In the longest-running study of its kind, researchers in Maryland and Florida are trying to piece together how plants will cope in 50 years - when the air is thicker with the products of our lifestyle.
Scientists with the Smithsonian Environmental Research Center have spent 17 years tracking how higher levels of carbon dioxide - a "greenhouse" gas produced by cars, factories and power plants - are affecting vegetation.
"We're measuring as many of the effects of carbon dioxide on an ecosystem as possible," said Bert Drake, a plant pathologist at SERC.
Drake has been monitoring the effects of carbon dioxide on plants since the 1980s and is considered a pioneer in the field. "He really was one of the first scientists to spot the significance of rising CO2 levels," said Bruce Hungate, a professor of environmental ecology at Northern Arizona University.
Drake's work, funded largely by the Department of Energy, focuses on two plants - spartina grass and sedges - grown in a marsh on a 10-acre site near the Rhode River in Edgewater and encased in 30 polyester chambers. Carbon dioxide is pumped into the chambers, doubling the concentration of the gas the plants are exposed to.
Monitors record a range of data about each plant's life cycle, including the amount of nitrogen and methane given off and the growth rates of roots and branches. Scientists track their progress from a cabin constructed near the marsh.
"It's not a question of whether plants will respond to carbon dioxide. We know they do. The question is how do they respond to these other factors - things like rainfall, the availability of nitrogen, salinity," Drake said.
So he and other scientists are trying to figure out which plants will thrive and which will disappear if the world's appetite for fossil fuels continues unabated.
Although politicians argue about the effect, scientists agree that increases in greenhouse gases are warming the planet. And one gas in particular - carbon dioxide - has been rising steadily since the Industrial Revolution began in the 19th century. Carbon dioxide levels are now at 400 parts per million, a huge increase in the past 100 years, experts say.
The rate of future increases will depend on whether alternative power sources are developed, such as hydrogen-powered vehicles. But scientists expect carbon dioxide levels to nearly double by 2100, and an understanding of the effects of that increase remains elusive.
For example, studies over the past decade have shown that rising carbon dioxide produces increased vegetation, which in turn might absorb much of the carbon dioxide being pumped out by the world's cars and industries.
But a study published last month in the journal Science - based on field work at the Smithsonian's satellite facility in Florida - found that increased carbon dioxide levels actually crippled the growth rates of some plants raised in the sandy soils of Coastal Florida.
In that study, researchers pumped carbon dioxide onto common white milkpea plants raised in eight enclosed chambers on a tract at the Smithsonian's satellite site at the Kennedy Space Center.
Over seven years, higher carbon dioxide levels hindered the legume's ability to grow and to process nitrogen, an essential process in the plant's development.
The milkpea was probably affected because increased carbon dioxide levels cut in half the plant's supply of molybdenum, a key nutrient in the plant, said Hungate, the study's lead author.
Increasing levels of carbon dioxide could have the same effects on plants in similar habitats because the plants in his experiment were exposed to carbon dioxide levels that will be common when levels double over the next century.
"What we're doing is simulating the atmosphere that we're likely to see in 50 or 100 years," Hungate said.
The results surprised scientists because higher levels of carbon dioxide mean more fuel for plants and previous studies had showed increased growth rates.
Some experts say the results show the earth's vegetation may not be able to absorb as much carbon dioxide as expected, making increased levels of the gas more of a threat than previously thought.
"It's really a novel result, and a significant one," said David Schimel, a scientist with the National Center for Atmospheric Research.
But Drake, a co-author of the study, is more cautious about the implications. He views the results as a small piece of a complicated puzzle.
"Just because one plant under certain conditions won't respond to carbon dioxide, doesn't mean that other plants won't," he said.
Drake's studies have shown that increased carbon dioxide make sedges more resistant to the stress produced by droughts. Another study two years ago showed that increased carbon dioxide, along with warmer temperatures, spurred growth of ragweed at three test sites in Maryland.
At the University of Illinois, researchers found higher carbon dioxide levels increased growth rates for corn and soybeans, but they also increased pest infestations.
"We think the pest problems will become bigger as we go through the rest of the century," said Stephen P. Long, a plant biologist at the University of Illinois.
