Last summer’s crippling Great Plains drought can’t definitively be linked with global warming, according to a team of federal scientists from various agencies. In a new report issued this week, the researchers said the drought was probably caused by a confluence of natural climate variations that might only come together in a similar constellation once a century.

Cyclical variations in ocean temperatures — especially the combination of a cooler-than-average Pacific Ocean and a warm phase of the North Atlantic Multidecadal Oscillation may have nudged the region toward drought conditions, but those factors tend to be more of a factor in suppressing winter precipitation.

And background global warming may increase the chances of high temperatures to begin with, but the research team couldn’t find a direct link between the drought and global warming — in fact, the region hit hardest by the drought has been a kind of global warming “hole” in the past few decades, said lead author Dr. Marty Hoerling, a researcher with the National Oceanic and Atmospheric Administration.

“The dominant control in this region is the amount of precipitation. When it’s dry, the ground gets really hot … This is one of those events that comes along once every couple hundreds of years,” Hoerling said, adding that the lack of El Niño conditions in the past 10 years may have been another small factor.

The report put the drought in the context of large-scale natural climate variability, against which it’s hard to pick out any man-made climate change signal.

At its peak last summer, drought spread across 61 percent of the Lower 48. According to the report, two key meteorological processes that bring rain were largely absent in the Plains last year. First, low pressure systems that normally produce widespread rains in May and June did not occur, as storm tracks produced by these systems were shunted northwards into Canada. Also, July and August thunderstorms, which are typically abundant in the region and deliver the majority of precipitation for the year, were infrequent and produced little rain.

“Together, these conditions conspired to create a four-month sequence of record rainfall reduction over the central Great Plains,” according to the report, which also attributes the drought to a “sequence of unfortunate events.”

Using climate models, the researchers found that global ocean conditions had only a modest effect on these weather patterns, and therefore seasonal forecasts were unable to foresee the drought.

The researchers said the Central Plains drought of 2012 was not a progression or northward creep of the previous year’s Southern Plains drought event centered on Texas. In fact, there were no strong indicators that an extreme drought event was poised to spread over the Central Plains in 2012, consistent with the diagnosis that the event was due mostly to natural variations in weather.

The report examined nine previous severe summer droughts since 1895, and found no strong evidence for a progression of drought from the Southern Plains in one year to the Central Plains in the next. Although large portions of the United States are again experiencing a third year of drought, each individual drought event over the last few years has had different causes – with and without warning signs.

In terms of intensity, rainfall deficits from the 2012 summertime drought eclipsed any previous summer season drought in the Central Great Plains since recordkeeping began in 1895, including those of 1934 and 1936 during the Dust Bowl era, and another severe event in 1988.

“The fact that an extreme drought did occur in 2012 may thus be largely coincidental, and by the very nature of extreme events, its occurrence was a low probability outcome,” the report concludes.