Rising greenhouse-gas levels have been making summers in the Northern Hemisphere hotter, even though global warming has been slowing in recent years.
Youichi Kamae of the National Institute for Environmental Studies in Tsukuba, Japan, and his colleagues compared the results of climate models that include the effects of human activity with those that do not.
They found that a rise in greenhouse-gas concentrations in the atmosphere has been the dominant cause of the increasing frequency of unusually hot summers in the Northern Hemisphere since the late twentieth century. In the middle latitudes, however, about half of the increase in hot summers can be attributed to natural climate variability over the Pacific and Atlantic oceans.
Heat extremes over land will probably become more frequent even if the global-warming hiatus persists, the authors conclude.
Geophys. Res. Lett. http://doi.org/tr3 (2014)