Richard Oswald did not need the latest US government report on the creeping toll of climate change to tell him that farming in the midwest is facing a grim future, and very likely changing forever.

For Oswald, the moment of realisation came in 2011.

The 68-year-old lives in the house he was born in and farms 2,500 acres with his son, some of it settled by his great-great-grandfather. The land sits where the Missouri river valley is about four miles wide.

Growing up, Oswald heard tales of a great flood in 1952 which prompted the army to construct levees.

“The next flood wasn’t for another 40 years, in 1993. Heavy rains day after day after day after day until the runoff water and the rain just overpowered the river and the levees,” said Oswald. “Both the ’52 and ’93 floods lasted three weeks. They were abnormal.”

Then came the big Missouri river flood in 2011.

“Heavy rains and heavy snow in the Dakotas and Montana created a huge amount of water. That flood lasted here almost four months. More rain than anybody really ever has any memory of coming down the river,” he said.

Oswald’s farm was underwater for much of that time and the corn lost. Missouri declared a state of emergency. Crops were ruined or never planted. Grain prices surged.

The flooding was memorable in its own right but Oswald also sees it as marking a shift in weather patterns which has forced him to farm differently.

“When I was a kid, my dad would say an inch of rain was a good rain. That’s just what we needed. Now we get four inches, five inches, six inches in one sustained wet spell that lasts two or three days. I don’t ever remember that as a boy. I’ve never seen the sustained wetness in the land that we have now. Even though the river hasn’t gone on the land it’s raised the water table so that the rains that we’ve had this fall, which have been unusually heavy, make it muddy. Continually muddy,” he said.

Oswald cracks open an unharvested soybean from his farmland in Langdon, Missouri. Photograph: Amy Kontras/The Guardian

On 23 November, the National Climate Assessment warned of “substantial damages” across the US in the coming years from increasing wildfires in the west to flooding in the east. But the sharpest rise in temperatures will be between the coasts as the midwest endures longer and hotter summers, heavier rains and droughts that collectively are predicted to significantly reduce US agricultural production.

“Expected increases in challenges to livestock health, declines in crop yields and quality, and changes in extreme events in the United States and abroad threaten rural livelihoods, sustainable food security, and price stability,” said the 1,600-page report, the work of 13 federal agencies.

Climate change is likely to make it harder to grow crops, and to make those that do grow more vulnerable to diseases and pests because of rising humidity. The report said heat and diminishing air quality will take its toll on livestock. Farmers will collectively have to spend billions of dollars to adapt. The effects are already seen from prolonged drought in Kansas and torrential rains in Iowa.

Before the flood in 2011, Oswald, a Missouri river valley crop farmer, was skeptical about the warnings that rising temperatures heralded a more difficult future. Since then, the routines of planting and harvesting that his family has pursued on the same land for five generations have given way to a haphazard cycle governed by waves of extreme heat and intense rains.

“The changes have become more radical. The way the rains come down and the temperatures. You’re constantly trying to manage it,” said Oswald, a former president of the Missouri Farmers Union. “There’s so much unknown about the weather now that it’s pretty hard to do much about it.”

As his son, Brandon, works a combine harvester up and down a field, Oswald kicks the soil with his foot.

“If you look at this, it’s pretty dry right on top but not too far down it’s mud. Two weeks ago there was water standing here from all the rain and the inability of the soil to absorb that much moisture because the level of the river was such that the water level was pretty close to the top of the ground here,” he said.

That mud makes it difficult to plant and to harvest. The rains narrow the number of days when Oswald can get a crop in the ground. If it forces him to delay planting the corn, that means the soybeans will go in late. Worst of all, Oswald said, is that it is all so unpredictable. Where planting was typically spread over the same few days in spring and summer every year, now it can vary by weeks.

Unharvested corn stands in the snow in Langdon, Missouri. Photograph: Amy Kontras/The Guardian

Gene Takle, one of the authors of the climate change report and director of the climate science programme at Iowa State University, traces the sharply increased rainfalls to rising temperatures in the Gulf of Mexico. He said that for centuries, the gulf’s waters have been carried as moisture into the midwest and delivered consistent rainfalls that made the region America’s breadbasket.

