WASHINGTON - Farm state senators and others soon will get a taste of what their colleagues from Missouri already have piled high on their desks: thousands of letters from farmers urging them to vote against the climate and energy bill.

The Missouri Farm Bureau started the letter campaign early, weeks before the bill was fully written and made public. It was followed this month with a pitch from the American Farm Bureau, the nation's largest agriculture lobby, to get farmers to take farm caps, sign their bills and send them to senators with notes that say, "Don't cap our future."

Agriculture is likely to have a central place in the debate on the bill later this year about the short-term costs of acting to curb climate change -- and the costs of failing to address the long-term risks.

Farm lobby groups and senators who agree with them argue that imposing limits on the nation's emissions of heat-trapping gases from coal, oil and natural gas would raise the cost of farming necessities such as fuel, electricity and natural gas-based fertilizer. A government report, however, warns of a dire outlook for farms if rising emissions drive more rapid climate shifts in the decades ahead.

The Senate bill includes provisions that would hold down energy costs for consumers, and some senators are working to add sections that would help farmers.

Agriculture Secretary Tom Vilsack said in written testimony while traveling in China this week that the bill would create opportunities for farmers to sell renewable energy and to earn money by selling credits for reducing emissions. He also said the bill contained provisions that would prevent fertilizer price increases before 2025, even though fuel prices would rise.

The benefits of the bill probably will outweigh the costs in the short run, and "easily trump" increased costs in the long run, he said.

Others are worried, however.

"I can understand in the political world why they're trying to get this under control," said Bill Wiebold, a University of Missouri agronomist, a scientist who specializes in crop production and soil. "What are the ripple effects? That's what farmers are concerned about. They understand that what's being passed in Washington, D.C., could have a direct effect on their bottom line."

Another side of the cost question, however, will be the burden on the daughters and sons who succeed today's farmers, and the generations after them. A comprehensive review of scientific literature and government data undertaken by a team of 19 U.S. scientists at the end of the Bush administration and released in June forecast a disturbing future for American agriculture as warming accelerates in the decades ahead.

The report, "Global Change Impacts in the United States," is the most comprehensive U.S. effort so far to move from a global view of rising temperatures due to accumulating greenhouse gases to a more regionally focused look at current and future changes.

The key messages on agriculture:

• Early on, some warming and elevated carbon-dioxide levels may be good for some crops, but higher levels of warming impair plant growth and yields. More frequent heat waves, for example, would be hard on crops such as corn and soybeans.
• Other more frequent extremes, such as heavy downpours and droughts, also would be likely to reduce crop yields.
• The quality of grazing land will decline, and heat and disease will be harder on livestock.
• Finally, warming will be good for something: pests and weeds.

"This is going to have profound effects on agriculture and forests around the world," said William Hohenstein, the director of the Global Change Program at the Department of Agriculture.

It's not clear how agriculture might adapt to a changing climate and at the same time improve productivity to help meet the needs of a growing population.

"We may not keep up," said Melanie Fitzpatrick, an Australian glaciologist and science adviser to the Union of Concerned Scientists. The environmental advocacy group recently produced reports on climate change in Midwestern states.

Jere White, the executive director of the Kansas Corn Growers Association, said that farmers might be leery of predicted climate changes because "they have a perspective of having to appreciate what occurred with the weather over a fairly long period of time. It's not an abstract issue to them. It's part of their livelihood."

Climate scientists, in reports such as those used in the government study, say that while the weather will keep varying from year to year, the long-term warming trend that's already being observed will continue and accelerate. The severity of the warming will depend on the amount of heat-trapping gases that build up in the atmosphere.

Richard Oswald, 59, grows corn and soybeans and raises cattle with his son on 2,000 acres in Rock Port, in Missouri's northwest corner. He's the chairman of the board of the Missouri Farmers Union, which is part of the National Farmers Union, a group that supports a mandatory cap on emissions and a trading scheme for pollution permits, as long as farmers' concerns are met.

"We can either get behind this and push this legislation in a direction that will help farmers, or we can sit back and fight it all the way and get something we really don't want," Oswald said.

