Geophys. Res. Lett. 34, L19709 doi:10.1029/2007GL030948 (2007)

Humans are primarily responsible for the increase in extreme temperatures observed over the US continent since the 1950s, a new study shows. Although the link between atmospheric CO2 emissions from fossil-fuel burning and increased global average temperature is well established, whether humans are influencing extreme weather events, such as heat waves, has remained something of a mystery.

Now, Gerald Meehl from the National Centre for Atmospheric Research in Colorado and colleagues have simulated the number of frost days and warm nights as well as growing season lengths and heat-wave intensity that would have occurred in the US throughout the twentieth century both with and without human greenhouse-gas emissions. Using climate simulations with human and natural factors separately, they found that only by including human-generated emissions could the model results match the observed trends of fewer frost days, more warm nights, greater heat-wave intensity and longer growing seasons since the 1970s.

Extreme weather events have the greatest potential to cause climate-related damage. This study is just one of a number to show the influence of human activity on temperature extremes regionally.