WASHINGTON — What can make a bad drought even worse? A sizzling summer, the likes of which the lower 48 states haven’t seen since record-keeping started in 1895.

Fields of crops that are shriveled or still green with unproductive plants are clear signs of how damaging the summer has been, particularly in the Midwest.

But in a warming world, climate scientists say, more hot days and heat waves are a virtual certainty. Humans have altered the background conditions of the atmosphere and increased the risks of extreme weather.

“There have been many severe droughts here before,” said Katharine Hayhoe, an atmospheric scientist at Texas Tech University. “What’s really unusual, though, is the heat. When it’s really hot, it actually makes the drought worse, because the hotter it is, the faster water evaporates, so the drier the soil gets.”

And the drier the soil, the hotter the surface temperatures, expert said.

“It’s a self-perpetuating event,” said Jake Crouch, a climatologist at the National Climatic Data Center in Ashville, N.C.

The proof is in the pasture.

This year’s corn harvest will be down 13 percent compared with last year, according to the U.S. Department of Agriculture, even though more acres were planted this year than at any time in the past 75 years. The same is true with soybeans; the USDA expects production to be down 12 percent.

What happened was the result of a triple whammy from the weather: the drought, 100-plus-degree days and little drop-off at night – which corn, in particular, requires – and high winds of 40-45 mph throughout July.

“The combination of the dryness, the heat and the wind has been lethal,” said Pam Johnson, a sixth-generation farmer in Iowa and the first vice president of the National Corn Growers Association. “We planted, took care of that crop, had healthy plants and then there are just certain things that are out of your control.”

The drought itself was the result of natural forces, according to Crouch. But it what it appears to tell us about the weather is this: “The bigger issue is it’s unpredictable now,” said Brian Depew, the assistant director of the Center for Rural Affairs in Lyons, Neb.

Indeed, he said that farms along the Missouri River in northeast Nebraska were underwater a year ago. This past March, temperatures there hit the 90s, followed by another frost.

“The longer-term concern is that climate science tells us this is exactly what we should expect from climate change,” Depew said. “So whether this year’s drought is a result of climate change or not, this is what we can expect more of. That’s really troubling, from an agricultural perspective.”

The National Academy of Sciences declared two years ago that climate change is “caused largely by human activities” and “poses significant risks.” The vast majority of climate scientists say that it’s due largely to the burning of fossil fuels. Scientists generally haven’t linked individual weather events with climate change because so many factors influence the climate.

Climate scientist James Hansen of NASA, however, said recently that last year’s drought in Texas and Oklahoma could be attributed directly to climate change, and that the same probably would be true for this year’s heat once the data were in.

The director of the Goddard Institute for Space Studies, Hansen reported in a new scientific paper that only 1 percent of the Earth’s surface had extreme hot weather from 1951 to 1980, compared with 10 percent typically now. Hansen and his colleagues wrote that extremes such as the 2010 Moscow heat wave and the Texas and Oklahoma drought last year “were a consequence of global warming, because their likelihood in the absence of global warming was exceedingly small.”

Scientists say that more heat waves as a result of climate change could be likely. Droughts, however, could vary by region. Climate models don’t show a strong pattern of continuous drought for the Midwest, for example, Hayhoe said. But experts have forecast more dry spells in the West.

The journal Nature Geoscience last month reported the drought in the West from 2000 to 2004 as the worst in 800 years but said even drier decades were ahead as a result of global warming.

“In 20-50 years of time, global warming may become the dominant drying factor over the U.S,” said Aiguo Dai, a scientist at the National Center for Atmospheric Research. “But we should not take the predictions as what will happen exactly.”

Natural forces, greenhouse gas emissions and climate model uncertainties will "alter the future drought," he said.