There's yet another downside to our warming climate: bigger and badder poison ivy.
When researchers adjusted carbon dioxide levels to match those anticipated during the next 50 years, poison ivy in an experimental pine forest grew much faster and was far more toxic than the plant that annoys us today.
In fact, the experimental plants grew twice as fast as normal poison ivy, and their leaves contained a more allergenic form of urushiol, the carbon-based compound that causes contact dermatitis, scientists said.
The findings, published this week in Proceedings of the National Academy of Sciences, apply to poison ivy - and possibly other vinelike plants that threaten larger trees. They are likely to become more abundant in the years ahead as cars and factories pump out more carbon dioxide, and levels of the greenhouse gas continue to rise.
Carbon dioxide levels have increased 21 percent since 1960, the researchers say.
The scientists from Duke and Harvard universities and the Henry A. Wallace Beltsville Agricultural Research Center focused on poison ivy because it's native to North America, found throughout the world and causes skin rashes for 350,000 people each year in the United States, according to lead author Jacqueline Mohan.
She began the project at Duke and is now an ecologist with the Ecosystems Center at the Marine Biological Laboratory in Woods Hole, Mass.
Also, previous studies showed that vinelike plants such as poison ivy, Japanese honeysuckle and kudzu will thrive in elevated levels of carbon dioxide.
"I've been concerned about the increasing abundance of these woody vines because of their effect on forest growth. They strangle the big trees and shade out the little ones," Mohan said.
Plants need four sources of energy to grow: sunlight, nutrients, water and carbon dioxide, experts say. Many plants thrive when carbon dioxide levels are elevated, said Lewis Ziska, a co-author who studies the effect of greenhouse gases on plants at the Beltsville Agricultural Research Center.
"We know there's positive effect on plant life, but not every plant species is going to respond the same way," Ziska said.
For the study, the researchers grew poison ivy for six years in the Duke Forest, a patch of woods near Chapel Hill, N.C., where vegetation grows in natural sunlight and soils under a variety of test conditions.
They fumigated six circular test plots, each about 30 yards in diameter, with carbon dioxide to raise levels in the air to the 550 parts per million. That's the concentration the Intergovernmental Panel on Climate Change expects us to reach by about 2050, Mohan said.
Current carbon dioxide levels vary, depending on location, but in Baltimore it's about 420 parts per million, Ziska said.
The scientists found that over six years, poison ivy exposed to elevated carbon dioxide levels grew from three grams to eight grams. A control sample grew from three grams to about 4.5 grams.
Experts say the findings further illustrate that climate change could have major effects on a variety of plants and on the insects and animals that depend on them.
"What they've done is confirm that CO2 is not only going to change the growth rate and overall size of many plants, but that it's going to change plant chemistry and have an impact on human health," said Evan DeLucia, an ecologist at the University of Illinois who was not involved in the study.
The study also raises questions about the effects of elevated carbon dioxide levels on the Earth's other 250 million species of plants, Ziska said. Many weeds, crops and garden plants will thrive as carbon dioxide levels rise in the years ahead, Ziska said. His studies have shown that ragweed, a common allergen, thrives when temperatures and carbon dioxide levels are elevated.
But scientists need to know more about the overall effects of higher CO2 levels and find ways to use it as a kind of airborne fertilizer, he said.
"We have to ask ourselves, what is it that's causing some varieties of plants to be such dramatic responders to rising CO2?" he said.