greenhouse gas and sulphate aerosol concentrations. The observed mean warming over the last 50 years is not consistent with internal climate variability, nor with the response to changes in solar forcing. Hence, it is likely that a significant fraction of the observed warming in the North American region over the last 50 years is due to human activity. This is the first study that has successfully detected an anthropogenic influence on recent climate change on a regional scale.