By Climate Central's Andrew Freedman:

Driven by a combination of natural climate variability, manmade global warming, and plain old bad luck, drought conditions are so widespread in the U.S. that it’s possible to take a cross-country flight from Washington, D.C. to San Francisco — a distance of approximately 2,400 miles — without once overflying an unaffected area. With about 81 percent of the lower 48 states experiencing at least abnormally dry conditions, and 63 percent mired in moderate-to-exceptional drought, it’s becoming harder and harder to find an oasis. And the dog days of August are yet to come.

The National Oceanic and Atmospheric Administration (NOAA) already ranks this drought as one of the worst on record, comparable to the drought events of the 1950s. The last time there were such widespread drought conditions in the corn-growing region of the country was in 1988, and that drought cost at least $40 billion.

Given this summer’s punishing 1-2 punch of dry weather and heat, this drought is also being compared to the Dust Bowl era of the 1930s.

According to Richard Seager, a professor at the Lamont-Doherty Earth Observatory at Columbia University and a prominent drought researcher, this drought has one key thing in common with the Dust Bowl and the 1988 drought events: its origins can be traced to the tropical Pacific Ocean, where a periodic cooling of sea surface temperatures — a phenomenon known as La Niña — helped reconfigure global weather patterns during the past two years.

Seager’s research has shown that La Niña is closely tied to many historical drought events in the U.S., since it shifts weather patterns in a way that favors a northern storm track, bypassing the southern tier of the country and depriving the already arid Southwest of much-needed moisture. Even though La Niña conditions are no longer present in the tropical Pacific — in fact, there is talk of a coming El Niño, with warmer sea surface temperatures — the weather pattern in the tropical Pacific is still behaving in a manner that is more consistent with La Niña. This may be giving the 2011-12 La Niña an extended lifespan in the U.S., Seager and other climate scientists said.

“La Niña conditions are perfect for getting drought in the southern parts of the U.S. There is absolutely no doubt that that has been a large part of what’s been going on here,” Seager said.

Seager and other experts noted that factors besides La Nina are also playing a part in this drought, however.

Unlike the droughts of the 1930s, this one is occurring in a much warmer climate, a byproduct of manmade global warming. Seager said that although it most likely didn’t trigger the drought, it’s possible that global warming is making this drought worse than it would otherwise be.

“I think what we’re seeing is largely a naturally occurring event, but it’s occurring against the background of a warming environment,” Seager said.

Global warming may be affecting the feedbacks that take place between dry soils and the atmosphere. When soils dry, very little evaporation occurs and very little moisture is released from vegetation to the air. This can accelerate warming, making droughts hotter, and therefore even drier. Mark Svoboda, a climatologist at the National Drought Mitigation Center in Lincoln, Neb., said this phenomenon is known as drought “feeding on itself.”

Such feedbacks have helped the current drought rapidly balloon in size, overtaking some regions so quickly that it’s referred to as a “flash drought.” At the start of the year, just 28 percent of the contiguous U.S. — mainly the southern tier — was in at least moderate drought. Between mid-April and mid-July, the moderate to exceptional drought area went from 37 percent to 63 percent, a clear indication of the toll that the hot, dry spring and summer have taken.

Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo., said global warming helps make droughts hotter, and therefore drier, than they would be without a human influence. Manmade global warming, he said via email, “. . . means more energy that has to go somewhere. In dry conditions it amplifies drying and goes into heating, creating heat waves. It is small on a day-to-day basis, but is always in one direction and it creates stronger, more intense, and longer-lasting drought. No doubt about it.”

Seager pointed to the lack of snow cover from the 2011-12 winter as another key factor in the drought’s expansion. In Colorado, for example, there was an unusually thin snowpack that melted earlier than average. This primed the state for destructive wildfires and record warm temperatures during June. The thin snow cover and early melt across the High Plains also may have helped dry soils in that region, and may have affected larger-scale weather patterns.

According to Svoboda, the below-average snowpack was one of the factors, along with a dry and warm spring, that made the U.S. more vulnerable to drought come early summer. “We had two strikes against us before even getting to the summer or the heat wave that would follow,” he said in an email conversation.

“I don’t attribute this drought to global warming, but we do see what increased temperatures can do to exacerbate droughts, and if higher temperatures become a more persistent feature in some regions not used to it, then this isn’t a good combination,” Svoboda said.

