Extreme summer heat linked to climate change, scientists say

Exceedingly high summer temperatures, longer summers and related catastrophes, such as wildfire and drought, are poised to be the norm, and they are driven by climate change, according to a new research paper published Monday in the Proceedings of the National Academy of Sciences.

In an opinion article over the weekend in the Washington Post that previewed the findings, the paper’s lead author, James E. Hansen wrote: “It is no longer enough to say that global warming will increase the likelihood of extreme weather and to repeat the caveat that no individual weather event can be directly linked to climate change. To the contrary, our analysis shows that, for the extreme hot weather of the recent past, there is virtually no explanation other than climate change.”

The longtime director of the NASA Goddard Institute for Space Studies, Hansen was among the first scientists to warn about climate change and its potential effects during a 1988 Senate hearing. He now says he was mistaken in one critical way: “I was too optimistic.” The effects of climate change are being felt now, not in a distant future, he said.

Hansen added: “The deadly European heat wave of 2003, the fiery Russian heat wave of 2010 and catastrophic droughts in Texas and Oklahoma last year can each be attributed to climate change. And once the data are gathered in a few weeks’ time, it’s likely that the same will be true for the extremely hot summer the United States is suffering through right now.”

Along with co-authors Makiko Sato and Reto Ruedy, Hansen looked at the period from 1951-1980 as the “base period” with which to compare more recent seasonal temperatures and variability of temperatures, over summer and winter. The 29-year base period was one of relatively stable global temperatures, the paper notes, and recent enough that many people, especially baby boomers, would likely remember it.

The researchers showed the chances of temperatures spiking past their normal variability are much greater now than during the base period. “They found that prior to the onset of human-caused global warming, there were very few of these [anomalous] events,” said John Abraham, professor of thermal sciences at the University of St. Thomas in St. Paul, Minn., and co-founder of the Climate Science Rapid Response Team, an information clearinghouse. “However, with each decade, the number of these very rare events has increased significantly. Not only have the average temperatures increased around the world but so too has the variability.”

Hansen and his team also showed that while all extreme “hot events” have increased globally, “the occurrence of cold events has virtually disappeared,” Abraham wrote.

Many scientists are researching the link between climate and weather, including Kevin E. Trenberth, distinguished senior scientist in the climate analysis section at the National Center for Atmospheric Research. Trenberth is looking into climate change’s day-to-day effect on weather.

Trenberth said that while other scientists had covered much of the ground that Hansen and his colleagues did, the new paper highlights that summer time is when we can expect more anomalies to occur.

“ ‘Climate dice,’ describing the chance of unusually warm or cool seasons, have become more and more ‘loaded’ in the past 30 years, coincident with rapid global warming,” Hansen and his colleagues write. “An important change is the emergence of a category of summertime extremely hot outliers. This hot extreme, which covered much less than 1% of Earth’s surface during the base period, now typically covers about 10% of the land area.”

All this, without 2012’s data in the mix.