Beltsville, Md. - For a peek at what the future holds for agriculture, go to the Agriculture Department's sprawling research campus outside the nation's capital and check out the stainless steel chambers in Building 10.

Resembling oversized refrigerators, the chambers simulate the atmospheric conditions plants could experience in coming decades. Inside the chambers, scientists are growing stands of both wild and conventional varieties of rice to see how they respond.

Stalks of wild rice are protruding from the chamber that simulates the carbon dioxide levels projected for 2030. After just three weeks, the wild rice plants have grown 3 to 4 feet tall - a foot more than they do in the chamber simulating today's atmosphere - and have filled the chamber to overflowing.

The wild rice plants show both the peril and promise facing American agriculture as the climate changes, said USDA scientist Lewis Ziska, who is running the experiment. Carbon dioxide is to plants what fuel is to cars. Higher carbon dioxide levels and rising temperatures could mean weeds will grow faster, taller and in places where they're not found now.

But many food crops, including wheat, rice and soybeans, also are likely to produce bigger yields as carbon dioxide levels increase. Experiments like those with the wild rice could someday help scientists learn how to transfer traits from weeds into cultivated crops to juice up their yields. The wild rice Ziska is experimenting with isn't the variety used for food but rather a weedy relative of conventional rice.

"It's both scary and incredibly interesting at the same time," he said.

Carbon dioxide - and its impact on crops and weeds - is the wild card in predicting the impact of climate change on agriculture. Higher temperatures cause plants to grow faster and can harm plants during critical periods - including pollination - that affect crop yields.

Because the planet is already warming, many of the projected changes to agriculture over the next few decades are likely to occur regardless of what is done to mitigate future greenhouse gas emissions, according to a USDA study.

The temperature increase expected in the Midwest over the next 30 years could decrease crop yields by about 4 percent, the study found.

However, higher carbon dioxide levels will offset the impact of that heat. The question is how much and for what crops. Crops that are known as C3 plants, including soybeans, wheat and rice, respond to carbon dioxide more vigorously than crops such as corn, a C4 plant which has a different photosynthesis process.

The USDA study estimates Midwest soybean yields could increase by nearly 10 percent over the next 30 years, mostly because of the increased carbon dioxide levels. Corn yields would fall 3 percent. The increased carbon dioxide only slightly offsets the impact of increased heat on corn.

Overall, U.S. crop production could rise by 7 percent or fall by 7 percent, depending on the growth-inducing effect of carbon dioxide, said William Cline, a senior fellow of the Washington-based Center for Global Development and author of the book, "Global Warming and Agriculture."

The impact of carbon dioxide may not be as great as first thought based on laboratory conditions. Recent studies using open-air conditions suggest that yields could be significantly lower.

"There are some questions about just how much that (increased carbon dioxide) can be relied on to mitigate effects of climate change," said Eugene Takle, director of Iowa State University's Climate Science Initiative.

There is a dark side to the increase in carbon dioxide, too: a potential increase in weeds that are resistant to herbicides. Think weeds on steroids.

Doubters can check out another experiment in Beltsville, this one on a small research farm just inside the Capitol Beltway that rings Washington, D.C.

Ziska and his colleagues were growing small plots of soybeans mixed with Canada thistle, a tenacious weed found across much of the country. The plots are enclosed with glass walls. Carbon dioxide is pumped into some of the plots to simulate the effects of climate change.

Canada thistle can normally be killed by spraying it with the popular herbicide Roundup. The soybean plants in the research plots, like most soybeans grown in the United States, are genetically engineered to be immune to the weed killer.

However, by accident, the plots were sprayed with 15 to 20 times the normal dose of Roundup. The thistle died off at first - but then came back to life in two of the three plots with extra carbon dioxide.

Ziska thinks the weeds grew deeper roots because of the carbon dioxide and so even the heavy doses of Roundup couldn't kill them.

But as with the wild rice, Ziska sees opportunity, not just doom, in these supercharged thistles. Plant breeders just need to find and harness the genetic changes that make them so resilient.

"There are some major, major issues facing agriculture and agricultural productivity," Ziska said. "That doesn't mean the world is going to end, but we've got to adapt to that change."