There is some evidence that this is already happening. A recent study found that global warming made the Texas drought and heatwave 20 times more likely to occur than under similar large-scale climate conditions 50 years ago. Other research shows that warming driven by human activities made the same heat wave and drought more severe than it otherwise would have been. As for whether global warming made the current drought more likely to occur, such "climate attribution" studies can take several months to complete, and the drought is still underway.

If indeed we’re already seeing more intense droughts as a result of global warming, it doesn’t bode well for the future, judging from what climate studies show. For example, climate research consistently shows the Southwest U.S. will become much drier as the climate continues to warm.

Seager’s own research has revealed a disturbing history of North American “megadroughts” that have lasted for decades, making the current event look quaint by comparison. And one study published in 2011 found evidence for a global warming-induced increase in aridity over global land areas since the 1970s, and noted that climate models project a huge expansion of drought areas in the coming decades, depending on whether and by how much greenhouse gas emissions are reined in.

In other words, if you think this drought is bad . . . just wait.

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  • In this Dec. 28, 2012 photo, corn stalks stand in a snowy field near La Vista, Neb. Despite getting some big storms in December, much of the U.S. is still desperate for relief from the nation’s longest dry spell in decades. And experts say it will take an absurd amount of snow to ease the woes of farmers and ranchers. (AP Photo/Nati Harnik)

  • In this Dec. 28, 2012 photo, Bales of corn stalks are covered with a dusting of snow near La Vista, Neb. Despite getting some big storms in December, much of the U.S. is still desperate for relief from the nation’s longest dry spell in decades. And experts say it will take an absurd amount of snow to ease the woes of farmers and ranchers. (AP Photo/Nati Harnik)

  • Severe Drought Threatens Midwest Corn Crops

    PRINCETON, IN - JULY 17: Drought-damaged corn grows in a field on July 17, 2012 near Princeton, Indiana. The corn and soybean belt in the middle of the nation is experiencing one of the worst droughts in more than five decades. Indiana was the nation's fourth largest corn producer in 2011. (Photo by Scott Olson/Getty Images)

  • Severe Drought Threatens Midwest Corn Crops

    FRITCHTON, IN - JULY 17: Corn plants dry in a drought-stricken farm field on July 17, 2012 near Fritchton, Indiana. The corn and soybean belt in the middle of the nation is experiencing one of the worst droughts in more than five decades. Indiana was the nation's fourth largest corn producer in 2011. (Photo by Scott Olson/Getty Images)

  • In this July 26, 2012 photo, dead fish float in a drying pond near Rock Port, Mo., as a turkey vulture paces the shore. Multitudes of fish are dying in the Midwest as the sizzling summer dries up rivers and raises water temperatures in some spots to nearly 100 degrees. (AP Photo/Nati Harnik)

  • Severe Midwest Drought Continues

    WYATT, MO - JULY 18: A buoy used to help guide barges rests on the bank after the water level dropped on the Mississippi River July 18, 2012 near Wyatt, Missouri. Some barge operators have lightened their loads or stopped running altogether on the lower Mississippi because of low water levels. (Photo by Scott Olson/Getty Images)

  • Midwest Farmers Continue To Struggle Against Extended Drought

    CUBA, IL - AUGUST 03: Grass, dried from heat and drought, struggles to survive in a cattle pasture August 3, 2012 near Cuba, Illinois. Cattle being raised in the pasture used to be self-sustaining. This summer's drought has forced the farmer to truck in water, after the pond dried up, and extra feed, to supplement the dry grass, from another farm nearly 20 miles away. Farmers in the Midwest and elsewhere continue to struggle after than half the counties in the United States have been designated disaster areas, mostly due to drought conditions throughout the Midwest. (Photo by Scott Olson/Getty Images)

  • Midwest Farmers Continue To Struggle Against Extended Drought

    NEW HARMONY, IN - AUGUST 03: Corn dead from drought sits in a field August 4, 2012 near New Harmony, Indiana. More than half of the counties in the United States have been designated disaster areas, mostly due to drought conditions throughout the Midwest. (Photo by Scott Olson/Getty Images)