Sen. Saxby Chambliss of Georgia, the Agriculture Committee's ranking Republican, said he'd oppose the bill because it would bring "economic pain for no benefit" and would "only hurt farmers, ranchers and forest landowners and provide them no opportunity to recoup the higher costs they will pay."

"The huge taxes on carbon would be devastating to Midwest farmers," said Sen. Kit Bond, R-Mo.

The bill would charge large sources of emissions, such as power plants, for the amount of greenhouse gases they produce. Farms wouldn't be required to reduce their emissions.

As those limits further tighten, businesses would have to find ways to comply or pay more.

Some of those penalty payments would be used to help vulnerable industries and consumers. Energy costs would rise, but how that would affect Americans would depend on the policies the law imposed.

Sen. Mike Johanns, R-Neb., who was the secretary of agriculture for several years during the Bush administration, said that higher energy costs were certain if the bill passed. He wasn't convinced by the government study that climate changes are equally certain.

It's important to know "the predictability of the studies relative to what climate change could look like," Johanns said. "That gets tougher. The USDA is only starting to dig into that."

He said the report on climate changes in the U.S. was "based on some studies I think are incomplete."

The USDA had a lead role in the agriculture section of the study. The report's conclusions drew from a large body of scientific reports.

Richard Krause, an American Farm Bureau lobbyist, said his group wouldn't dispute the study, but he stressed that it was "about future events, based on models and assumptions."

Unless China, India and other developing countries also reduce emissions, "we're going to be spending money on something for very little return," Krause said. "All the impacts are going to happen anyway."

The U.S., China and other countries have started to move toward cleaner sources of energy, but studies conclude that more changes will be needed to prevent dangerous climate shifts. Climate scientists, meanwhile, say that climate disasters aren't a given but can be averted by large reductions starting soon.

"Most farmers are just sort of skeptical," said Oswald, the farmer and Missouri Farmers Union board chairman. "You're out every day working to overcome adversity from the government, adversity from Mother Nature, adversity from the market. You learn not to put all your eggs in one basket. That's where we are now with climate change. Farmers aren't willing to sign off on all of it."

BOX

Global warming would be bad news for all those amber waves of grain, and for the corn and soybeans that are plentiful throughout the Midwest.

"The grain-filling period" -- the time when the seed grows and matures -- "of wheat and other small grains shortens dramatically with rising temperatures. Analysis of crop responses suggests that even moderate increases in temperature will decrease yields of corn, wheat, sorghum, bean, rice, cotton and peanut crops," according to "Global Climate Change Impacts in the United States," a report based on a comprehensive review of scientific literature and government data by a team of American scientists.

Other details from the study:

- Plant winter hardiness zones -- each of which represents a 10-degree Fahrenheit change in minimum temperature -- in the Midwest are likely to shift by a half- to a full zone about every 30 years. By the end of the century, plants now associated with the Southeast are likely to become established throughout the Midwest.

- "Higher temperatures will mean a longer growing season for crops that do well in the heat, such as melon, okra and sweet potato, but a shorter growing season for crops more suited to cooler conditions, such as potato, lettuce, broccoli and spinach."

- Fruits that require long winter chilling periods, such as apples, will experience declines.

- "Higher temperatures also cause plants to use more water to keep cool . . . . But fruits, vegetables and grains can suffer even under well-watered conditions if temperatures exceed the maximum level for pollen viability in a particular plant; if temperatures exceed the threshold for that plant, it won't produce seed and so it won't reproduce."

- Climate change is expected to result in less frequent but more intense rainfall. One consequence is expected to be delayed spring planting. In the Midwest, heavy downpours are now twice as frequent as they were a century ago.

In the Great Plains, most water comes from the High Plains aquifer. Water withdrawals outpace natural recharge. Increasing temperatures, faster evaporation rates and more sustained droughts will stress the water resource further.

ON THE WEB

"Global Climate Change Impacts in the United States" report

A guide for all ages by U.S. scientists, "Climate Literacy: The Essential Principles of Climate Science"