Nature Stunner: As Climate Change Speeds Up, The Number Of Extremely Hot Days Is Soaring

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The number of very hot days have soared in the past 15 years, a new study in the journal Nature Climate Change reports. Based on observations, the authors conclude that “the term pause, as applied to the recent evolution of global annual mean temperatures, is ill-chosen and even misleading in the context of climate change.”

As Climate Progress has repeatedly reported, mean surface temperatures have slowed only a little in recent years, the factors causing that are well understood, and when they reverse, warming will accelerate. But the authors of this new study reject the term “pause” entirely:

it is land-based changes in extreme temperatures, particularly those in hot extremes in inhabited areas, that have the most relevance for impacts. It seems only justifiable to discuss a possible pause in the Earth’s temperature increase if this term applies to a general behaviour of the climate system, and thus also to temperature extremes.

However, we show that analyses based on observational data reveal no pause in the evolution of hot extremes over land since 1997.

What matters most to humans directly is not average temperatures, but hot extremes, such as the monster 2003 heat wave that killed more than 30,000 Europeans or the off-the-charts 2010 Russian heat wave that caused an excess of some 50,000 deaths and led the country to ban grain exports for over a year after their crops shriveled.

The Nature authors underscore this point in their conclusion:

Furthermore, the available evidence suggests that the most ‘extreme’ extremes show the greatest change. This is particularly relevant for climate change impacts, as changes in the warmest temperature extremes over land are of the most relevance to human health, agriculture, ecosystems and infrastructure.

This finding matches recent research from NASA (see here and video below). Similarly, the World Meteorological Organization said Tuesday that in March they “will publish a major report showing the likelihood of extreme heatwaves is increased 500% [with climate change].”

What do the Nature authors mean by “extreme extremes”? They first look at the “annual number of days with daily maximum temperature above the 90th percentile” in a given location averaged over the base period of 1979-2010. That is, what is the average number of annual extreme warm days in a region during that 32-year period? Call that reference number of days for a given location ExDref. The authors then calculate the total land area in a given year that exceeds ExDref by 50 extreme warm days (ExD50) or by 30 days (ExD30) or by 10 days ExD10). They also look at two different, relatively different data sets, ERA-Interim and HadEX2 datasets. And they look at two different slightly reference periods.

And all that means their figures are quite complicated. But here’s the key one, which is worth understanding:

extreme heat

Time series of the ratio of land area affected by exceedances of 10, 30 and 50 extreme warm days per year relative to 1979–2010 average (ExD10, ExD30 and ExD50) in ERA-Interim (E-Int). The respective tendencies over the time period 1997–2012 are overlaid on the time series (trend lines) and displayed in the left panel of the inset plot. The corresponding values over 1997–2010 for HadEX2 are provided in the right panel of the inset plot. The grey dashed line indicates a ratio of 1. The grey dotted line indicates a ratio of 2 (that is, a doubling of the affected area compared with the reference period).

This is a stunning result. During the so-called hiatus, the amount of land that saw more than 50 extreme warm days above the long-term average increased multi-fold. This is rapid climate change, and if it continues it is a very worrisome trend. Certainly the threat from extreme heat is one of the greatest we face due to human-caused climate change (see our 11/13 post, “Deaths From Heat Waves May Increase Ten Times By Mid-Century”).

Finally, back in 2012, NASA examined its global temperature data set and found a similar result — a rapid growth in the land area experiencing “extremely hot” summers.

Earth’s Northern Hemisphere over the past 30 years has seen more “hot” (orange), “very hot” (red) and “extremely hot” (brown) summers, compared to a base period defined in this study from 1951 to 1980. This visualization shows how the area experiencing “extremely hot” summers grows from nearly nonexistent during the base period to cover 12 percent of land in the Northern Hemisphere by 2011. Watch for the 2011 heat waves in Texas, Oklahoma and Mexico, or the 2010 heat waves the Middle East, Western Asia and Eastern Europe. Credit: NASA