  • The last remaining water in a drought-stricken rural pond reflects the sky and clouds near Calumet, Okla., Friday, July 20, 2012. The nation's widest drought in decades is spreading. More than half of the continental U.S. is now in some stage of drought. (AP Photo/Sue Ogrocki)

  • Cattle graze in a dry field near Calumet, Okla., Friday, July 20, 2012. (AP Photo/Sue Ogrocki)

  • Rows of corn stalks stand under a cloudless sky south of Blair, Neb., Monday, July 23, 2012. The drought-damaged field was cut down for silage. (AP Photo/Nati Harnik)

  • Central Illinois cattle stand in a pasture struggling from lack of rain and a heat wave covering most of the country, Friday, July 20, 2012, in Farmingdale, Ill. The nation's widest drought in decades is spreading. More than half of the continental U.S. is now in some stage of drought, and most of the rest is abnormally dry. (AP Photo/Seth Perlman)

  • In this July 19, 2012 photo, a herd of cattle belonging to Kendal Grecian drink from a water tank at his ranch in Palco, Kan. Grecian spent years meticulously breeding his cows to improve the genetics in each generation, but with Kansas in one of the worst droughts seen in decades, he's struggling to find enough grazing to feed 300 cows, plus their calves. He hopes to get by with selling only a quarter of his herd, but there are no guarantees with the drought expected to linger through October. (AP Photo/John L. Mone)

  • A field of corn withers under triple-degree heat north of Wichita, Kan., in Sedgwick County Monday, July 16, 2012. The drought gripping the United States is the widest since 1956, according to new data released Monday by the National Oceanic and Atmospheric Administration. Fifty-five percent of the continental U.S. was in a moderate to extreme drought by the end of June, NOAA's National Climatic Data Center in Asheville, N.C., said in its monthly State of the Climate drought report. That's the largest percentage since December 1956, when 58 percent of the country was covered by drought. (AP Photo/The Wichita Eagle, Mike Hutmacher)

  • Boats sit on the bottom in a dry cove at Morse Reservoir in Noblesville, Ind., Monday, July 16, 2012. The reservoir is down nearly 6 feet from normal levels and being lowered 1 foot every five days to provide water for Indianapolis. (AP Photo/Michael Conroy)

  • A Great White Egret looks for food on a lake drying up from lack of rain Saturday, July 21, 2012 in Chandlerville, Ill. Wildlife as well as livestock, and crops are struggling from the dry weather and a heat wave covering most of the country. The nation's widest drought in decades is spreading. More than half of the continental U.S. is now in some stage of drought, and most of the rest is abnormally dry. (AP Photo/Seth Perlman)

  • Boats sit on the dry, cracked bottom in a dry cove at Morse Reservoir in Noblesville, Ind., Monday, July 16, 2012. The reservoir is down nearly 6 feet from normal levels and being lowered 1 foot every five days to provide water for Indianapolis. (AP Photo/Michael Conroy)

  • A cow looks for something to eat as it grazes in a dry pasture southwest of Hays, Kan., in a July 6, 2012 photo. The nation's widest drought in decades is spreading. More than half of the continental U.S. is now in some stage of drought, and most of the rest is abnormally dry. (AP Photo/The Hays Daily News, Steven Hausler)

  • Corn stalks struggling from lack of rain and a heat wave covering most of the country lie flat on the ground Monday, July 16, 2012, in Farmingdale, Ill. The nation's widest drought in decades is spreading. More than half of the continental U.S. is now in some stage of drought, and most of the rest is abnormally dry. (AP Photo/Seth Perlman)

  • The gate is closed on a boat ramp leading to a dry cove at Morse Reservoir in Noblesville, Ind., Monday, July 16, 2012. The reservoir is down nearly 6 feet from normal levels and being lowered 1 foot every five days to provide water for Indianapolis. (AP Photo/Michael Conroy)

  • Farmer Joe Fischer holds ears of corn showing the variety of kernal development Thursday, July 12, 2012, at Fischer Farms Inc. in Owensboro, Ky. Normally the silks would already be brown, Fischer said. "There is no pollen left because the silks were delayed. . . because it has been too hot and dry," Fischer said. All five Owensboro-area counties have been designated primary disaster areas because of drought. (AP Photo/The Messenger-Inquirer, John Dunham)

