In late July 2011 the US drought had a parched core centered in the southern Plains and stretched along much of the southern tier. About 30 percent of the continental US was experiencing drought at varying degrees of severity, and another 11 percent was considered abnormally dry.
This year it has overspread most of the country.
Sixty-three percent of the continental US is now drought-stricken, a figure that grows to 79 percent when abnormally dry areas are included, according to the latest figures from the National Drought Mitigation Center in Lincoln, Neb.
"It's kind of like the blob in the old Steve McQueen movie,” says Martin Hoerling, a research meteorologist at the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory.
Summer monsoons have brought some relief to the Southwest and up into the Rocky Mountain states. Early tropical storms also eased dryness along the Gulf of Mexico and into the southeastern corner of the country.
But forecasters are calling for the drought to expand its boundaries through the end of October to cover more of Michigan, Minnesota, and the Dakotas, while to the east drought is expected to move deeper into Pennsylvania and West Virginia.
Several factors have contributed to the expanded drought, meteorologists say.
The lingering aftereffects of two years' worth of colder-than-normal sea-surface temperatures in the eastern tropical Pacific – a condition known as La Niña – set the stage. La Niña events drive average storm tracks farther north than usual as they snake across North America. La Niña also tends to encourage hurricane formation in the Atlantic and Caribbean, but with a couple exceptions, that hasn't helped the southern tier much.
[Editor's note: The original version of this story incorrectly described the effect that La Niña has on hurricane formation.]
But for some parts of the United States, some researchers add, the dryness encouraged by this natural climate cycle appears to be reinforcing a longer-term drying that is consistent with climate models gauging the effects of global warming. For the West in particular, conditions may be setting up for what researchers call a “megadrought” by the end of the century.
For the Midwest, the recent onset of drought was sudden.
"It's been kind of a flash drought," says David Miskus, a senior meteorologist specializing in agricultural weather forecasting at NOAA's Climate Prediction Center in Camp Springs, Md. When you look back to February of this year, there was no drought at all in the Midwest Corn Belt area.
But the winter was unusually warm and dry and was capped by a period in early March when daytime high temperatures in several Midwestern cities ranged from the upper 70s to the mid-80s.
The heat, combined with sparse snow cover, set the stage for drier soils heading into the growing season. Drier soils also meant less evaporation to ease the warmth.
The drought began to spread in April, gathering intensity to strike in force in May, June, and July, Mr. Miskus says. Its timing couldn't have been worse. High temperatures that accompanied the growing drought came as corn and soybeans were set to pollinate.
"When temperatures reach above 95 degrees, corn doesn't pollinate," he says. "We haven't seen a drought like this hit that area since 1988."
The '88 drought triggered congressional hearings on global warming that significantly raised the issue's public profile.
To explain the current drought's persistence, meteorologists point to a dome of high pressure that has been doing a rumba back and forth across the country's midsection.
Storms moving in off the Pacific have moved up over the dome into southern Canada before moving back down the eastern side of the dome to moisten the East Coast.
Given the history of drought in North America – including three decade-long droughts in the 1800s and one in the 12th century that lasted 40 years – it's difficult to spot changing trends that would clearly point to human-triggered climate change as a significant factor, says Richard Seager, a climate researcher at the Lamont-Doherty Earth Observatory in Palisades, N.Y., who focuses his research on multiyear droughts.
"The climate system has a tremendous amount of noise in it," he says. "You're trying to detect an emerging human-induced signal amongst this colossal natural variability. It's very hard to do."
Researchers also note that the Intergovernmental Panel on Climate Change's latest report on global warming and severe weather, released earlier this year, found that for central North America, droughts have become shorter, less frequent, and less severe.
Drought represents the combined effects of high temperature and a shortage of rain or snow. Global warming's fingerprint stands the best chance of showing up in temperatures.
A study published Monday attributes record-smashing summer temperatures in Russia in 2010 and in Texas and Oklahoma last year to global warming, based on a statistical study of global temperature records since 1951.
The study, led by James Hansen at NASA's Goddard Institute for Space Studies in New York and appearing in this week's issue of the Proceedings of the National Academy of Sciences, used 1951 to 1980 as a base period to determine how summer-temperature extremes vary naturally. Then they looked at how trends in those extremes change year by year.
Regardless of cause, the team notes, a generally warming global climate would "load the dice" toward more extreme summertime highs in the northern hemisphere, as well as globally. That's what's happening now as the climate has warmed, particularly since 1980. Climate scientists have indicated that it's "very likely" that the warming is mostly the result of carbon-dioxide emissions from fossils fuels and changes in land-use patterns.
Temperature is only half of the equation, however. Natural climate patterns, such as El Niño and La Niña play a significant role in where ocean storms travel as they move east over North America.
For the US, global warming is contributing only about a degree to the record highs at the moment, Seager says. The balance is still very likely due to natural variability and to the effect already dry soils can have warming an atmosphere already hot and dry.
Still, on a global scale, the drying of subtropical climate zones and their migration toward the poles in the Northern and Southern hemispheres – as models have projected – seem to be occurring.
The southern US already is naturally affected by the northern reaches of this zone. Dr. Seager notes that droughts have become "severe and very common" in the southern US over the past few decades. While finding a clear tie to global warming in the droughts is difficult, he suggests, the southern tier's droughts appear to be consistent with what would be expected as the zone begins to shift north.
A new post-mortem on a five-year drought that struck the US West from 2000 to 2004 indicates that the event – which a research team led by Northern Arizona University researcher Christopher Schwalm identified as the most severe drought in the region in more than 800 years – also is consistent with this long-term drying pattern.
If climate projections pan out, conditions in the West by the end of the century could make that 2000-04 drought look positively moist, says Christopher Williams, a climate scientist at Clark University in Worcester, Mass., who took part in the research.