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. Plain Language Summary: Extreme events can be studied from local and large‐scale perspectives. The two perspectives provide different insights into the likelihood of extreme events. For example, several notable recent heat waves in the Northern Hemisphere exceeded four standard deviations about the long‐term climatology. Locally, such events are expected less than once every century. But based on high spatial resolution temperature data, such events occur somewhere in the Northern Hemisphere roughly once every 10 days. The increased likelihood of extreme heat events when summed over the hemisphere is not well represented by normal statistics because it is strongly dependent on the shapes of the temperature distributions. The large effective sample size afforded by the hemispheric perspective is important, as it provides a robust estimate of the influence of climate change on the frequency of occurrence of heat events. The hemispheric perspective confirms previous findings that long‐term trends in extreme heat events summed over the Northern Hemisphere can be explained by recent increases in mean temperature. Key Points: Heat events expected less than once every century locally occur roughly once every 10 days somewhere in Northern Hemispheric midlatitudesHemispheric incidence is underestimated by a factor 10 by normal statistics and strongly influenced by the shape of frequency distributionsThe large sample size of the hemispheric perspective clarifies the key role of mean temperature in driving observed trends in heat events [ABSTRACT FROM AUTHOR]

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