  • Devin Davis of Paul Tree Farms uses a special water canon to water 30,000 trees on the 60 acre farm Saturday, July 21, 2012 in Pleasant Plains, Ill. The trees as well as livestock, wildlife and crops are struggling from lack of rain and a heat wave covering most of the country. The nation's widest drought in decades is spreading. More than half of the continental U.S. is now in some stage of drought, and most of the rest is abnormally dry. (AP Photo/Seth Perlman)

  • A pontoon is anchored on a mud flat as the owner could not reach their dock at Morse Reservoir in Noblesville, Ind., Monday, July 16, 2012. The reservoir is down nearly 6 feet from normal levels and being lowered 1 foot every five days to provide water for Indianapolis. (AP Photo/Michael Conroy)

  • This Thursday, July 5, 2012 photo shows dry soil in a corn field in western Kentucky. Persisting drought conditions have endangered corn fields in western Kentucky. (AP Photo/The Paducah Sun, Allie Douglass)

  • Illinois Farms Hurt By Continued Midwest Drought

    OLMSTED, IL - JULY 26: A corn plant grows in a field parched by drought on July 26, 2012 near Olmsted, Illinois. The field, farmed by Kevin Ulrich, was one of several in the drought stricken region of Southern Illinois that were visited by officials from the United States Department of Agriculture's (USDA) Farm and Foreign Agriculture Services (FFAS) department and Farm Service Agency (FSA). Seventy percent of Illinois, the nation's number two corn producing state, is classified as experiencing some level of drought.(Photo by Scott Olson/Getty Images)

  • Burnt stalks lie on the ground among rows of corn damaged by drought in a parched field in Louisville, Ill. on Monday, July 16, 2012. Over ten days of triple digit temperatures with little rain in the past two months is forcing many farmers to call 2012 a total loss. Rows of corn sit under high temperatures, burning and crisping until the stalks eventually fall, burning into the dry soil. (AP Photo/Robert Ray)

  • Four rows of corn left for insurance adjusters to examine are all that remain of a 40-acre cornfield in Geff, Ill. that was mowed down Monday, July 16, 2012. Over ten days of triple digit temperatures with little rain in the past two months is forcing many farmers to call 2012 a total loss. (AP Photo/Robert Ray)

  • Jack Maloney checks on corn on his farm in Brownsburg, Ind., Monday, July 16, 2012. With no significant rainfall since May 3 and the bleak outlook for rain, Maloney expects a total loss on his corn and soybean crop. (AP Photo/Michael Conroy)

  • A dry field of corn is seen near Fremont, Neb., Monday, July 16, 2012. The drought gripping the United States is the widest since 1956, according to new data released by the National Oceanic and Atmospheric Administration. (AP Photo/Nati Harnik)

  • Jack Maloney displays a drought-damaged ear of corn on his farm in Brownsburg, Ind., Monday, July 16, 2012. With no significant rainfall since May 3 and the bleak outlook for rain, Maloney expects a total loss on his corn and soybean crop. (AP Photo/Michael Conroy)

  • An empty dock sits on the bottom of a dry cove at Morse Reservoir in Noblesville, Ind., Monday, July 16, 2012. The reservoir is down nearly 6 feet from normal levels and being lowered 1 foot every five days to provide water for Indianapolis. (AP Photo/Michael Conroy)

  • Leaves become dry and brittle on stalks of corn in a parched field outside Effingham, Ill., Monday, July 16, 2012. The drought gripping the United States is the widest since 1956, according to new data released Monday by the National Oceanic and Atmospheric Administration. This summer, 80 percent of the U.S. is abnormally dry, and the report said the drought expanded in the West, Great Plains and Midwest last month with the 14th warmest and 10th driest June on record. (AP Photo/Robert Ray)

  • A dry field of corn is seen near Fremont, Neb., Monday, July 16, 2012. The drought gripping the United States is the widest since 1956, according to new data released by the National Oceanic and Atmospheric Administration. (AP Photo/Nati Harnik)

  • The sun rises Sunday, July 15, 2012, in Pleasant Plains, Ill. Corn stalks are struggling in the heat and continuing drought that has overcome most of the country. All of Illinois is officially in a drought, and Illinois Gov. Pat Quinn plans a trip to southern Illinois to discuss the state's plans for responding to dry conditions. (AP Photo/Seth Perlman)

