With a projected increase of globally averaged temperatures in the future, it could be expected that heat waves would generally worsen. However, a global coupled climate model shows that there is a pattern to the changes in heat waves associated with large-scale atmospheric circulation anomalies in future climate due to the increase of greenhouse gases. The Mediterranean region, and areas of western and southern North America, show comparatively more severe heat waves in the future. Areas of Europe and North America, associated with the severe heat waves in Chicago in 1995 and Paris in 2003, are examined in detail in the model. We show that future heat waves in these areas, defined in two ways, become more intense (by about 2.5°C), more frequent (by 25 to 31%) and longer-lasting (by 20 to 36%) in the second half of the 21st century. Midtropospheric circulation anomalies (positive 500 hPa height anomalies) that are associated with present-day heat waves in observations and the model over Europe and North America coincide with corresponding future increases of 500 hPa height due to an increase in greenhouse gases. This intensifies those anomalous circulation patterns in the model, and produces more severe heat waves in those regions in the future as a consequence.