Science Source

Attributing the increase in Northern Hemisphere hot summers since the late 20th century

States that anomalously high summertime temperatures have occurred with increasing frequency since the late 20th century
States it is not clear why hot summers are becoming more frequent despite the recent slowdown in the rise in global surface air temperature
Examines factors affecting the historical variation in the frequency of hot summers over the Northern Hemisphere (NH)
Conducts three sets of ensemble simulations with an atmospheric general circulation model
Finds that decadal variabilities in the Pacific and Atlantic Oceans accounted for 43 ± 27% of the recent increase over the NH middle latitudes
Finds that the direct influence of anthropogenic forcing also contributes to increasing the frequency of hot summers since the late 20th century
Results suggest that the heat extremes can become more frequent in the coming decade even with the persistent slowdown in the global-mean surface warming