Summer Heat Extremes Will Quadruple In Intensity By 2040
August 15, 2013

Summer Heat Extremes Will Quadruple In Intensity By 2040

Lawrence LeBlond for - Your Universe Online

Extreme heat waves, such as the one that blanketed the US last year and baked Russia in 2010, are expected to continue, becoming much more frequent and widespread in the coming years. Within three decades, extreme heat waves could triple to quadruple in intensity, in frequency and in area regardless of the amount of carbon dioxide we try to keep out of the atmosphere.

Just 20 years ago heat waves were few and far between. But as man-made climate change continues to dominate, summer heat extremes will likely continue for decades to come. According to researchers, led by Dim Coumou, from the Potsdam Institute for Climate Impact Research (PIK), summer heat extremes are already observed over five percent of the land area. This is projected to double by 2020 and quadruple by 2040.

However, if a cap is set on global greenhouse-gas emissions now, heat extremes beyond mid-century may be reduced substantially. For now, we will have to deal with these three-sigma events -- as they are called by the researchers. As time progresses, even more severe heat waves, classified as five-sigma events, “will go from being essentially absent in the present day to covering around three percent of the global land surface by 2040,” said the researchers.

In a new study, published Thursday in the journal Environmental Research Letters, researchers find severe heat extremes will continue to dominate during the first half of the 21st century regardless of the world’s CO2 emissions controls.

By mid-century, the extreme conditions will alter depending on whatever emission scenarios are adopted. If low-emission standards are adopted, the summer heat extremes should stabilize by 2040. On the other hand, a high-emission standard will see heat extremes continue to dominate land area, increasing by one percent per year after 2040.

"In many regions, the coldest summer months by the end of the century will be hotter than the hottest experienced today -- that's what our calculations show for a scenario of unabated climate change," said Coumou. "We would enter a new climatic regime."

Coumou and colleagues noted their focus has been on heat waves that exceed the usual natural variability of summer month temperatures in a given region by a large margin (three-sigma events). These events are marked by “several weeks that are three standard deviations warmer than the normal local climate -- often resulting in harvest losses, forest fires, and additional deaths in heat-struck cities.”

Under the research team’s scenario of a high-emission standard, three-sigma events can be expected to cover 85 percent of the land area by 2100, with five-sigma events covering as much as 60 percent of global land.

"A good example of a recent three-sigma event is the 2010 heat wave in Russia, which expanded over a large area stretching from the Baltic to the Caspian Sea. In the Moscow region, the average temperature for the whole of July was around [12.6°F] warmer than normal -- it was around [77°F]. In some parts, temperatures above [104°F] were measured," explained Coumou.

Coumou and coauthor Alexander Robinson, from Universidad Complutense de Madrid, said their state-of-the-art climate models have accurately reproduced results of observed monthly heat extremes over the past 50 years, and should be accepted for their projected changes in the trend of heat extremes for the future.

"Our three- and five-sigma thresholds are defined by the variability a region has experienced in the past, so the absolute temperatures associated with these types of event will differ in different parts of the world. Nonetheless these events represent a significant departure from the normal range of temperatures experienced in a given region," said Robinson.

They said tropical regions will see the strongest increase in heat extremes, exceeding the threshold defined by the historic variability in the specific region. Their results show these changes can already be seen when analyzing observations between 2000 and 2012.

"In general, society and ecosystems have adapted to extremes experienced in the past and much less so to extremes outside the historic range," Robinson said. "So in the tropics, even relatively small changes can yield a big impact -- and our data indicates that these changes, predicted by earlier research, in fact are already happening."

The scientists said they had combined all their results into one model ensemble (CMIP5) to reduce the likelihood of uncertainty when reading each individual model.

"We show that these simulations capture the observed rise in heat extremes over the past 50 years very well." Robinson noted. "This makes us confident that they're able to robustly indicate what is to be expected in future."