  • Joe Fischer checks on his corn field Thursday, July 12, 2012, at Fischer Farms Inc. in the 3700 block of Fisher Road in Owensboro, Ky. "We've been in a drought for the last three weeks," he said. Fischer farms the property with his brother Tony Fischer. They planted 900 acres of corn with 30,000 plants per acre. "We have no idea what our yield will be," Joe Fischer said. U.S. Agriculture Secretary Tom Vilsack has designated 26 Kentucky counties among more than 900 counties in 29 states as disaster areas. (AP Photo/Messenger-Inquirer, John Dunham)

  • In this photo taken June 27, 2012, farm worker Juan Carlos walks to an irrigated soybean field near England, Ark. The U.S. Agriculture Department has granted a disaster declaration for 69 of Arkansas' 75 counties due to the drought. (AP Photo/Danny Johnston)

  • Midwest Farmers Continue To Struggle Against Extended Drought

    CUBA, IL - AUGUST 03: Cattle try to keep cool in the remains of a farm pond in a pasture heavily damaged by drought August 3, 2012 near Cuba, Illinois. Farmers in the Midwest and elsewhere continue to struggle after than half the counties in the United States have been designated disaster areas, mostly due to drought conditions throughout the Midwest. (Photo by Scott Olson/Getty Images)

  • Midwest Farmers Continue To Struggle Against Extended Drought

    CUBA, IL - AUGUST 03: Cattle nibble the remains of grass in a pasture heavily damaged by drought August 3, 2012 near Cuba, Illinois. Farmers in the Midwest and elsewhere continue to struggle after than half the counties in the United States have been designated disaster areas, mostly due to drought conditions throughout the Midwest. (Photo by Scott Olson/Getty Images)

  • Steve Niedbalski chops down his drought and heat stricken corn for feed Wednesday, July 11, 2012 in Nashville Ill. Farmers in parts of the Midwest are dealing with the worst drought in nearly 25 years. (AP Photo/Seth Perlman)

  • Illinois Gov. Pat Quinn, second left, peels away the husk of a drought-ravaged ear of corn, only to find it had no kernels, as Illinois Farm Bureau President Philip Nelson, right, looks on during visit to the Laird Family Farm in Waltonville, Ill. on Monday, July 16, 2012. Quinn says the state will offer an array of debt restructuring and loan programs to farmers and ranchers affected by the drought. Drought is affecting much of the Midwest, where almost a third of the nation's corn crop has been damaged by heat and drought so severe that some farmers have cut down crops midway through the growing season. (AP Photo/Jim Suhr)

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  • Severe Midwest Drought Continues

    WYATT, MO - JULY 18: Corn plants struggle to survive in a drought-stricken farm field on July 18, 2012 near Wyatt, Missouri. The corn and soybean belt in the middle of the nation is experiencing one of the worst droughts in more than five decades. All 114 of Missouri's counties have received disaster designations because of drought. Last year this area, which sits across the Mississippi River from Illinois, was ravaged by flooding after the Army Corps of Engineers blasted a hole in a levee to save the town of Cairo, Illinois from the rising Mississippi. (Photo by Scott Olson/Getty Images)

  • Severe Midwest Drought Continues

    VINCENNES, IN - JULY 18: Corn plants struggle to survive in a drought-stricken farm field on July 18, 2012 near Vincennes, Indiana. The corn and soybean belt in the middle of the nation is experiencing one of the worst droughts in more than five decades. Indiana was the nation's fourth largest corn producer in 2011. (Photo by Scott Olson/Getty Images)

  • In this July 26, 2012 photo, dead fish decompose in a drying pond near Rock Port, Mo. Multitudes of fish are dying in the Midwest as the sizzling summer dries up rivers and raises water temperatures in some spots to nearly 100 degrees. (AP Photo/Nati Harnik)

  • This photo from July 31, 2012 shows a beached air boat as bathers walk in the nearly dry Platte River near Yutan, Neb., Tuesday, July 31, 2012. The latest U.S. Drought Monitor survey shows an increase in extreme drought conditions in four Plains states but a slight decrease in the overall area of the lower 48 states experiencing some form of drought. (AP Photo/Nati Harnik)