-- So what's up with the U.S. weather this year, a warm winter, early droughts and a multistate "derecho" windstorm before July.
Is global warming cooking our goose with extreme weather events, or not?
At the same time, they caution against pointing to a warming climate as the direct cause of any one bit of wild weather this year, even as much of the nation sweated out a record-breaking heat wave through the start of July, one expected to break by Monday.
"There is a little bit of truth in both views," says Princeton climate scientist Ngar-Cheung Lau. "What we can say is that the long-term trend is for heat waves to have longer durations and higher temperatures."
While U.S. temperature records fall, this year has globally only been the 11th-warmest one on record as of May, according to the National Oceanic and Atmospheric Administration. Still, there is little doubt that global warming is occurring, most likely increasing average surface temperatures worldwide about 1.4 degrees Fahrenheit over the last century, according to the U.S. National Academy of Sciences. In 2011, the academy concluded in a final report on U.S. global warming impacts that " Climate change is occurring, is very likely caused primarily by the emission of greenhouse gases from human activities, and poses significant risks."
Extreme weather such as heat waves has been more challenging to tie directly to global warming. Last week's heat wave broke or tied 65 temperature records nationwide for July alone, according to NOAA weather station data. Although June last year was warm, "This June has blown the doors off the daily record highs across the country," says NOAA storm expert Greg Carbin.
Projections readily show some types of weather look inevitably more common in a warming climate, including heat waves, extremes of both flooding rainfalls and drought, and temperature records, according to a report earlier this year from Potsdam Institute for Climate Impact Research scientists Dim Coumou and Stefan Rahmstorf
Some climate scientists, such as Georgia Tech's Judith Curry, dismiss connections between global warming and U.S. heat waves. "We saw these kinds of heat waves in the 1930s, and those were definitely not caused by greenhouse gases," she noted recently on her website, Climate Etc. "I don't think that what we are seeing this summer is outside the range of natural variability for the past century."
"That such outliers are mere freak events, so called black swans, remains a possibility," Coumou and Rahmstorf wrote in the journal Nature Climate Change about extreme weather events. "However, the recent clustering of outliers makes this highly unlikely."
For more complex and local events, say storms or tornadoes, it's even more difficult to point to global warming as the culprit. However, even June's derecho, a series of thunderstorm downbursts that unleashed 80 mph winds that knocked out power for millions from Ohio to Virginia, has been foreseen as a consequence of global warming. Derechos don't happen very often but with heat waves more common under climate projections, they would most likely increase in frequency and severity, says forest ecologist Chris Peterson of the University of Georgia in Athens. He pointed to likely extreme weather impacts on forests in a 2000 study.
While scientists study the question, the past six months have been the warmest on record for the United States, taking into account the recent warm winter, Carbin says. "Last summer was hot, but I'm thinking it was just a warm-up for this summer."