Heatwaves and climate change

From Yale Climate Connections:

Extreme heat may not trigger the same visceral fear as a tornado, but according to NOAA’s natural hazard statistics, it causes nearly twice as many fatalities in the United States  each year – more than any other weather hazard. As the climate continues to warm, that number could rise dramatically in the U.S. and around the world.

Since the late 1800s, human-caused climate change has warmed the Earth’s average temperature by around 1 degree Celsius (1.8 degrees Fahrenheit). That doesn’t sound like much, but a relatively small warming of the average temperature results in a large jump in extreme heat.

Why does a modest increase in the average temperature lead to extreme heat?

Consider a graph of temperatures plotted on a bell curve.

Most temperatures fall near the middle of the curve, and those temperatures would be considered typical. Any temperature that falls on the edges of the curve is considered extreme.

This animation shows that as the middle of the curve shifts just slightly to the warm side, a much larger chunk of the curve moves into extreme territory. In other words, extremely hot days occur more often.

Extreme heat occurred very rarely 50 years ago in the United States.

But as a result of climate change, the bell curve has already shifted by one standard deviation interval – a measure that tells you how spread out the values are – according to a 2016 paper by climate scientist James Hansen. As a result, extreme summer heat now occurs about 7% of the time.

The U.S. still sets some record lows, but it’s been setting far more record highs. In fact, recent record highs have been outpacing record lows at a ratio of two to one. This difference could grow to 20 to 1 by mid-century and 50 to 1 by the end of the century.

Hansen’s paper reports that the warming effect has been even larger for the Mediterranean and Middle East. In that area, the bell curve shift in summer is even more dramatic – nearly 2.5 standard deviations. Consequently, every summer is now warmer than average, and the summer climate now lasts considerably longer.

[Read more here]