The type of heat waves that wilt crops, torch forests — and kill people — are expected to become more frequent and severe over the next 30 years regardless of whether humans curb emissions of the greenhouse gas carbon dioxide, according to a new study.
These are heat waves akin to those that baked many regions of the U.S. in 2012 and devastated crops in Russia in 2010. Such bouts of extreme heat are so-called "three-sigma events," meaning they are three times warmer than the normal climate of a specific region for weeks in a row.
Since the 1950s, the frequency of these events has "strongly increased and right now they cover about 5 percent of the global land area," Dim Coumou, a climate scientist with the Potsdam Institute for Climate Impact Research in Germany, told NBC News. The findings are published Thursday in the journal Environmental Research Letters.
He and colleagues used a suite of 29 models that they say accurately represent the historic, observed trend, giving them confidence the models' ability to project the trend into the future.
"If we look at the near-term, up to 2040, we see that this increase will continue and that by 2040 we will see about 20 percent of the land area affected, so about another fourfold increase compared to today," Coumou said.
If levels of carbon dioxide continue to increase in the atmosphere as they are today, the researchers find heat extremes might cover 85 percent of the planet's land area by 2100.
What's more, even hotter — so called five-sigma events, which are virtually non-existent today — would affect 60 percent of the global land area, according to the research.
"Possibly even a more important message from this study is that a further increase during the second half of the 21st century can be stopped if we reduce CO2 emissions fairly soon," Coumou said.
However, the impact of such reductions will not be felt for several decades given an inherent time lag in the climate system "and this is, of course, something that we have to deal with," he said.
Dealing may mean breeding crops that are more resilient to heat and drought, for example, and preparing the healthcare system to handle an increase in heat-stressed patients.
The findings in and of themselves contain "nothing especially new," Martin Hoerling, a research meteorologist who studies climate variability at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo., said in an email to NBC News.
He noted other published studies have reached a similar conclusion — "a warming climate increases the frequency of temperature exceeding a high threshold."
What troubles Hoerling, who was not involved with the new study, is that it fails to consider the possibility that temperature variability will decrease in a warming world, as indicated by a recent study in Nature. If so, temperatures may not exceed a high threshold as often as projected by the new study.
"Of course, if temperature variability increases, then the frequency of exceeding a high threshold increases," he added.
Similarly, Martin Tingley, a climate scientist at Pennsylvania State University whose own research indicates recent summer heat waves are unprecedented, noted that the new paper is based on a short reference interval — 1951 to 1980, as was done in a 2012 paper by former NASA climate scientist James Hansen.
"It is quite a dangerous proposition" as discussed in commentary on the Hansen paper, Tingley told NBC News. "Because what happens is using a short reference interval, you underestimate the variance within that interval and artificially inflate them outside of that baseline. So it actually gives us more extreme behavior as you extrapolate."
Coumou said he is confident in the ability of the suite of models used in the new research to portray historic, observed temperature trends and thus what they say about the likelihood of increased frequency and severity of extreme heat in the coming decades.
"We know that such events can have strong impacts on society as well as ecosystems," he said. "Our study shows that in the near-term such events will become more regular, but it doesn't mean that we cannot adapt."
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