STUART, Neb. — For lifelong rancher Susan Straka, 46, the most potent image of climate change isn't a melting glacier or the rising sea.

It's a burning cow.

At the height of a crippling drought last year, an errant spark touched off a vast wildfire near Straka's ranch in the sand hills 200 miles west of Omaha. Fire consumed more than 300,000 acres of Nebraska's plains in 2012, including one pasture where Straka and her neighbors failed to reach their cattle in time.

The mother-calf pairs were nearly dead when she found them, tails and ears scorched to stumps. Shooting the animals was an act of mercy.

“I used a rifle on the cows, a pistol on calves,” Straka said. Her voice caught and trailed off as she remembered, “To see things suffer like that ... ”

She sat on the flatbed of her Chevy pickup, rubbing a calloused palm into her eye. A hardy soul who once rode out a tornado outdoors, on horseback, here she was, choking up in front of strangers.

Ranching has long been a trade for tough-minded gamblers, obliging producers to routinely bet their savings on future prices and patterns of rain. But in Straka's view, Western Nebraska's fiery dry spell in 2012 marked a shift to an entirely new game, where the weather has stopped following any recognizable pattern at all.

“We had two bad years of flooding and then got slapped in the face with two really bad years of drought,” Straka said. “We got people who never believed in climate change who are really scared right now.”

In the last five years, weather across the American heartland has whipsawed between extremes. Fires and downpours, 100-year floods and record-breaking dry spells have all seemed to hit harder and more frequently than before.

While not all farmers believe the mayhem represents evidence of a warming world, many climate scientists do. Their research increasingly suggests that violent weather will become the new normal in America's breadbasket — a shift with potentially profound implications for our food supply.

The United States leads the world in staples like corn, beef and soybeans. Because the vast majority of these US foodstuffs are produced in the heartland, changes to climate here can affect dinner tables around the world.

“It's one of the preeminent issues facing society today,” said Nebraska Farmer's Union President John Hansen, who says his group's public backing of climate science has found friendlier ears of late, even in a solidly conservative state.

“There's still a lot of climate change deniers out there, but there's not very many weather deniers,” Hansen said. “When it's hot and dry, you don't go out and give your corn ideology. You give it water, or it dies.”

Weirder, not just warmer

The hotter the world's climate gets, the more energetic its weather tends to become.

But Jennifer Francis, a Rutgers University climate researcher, says the link between warmer global temperatures and wilder American weather may be even more direct.

To begin with, she says the warming trend is no longer a matter of debate.

“We're at our 340th month in a row of above-average temperatures, in terms of the global mean, which is a pretty mind-boggling statistic,” Francis said. “Anybody who's younger than 28 years old has never seen an average temperature month.”

These higher global temperatures are having all kinds of effects. One of the easiest to measure is the Arctic's melting sea ice. In the summer of 2012, the ice dwindled to the smallest area recorded since satellite tracking began 35 years ago.

Ice reflects sunlight, so it helps keep the Arctic colder than the regions directly to the south. As anyone who's felt the blast from an open window in winter knows, differences in temperature cause air to move.

Similarly, the temperature gap between the Arctic and the rest of the hemisphere gives rise to a band of steady winds called the jet stream that governs weather patterns across the region.

Your location relative to the jet stream “says everything about the weather conditions that you're experiencing,” Francis said.

As the Arctic warms, the temperature gap between north and south narrows. Francis' research suggests this causes jet stream winds to slow down, and the path of the jet stream to meander.

That prompts weather in America and elsewhere to change less quickly. It becomes hung up, essentially, in extreme loops that can lead to unusually long periods of heat or cold, rain or drought.

Weather tends to stick around longer, Francis said, adding, “It really doesn't matter what the weather condition is, if it lasts a long time, it can end up being unusual and perhaps extreme.”

The implications for farmers in middle America are becoming increasingly clear, said Robert Robert Ogelsby, a climate scientist who worked for NASA before joining the University of Nebraska, in Lincoln.

