Condensed summary

The spatial scale of a heat wave is an important determinant of its impacts. Extensive summer hot and cold spells in Europe and North America are studied through observations and coupled model projections. Recent trends towards more frequent and extensive hot spells as well as rarer and less extensive cold outbreaks follow global warming trends, but they are regionally modulated on decadal timescales. Coupled model projections reflect these natural and anthropogenic influences, with their relative contributions depending on the particular scenarios assumed for global socioeconomic development. Europe appears to have had an early warning in 2003 of conditions that are projected for the second half of the twenty-first century, assuming a “business as usual” emissions scenario. North America, on the other hand, in spite of a general summer warming, has not seen the extent of summer heat that it can potentially experience even if global emissions of carbon dioxide and sulfate aerosols remain fixed at their current levels. Extensive and persistent heat waves naturally occur in association with widespread drought. The recent warming over North America is unusual in that it has occurred without the large-scale encouragement of a dry soil associated with precipitation deficit. Regional precipitation anomalies, together with global anthropogenic influences, can explain the atypical spatial pattern of recent North American summer warming. A decrease of precipitation to more normal amounts over the central and eastern United States is expected to result in a substantial summer warming over that region.