Last summer's record-smashing drought in the US heartland was driven far more by natural variability in weather patterns than by global warming, according to a new analysis by a team of federal and university researchers.
The study represents what its authors call a first cut at untangling the factors contributing to the drought – particularly to the hardest-hit region in the Central Plains. The analysis does not explicitly exclude global warming as a player.
Instead, the researchers say that any one effect was too small to contribute to the time, place, and intensity of the drought in any significant way.
"The peculiar severity of summer 2012 can only be explained by an additional heavy role for random weather variability," the team concludes in a paper submitted to the Bulletin of the American Meteorological Society. The team's analysis also was released on Thursday as a 50-page report under the aegis of a federal drought task force led by the National Oceanic and Atmospheric Administration (NOAA).
The study touches on an event that has become a touchstone in the political debates over global warming's effect on the US.
It follows a report on weather extremes produced by the United Nations-sponsored Intergovernmental Panel on Climate Change. The report noted that on average, droughts in central North America "have become less frequent, less intense, or shorter." But the report also noted that over the course of this century, global warming's influence is likely to intensify droughts in the region.
At its height, the 2012 drought covered not only the six states where the dry-out was most intense – Colorado, Wyoming, Kansas, Nebraska, Iowa, and Missouri. More than 75 percent of the continental US was bathed in yellow, orange, and russets on drought maps denoting conditions ranging from abnormally dry to exceptional drought.
Crops shriveled; the corn yield was 26 percent below initial US Department of Agriculture projections – representing a loss equivalent to the entire harvest in 1961, the research team calculates. Low water curtailed barge traffic on the Mississippi River. The economic losses are still being tallied, although by July 2012, the event had cost the US economy $12 billion.
In the six-state region the team analyzed, the May through August period was the driest in 117 years. Overall, the team put the recurrence rate for a drought of that severity at once every few hundred years.
The country, meanwhile, is heading into another warm season with a higher percentage of the continental US experiencing dry conditions than it did last year. Federal drought statistics released April 9 show that in each of four out of five severity categories, abnormal dryness or drought covers about 12 percent more of the continental US today than it did this time last year. The fifth category, exceptional drought, covers only 2 percent of the nation, but that's double the level for this time last year.
The current drought forecast, which covers April 4 to June 30, shows drought conditions easing from portions of northeast Texas through western Wisconsin and Minnesota. Forecasters expect modest improvement into the Central and Northern Plains. But from central and western Texas north through the Rocky Mountain states to California and eastern Oregon, drought is expected to continue or expand its reach.
It's unclear how well that will hold up. Where seasonal forecasters had a decent bead in advance on the drought that covered the US southern tier in 2011, last year's event was distinct and unforeseen, the researchers say.
Indeed, while much attention has focused on what the new study has to say about global warming's impact on the drought, the study's main intent was to look for features in the drought that seasonal forecasters could have predicted a season or more in advance.
"To speak about climate change, which has altered the odds of something, is not the same as speaking about the predictability of an event" such as last summer's drought, Martin Hoerling, a researcher who focuses on climate variability at NOAA’s Earth Systems Research Laboratory in Boulder, Colo., said at one of two briefings on the report on Thursday.
Ironically, despite shortfalls in winter snows on the Plains and in large sections of the Rockies generally, and an early, warm spring, precipitation in the area that later was hit worst by the drought was near to above-normal coming out of the 2011-12 winter.
Soil moisture across the six states "was not severely stressed," says Richard Seager, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., and a member of the team that performed the analysis.
But as May arrived, things changed. Three distinct shifts in circulation patterns between May and August collectively conspired to keep the Central Plains virtually rain-free. During the spring and early summer, storm systems that move through the Plains and typically draw their moisture from the Gulf of Mexico were diverted north into Canada. These patterns also prevented the formation of summer-evening thunderstorms that bring needed rain later in the season.
The researchers also noticed that a pattern in sea-surface temperatures in the Pacific and Atlantic that had been present during the Dust Bowl droughts of the mid 1930s reappeared in 1999 and has persisted ever since – through the hottest decade globally on record.
This change in sea-surface temperatures could suggest a long-term predisposition for severe drought in the Central Plains, the team suggests.
The report and submitted paper will hardly be the last word on what was a historic drought or on the potential contribution it received from global warming, Mr. Hoerling adds.
Indeed, other researchers note that the study has an element of "what's new?" to it.
"The report certainly did not ask the right questions if it was interested in the role of global warming or human influences," writes Kevin Trenberth, a climate researcher at the National Center for Atmospheric Research in Boulder, Colo., in an e-mail. "We know that natural variability plays a major role, and the question then is what did the global warming aspects do, and why did we break so many records?"
He notes that among the hallmarks that models have identified in droughts influenced by global warming are faster starts and more intense conditions. These contribute to the withering vegetation and wildfires that further reinforce the warming that accompanies drought.
"This is why we broke so many records," he says of the summer's temperatures.
As for drought-forecasting improvements, Dr. Trenberth notes that among other avenues researchers are pursuing, they are trying to upgrade forecasting models, which currently do a better job handling factors that influence drought in the winter than they do handling those same influential factors in the summer.
The report "is not definitive," Hoerling acknowledged during one of Thursday's briefings. "We will spend years on this question. So come back in 80 years and you'll hear me again – I'll look a little bit different – talking about the 2012 drought, I'm sure."