Attributing the Increase in Northern Hemisphere Hot Summers during the Last Half of the 20th Century and the Recent Climate Hiatus

Thursday, 18 December 2014
Youichi Kamae1, Hideo Shiogama1, Masahiro Watanabe2 and Masahide Kimoto2, (1)National Institute for Environmental Studies, Center for Global Environmental Research, Tsukuba, Japan, (2)Atmosphere and Ocean Research Institute University of Tokyo, Tokyo, Japan
Summertime anomalous high temperatures leading to enormous damages to public health and society are increasing continuously since late 20th century. However, the rise of global surface air temperature (SAT) has apparently been slowed in these 15 years. It is not clear why hot summers become more frequent despite the recent global-warming hiatus. Here we present, using ensembles of an atmospheric general circulation model (AGCM), that the continuous increase of hot summers over the Northern Hemisphere (NH) was largely due to direct effect of radiative forcing by the increasing concentration of greenhouse gases (GHGs) and to natural decadal climate variability. The 10-member AGCM ensemble, when forced by 63-year histories of observed sea surface temperature (SST), aerosols, land cover and GHGs, reproduced well the increasing hot summers, but another ensemble without changes in anthropogenic forcings and ocean surface warming still showed a small increase. Decadal SST variations in the Pacific and Atlantic Ocean contributed to the recent increase of hot summers over North America through atmospheric teleconnections. Analyses of the two and an additional ensembles show that local radiative heating over land due to increasing GHGs contributed by 39% to continuous increase over the mid- and high-latitude land areas. The contribution of the direct effect of anthropogenic forcing is independent of the global SST hiatus, suggesting a continuous increase in heat extremes over the land due to this effect even if the climate hiatus persist for the coming decades.


Kamae, Y., H. Shiogama, M. Watanabe, and M. Kimoto (2014), Attributing the increase in Northern Hemisphere hot summers since the late 20th century. Geophys. Res. Lett., doi:10.1002/2014GL061062.