But as temperatures have risen so has the amount of moisture in a dome of vapour over the Gulf of Mexico. Takle said that at the same time climate change has moved a pressure centre in the Atlantic, known as the Bermuda high, westwards and closer to a band of low pressure over the Rocky mountains creating higher windspeeds across parts of the midwest. That, in turn, has intensified the flow of moisture from the Gulf of Mexico.

The result is heavier rains dumping huge amounts of water on to fields, alongside rising temperatures.

“Humidity is the the key, the smoking gun, for the increase in our rainfall,” said Takle, a professor of agronomy and of geological and atmospheric sciences.

Takle said that heavy rains in 2013 forced farmers in north-western Iowa to abandon planting altogether on more than 700,000 acres, more than 10% of the state’s land. That came a year after Iowa was hit by widespread drought that also hit the crop harvest.

Some farmers are installing drainage systems to cope with the higher rainfalls, a sign that they know climate change is here to stay. Others are buying expensive new equipment that allows them to plant more seeds in the narrowing windows between rains.

Takle, who grew up on a farm and whose brother still farms in Iowa, said the intense rains have a long-term impact, too. Scientists calculate that dry soil is unable to absorb the water from a rainfall of more than about 1.25in over 24 hours. After that the water starts to erode the soil.

“You start to get excess water moving down slope surfaces and carrying with it any nutrients and nitrogen and phosphates or soil particles,” he said. “We’re getting more of the soil erosion promoting rainfalls. Farmers are using cover crops in the spring period to have some some residual vegetation on the surface to protect it from these extreme events.”

Richard Oswald looks out at the Missouri river. The river runs two miles west of his farmland. Photograph: Amy Kontras/The Guardian

Standing amid his soybeans, Oswald nods north toward the hills on the edge of the valley.

“We farm some upland fields up over that hill a couple of miles,” he said. “They’re rolling hills and they’re terraced. They have structures on them to help control the runoff of the water. But when you have those big rains, none of the terraces or the dams can keep up with that as well as they should. So you have more erosion.”

The size of harvests is already falling. This year, heavy rains have hit soya bean crops, delaying planting or washing out the plants in parts of the midwest, including Iowa.

Alongside the rains are increasing temperatures.

“This year we saw 100-degree temperatures in May which is very unusual,” said Oswald. “I don’t ever remember that in my lifetime or even heard about it. That’s a first ever.”

The Missouri Climate Center recorded that temperatures were above average throughout the spring and summer this year, with September on average 3F warmer than during the 20th century

In the short term, higher temperatures have had some benefit because in parts of the region they extended the growing season and contributed to bigger harvests.

“In the midwest we have about nine days more now than we did 50 years ago,” said Takle. “Part of the increase in yields that we’ve been experiencing over the last 20 or 30 years definitely has been due to the earlier planting and the longer growing season.”

But Takle said those benefits are being lost, and will be reversed in the coming years, as rising temperatures combine with the heavy rains to make growing ever more difficult. He said corn is vulnerable to high temperatures during pollination.

Richard Oswald walks out into a field of unharvested corn on his farm. Photograph: Amy Kontras/The Guardian

A decade ago, Oswald was on the fence about climate change.

“At a certain point you just have to look at what’s going on in your own world and try to decide what you think the impacts of that are,” he said.

As Oswald’s thinking changed, so did his determination to persuade others of the reality of climate change. As president of the Missouri Farmers union, he had some success in getting a discussion going among its members. But he said climate change is politically charged among farmers in part because some see it as a stick to beat them over their practices.

“One of the problems farmers have is when we start talking about environment, a lot of times Sierra Club comes to mind and Sierra Club is pretty radical in their approach. When you have a group that says cows are the problem, you need to get rid of all the cows, and raising corn is a problem, we need to get rid of all the corn, then you’re not going to have a lot of farmers who want to join in and follow you,” he said.

Still, Oswald believes that denial is in retreat. Where farmers, including him, were once skeptical they now see the change with their own eyes. The problem is what to do about it.

“A lot of them will say there’s nothing we can do about it so we might as well not worry because we can’t have an impact, we just have to live with it,” he said.

But he said as climate change bites, farmers are increasingly accepting of the science as they are forced to spend more money on equipment and seeds to maintain current crop yields.

“It’s become almost an annual assault on their ability to produce good crops. So they are now starting to ask questions and I think are listening a little more to what the scientists are saying about the potential future.”