Heat waves, droughts, blizzards and the the rest of the year’s U.S. record-breaking extreme weather, likely enjoyed a boost from global warming, suggests a climate report.
Hurricane Irene this year pushed the U.S. yearly record for billion-dollar natural disasters to 10, smashing the 2008 record of nine. In the “Current Extreme Weather and Climate Change” report, released today by the Climate Communication scientific group, leading climate scientists outlined how increasing global atmospheric temperatures and other climate change effects — triggered by industrial emissions of greenhouse gases including carbon dioxide and methane — are loading the dice for the sort of extreme weather seen this year.
“Greenhouse gases are the steroids of weather,” says climate projection expert Jerry Meehl of the National Center for Atmospheric Research, at a briefing held by the report’s expert reviewers. “Small increases in temperature set the stage for record breaking extreme temperature events.” Overall, says the report, higher temperatures tied to global warming, about a one-degree global average temperature rise in the last century, have widely contributed to recent runs of horrible weather:
- In 1950, U.S. record breaking hot weather days were as likely as cold ones. By 2000, they were twice as likely, and in 2011 they are three times more likely, so far. By the end of the century they will be 50 times more likely, Meehl says.
- With global warming’s higher temperatures packing about 4% more water into the atmosphere, total average U.S. snow and rainfall has increased by about 7% in the past century, says the study. The amount of rain falling in the heaviest 1% of cloudbursts has increased 20%, leading to more flooding.
- Early snow melt, and more rain rather than snow, has led to water cycle changes in the western U.S. in river flow, winter air temperature, and snow pack from 1950 to 1999. The effects are up to 60% attributable to human influence.
Rather than totally triggering any extreme event, global warming just makes it worse, says meteorologist Jeff Masters of Weather Underground, a report reviewer. “A warmer atmosphere has more energy,” he says, contributing to heat waves, tornadoes and other extremes. Even heavy blizzards come from an atmosphere packed with extra moisture by global warming he adds. “Years like 2011 may be the new normal.”
The report notes scientific disagreement exists over the role of global warming in some severe weather events, such as hurricanes, or the frequency of El Nino weather patterns.
“There’s really no such thing as natural weather anymore,” says climate scientist Donald Wuebbles of the University of Illinois, who was not involved with the report, but said he largely agreed with its conclusions. “Anything that takes place today in the weather system has been affected by the changes we’ve made to the climate system. That’s just the background situation and it’s good for people to know that,” Wuebbles says. Although scientists cannot immediately tie what percentage of an extreme weather event relies on global warming to make it more severe, he says. “It’s always a factor in today’s world.”
Another outside climate scientist, Gavin Schmidt of Columbia University, said by email that public discussions of the role of climate change in extreme weather events, “oscillate between two equally unlikely extremes – that all weather events are caused by global warming or that global warming has no effect on weather at all.” Too often, the discussion finally descends to name-calling (“alarmist” or “denier”) between disagreeing sides, he adds:
The facts of the matter are this: the planet’s climate has changed over the last 30 years, chiefly because of human activities. This will impact the weather – in the trivial sense that the specific weather we are having is not the same as the weather that we would have had without human actions, but also in the non-trivial sense that probabilities of various outcomes will shift – sometimes towards more extremes and sometimes towards less. We have a great deal of difficulty characterizing these changes because of insufficient observations (not enough 100 year periods to properly estimate 100 year events), insufficient attention to extremes in modeling and theory, inaccessibility of model results for extremes, and the basic statistical difficulty in attributing infrequent occurrences.
Nonetheless, the data are good enough to say some things about certain kinds of extremes (heat waves, rainfall intensity (both going up), cold snaps (going down) etc.). In far more cases however, the studies simply have not been done, or the data are simply not good enough to say much, and pundits are tending to extrapolate. That is something most scientists are loath to do.
For researchers, he adds, tracing the role of global warming in extreme weather presents an intriguing problem. “However, the portrayal of this nuanced field in public as either proving that global warming is bad, or that scientists are alarmists, is a travesty. The impact on extremes from human emissions is one of a myriad reasons why we probably don’t want to continue to mess with the planetary energy balance.”