Comparing Local Versus Hemispheric Perspectives of Extreme Heat Events

We compare insights provided by local and large-scale perspectives of extreme heat events in ERA5 near-surface temperature data. Heat waves where temperatures exceed four standard deviations about the climatological-mean are expected less than once a century locally but occur roughly once every 10 days somewhere in the Northern Hemisphere midlatitudes. The high frequency of occurrence indicated by the hemispheric perspective is not well represented by normal statistics because it strongly depends on the shapes of the local temperature distributions. The large effective sample size afforded by the hemispheric perspective provides robust evidence of trends in the frequency of occurrence of extreme heat events integrated over the Northern Hemisphere. It also confirms that trends in heat events summed over the hemisphere can be explained by changes in mean temperature alone.