Global warming dramatically increases frequency of the most extreme heat events

Extreme heat events, such as this heat wave, are the kind of weather events that increase the most as the climate warms. The more extreme the heat wave, the more likely it is due to the change in the climate. And global warming has led to a dramatic surge in the frequency of the most extreme heat events.

Because of the way global warming shifts the climate, the very most extreme events are the weather events most affected by climate change. As the average global temperature rises and the climate shifts, temperatures that were extreme under the old climate are closer to the middle of the new temperature range. Under the earth's climate system events closer to the midpoint occur much more frequently than events closer to the extremes, as shown in the graphic on the right. Thus a small shift in climate leads to a dramatic increase in the frequency of temperatures at the high end. The shifting bell curve also leads to the occurrence of never-before-seen extremes in high temperatures.[1][2][3]

...it is the rarest and the most extreme events - and thereby the ones with typically the highest socio-economic impacts - for which the largest fraction is due to human-induced greenhouse gas emissions.[4]

Due to global warming, the most extreme heat events now impact a global area 10 times greater than in the period 1951-1980.[1]

The impact on moderate heat waves is also dramatic, with a seventy-five percent share of moderate heat events now attributed to climate change.[4]

Many urban areas across the globe have witnessed a significant increase in the number of heat waves, with the largest number of heat waves occurring in the most recent decade studied, 2003-2012.[5]

Heat waves have generally become more frequent across the US in recent decades, with western regions setting records for numbers of these events in the 2000s. Recent multi-month extreme heat events in the US are unprecedented since the start of reliable instrumental records in 1895. There has also been a dramatic increase in nighttime temperatures in the US, reducing the number of critically important relief windows during heat waves.[6]