The day’s news surely confirmed that prejudice. The second day of this year’s annual meeting was devoted to a symposium on weeds and global climate change, and the speakers were outlining a future in which many of the members’ current strategies will be irrelevant or ineffective.

The keynote speaker, Cameron Wake of the University of New Hampshire’s Climate Change Research Center, did little to put the audience at ease. Wake is a charismatic man who has traveled the colder regions of the world  the Canadian arctic, the Greenland ice sheet, Antarctica and the high mountains of Central Asia. On these trips, he collects ice cores, whose analysis enables him to reconstruct histories of past atmospheric and climatic changes. His soul patch, pink shirt and pink tie made him a minority of one in this room. He dealt firmly with an audience member who asserted that the climatic warming is nothing new, that records from imperial Rome indicate that citrus and other warm-weather crops were then far to the north of their current ranges. Wake pointed out that local archaeology can’t change the global data set, which proves that the level of CO2 in the atmosphere is at its highest point in more than 650,000 years and that the rate of increase is accelerating.

Subsequent speakers got down to cases. Andrew McDonald, an agricultural scientist at Cornell University, had used the Intergovernmental Panel on Climate Change’s high projections for CO2 levels at the middle and end of the century to create an atlas of potential weed migrations in cornfields in the Eastern United States. If these projections prove accurate, Kentucky, by the end of the next one to three decades, should have a climate (and weed flora) resembling that of present-day North Carolina; by century’s end, it will have shifted to a regime more like that of Louisiana. Delaware, over the same period, will be transformed to something first like North Carolina and then Georgia, while Pennsylvania will metamorphose into West Virginia and then North Carolina. Florida will become something unprecedented in this country. Field observations indicate that these transformations are already under way: another speaker pointed out that kudzu, “the weed that ate the South,” has already migrated up to central Illinois and by 2015 could be extending its tendrils into Michigan’s Upper Peninsula.

Even more sobering were the figures that the biologist Brent Helliker of the University of Pennsylvania flashed on the screen. First, he used maps taken from an ecology textbook to show the way the last ice age drove various forest types southward. Then came a map Helliker created, suggesting that the current warming seems most likely to change the ranges to which forest trees are adapted  the areas where the black spruce, for example, grows now, are likely to become better suited to broadleaf trees. He asked the question that was on the lips of every one of his listeners: Can the forest adapt so drastically in a space of just decades? Helliker announced that he had no answer to that question, and his talk was over.

During a break between talks, Lewis Ziska was surprisingly upbeat. With the challenges, he insisted, come opportunities. Kudzu, for instance: Ziska has been seeking financing to study its potential as a source of biofuel. Kudzu roots, as much as 50 percent starch by weight, seem ideal for ethanol production, while the plant’s supercharged vines, which can grow a foot a day, would be an abundant source of alternative energy. This would be win-win: we develop an alternative to fossil fuels and, at the same time, create a financial incentive to root out a particularly troublesome weed.

Developing techniques for managing weeds in a time of global climate change will be essential to the world’s agricultural future, and the U.S.D.A. researchers, though they have been starved of essential financing, lead the world in this field. (There is one exception, Ziska admits; his Web searches have revealed that marijuana growers have an amazingly detailed knowledge of how CO2 enrichment affects their crop. But as Ziska points out, they don’t publish in scientific journals.) Possession of this expertise could be a great economic asset to the United States, both for the protection it could provide to our own harvests and as an intellectual export that is sure to be much in demand in other countries.

Ziska says that he worries about mankind’s ability to feed itself in a fast-changing future. Paradoxically, it is weeds, he says, that can provide solutions. They have helped us deal with lesser crises in the past. When diseases and pests overwhelmed our domesticated food crops, it was to their wild relatives  plants that mankind has been battling for millennia  that plant breeders turned. Because weeds have more diverse genomes, it is easier to find one with the proper genetic resistance to a given threat  and then to create a new hybrid by breeding it with existing crops. An answer to the Irish potato blight of 1845-6 was eventually found among the potato’s wild and weedy relatives; a wild oat found in Israel in the 1960s helped spawn a more robust, disease-resistant strain of domesticated oats.

Weedy ancestors of our food crops, Ziska predicts, will cope far better with coming climatic changes than their domesticated descendants. Coping, after all, is what weeds have always done best. As last year’s climate- change panel report, Climate Change 2007, made clear, we have already set in motion far-reaching and unstoppable changes in regional temperatures and precipitation and in the composition of our atmosphere. No matter what actions we take, these changes will continue for decades. If we are to avoid disaster, experts agree, we will need to be tenacious but flexible, ready to identify and exploit any opportunity in what will be a challenging, even hostile situation. In this new world that we have made, weeds, our old adversaries, could be not only tools but mentors. At which point, if Ralph Waldo Emerson is to be believed, weeds by definition will cease to exist.