“If this extra loopy jet stream is indeed a response to the reduced snow and sea ice in the summer,” Ogelsby said, “we can expect even more extremes of hot, dry and wet.”

That is, wetter farmland in the northeast will see more rain and floods. The drier regions to the southwest ought to brace for longer and more frequent droughts.

“They're both gonna get worse,” Ogelsby said. “The rich get richer, but the poor get poorer.”

Draining America's reservoir

In dry spots like the Texas panhandle, that's likely to mean less water falling from the sky and, in the long term, less water available to pump from the ground. Much of the agriculture in that corner of the state relies on water drawn from the Ogallala Aquifer, a vast underground water table that's pumped onto the most productive farmland in America, from New Mexico to North Dakota.

“I moved here 56 years ago, we had enough water in this aquifer, we had wells that would pump 1,000 gallons a minute,” said Carol Webb, 86, who runs an irrigation equipment business in Dimmitt, Texas, about 50 miles from the New Mexico border.

“Now some of us don't have any water left,” Webb said. “Just enough for house wells, not enough to grow anything.”

As the amount of irrigated land has grown, so too has the Ogallala Aquifer's depletion rates — from less than half a cubic kilometer per year a century ago, to 10 cubic kilometers per year by 2008.

Nationwide, pumping has surged far beyond recharge rates. Since 1900, America has lost enough groundwater to fill Lake Erie two times over, according to the US Geological Survey.

In 2013, as the Texas panhandle entered its third straight year of drought, the Ogallala Aquifer there suffered some of its worst drops in a decade. Wells in a 16-county area dropped nearly two feet on average from 2012 to 2013, according to data High Plains Underground Water Conservation District, which monitors water use there.

During the record-breaking dry spell of 2011, one county in the district saw its wells drop by more than 25 feet as farmers struggled to keep crops alive through weeks of 90- and 100-degree temperatures.

“It was hard to live through,” said Jason Coleman, manager for the water conservation district. “My biggest worry would probably have to be, how many 2011s are we gonna have?”

In parts of Iowa earlier this year, farmers contended with the opposite problem. The 2013 planting season was so heavily rained out that growers there filed insurance claims for more than 700,000 acres of corn and soybeans that went unplanted, according to agronomists at Iowa State University.

“I've never seen a year like this around here. Never,” said Chris Petersen, 58, who has farmed since boyhood on a spread near the town of Clear Lake.

In his part of the state, Petersen said breaks in the rain came so seldom that growers were only able to enter their waterlogged fields for four days during the peak planting month of May.

“We're becoming like India and Pakistan now,” Petersen said. “We have the monsoon season.”

Once the muck dried out, many Iowa farmers did manage to bring in a record harvest — helped in part by an unexpected dry spell toward the end of summer.

“Crops here turned out pretty good,” Petersen said. “But, hell, with the climate extremes, God knows what next year is going to bring.”

That's precisely what has some analysts worried.

Less stable weather means less stable crop yields, says Dan Glickman, a former US Secretary of Agriculture. Combine that with rising standards of living and a world population that's set to top 9 billion, and humanity's relationship with the food supply is in for a major change.

“Barring a revolution in agricultural production techniques, the era of surpluses is over,” Glickman said. “It's just over.”

That could mean that the struggles of a rancher in Nebraska's sand hills may soon enough be felt by customers stepping up to the meat counter.

“Eventually, the people in the city are gonna see that hit in the grocery store,” Straka said, as she watched a small herd of her Black Angus eating by a stand of cottonwoods. “That meat is gonna cost a lot of money and they're gonna be wondering why.”

And while isolated events like a wildfire can never be directly linked to global warming, they aren't likely to become more rare in a hotter world. The Intergovernmental Panel on Climate change predicts Nebraska will see a 4- to 10-degree Fahrenheit overall temperature increase in the next century.

“It definitely can give you an ulcer if you stop to think about it,” Straka said. “Right now we're just waiting to see what God does to us.”