"This is bad news for those of us who suffer from poison ivy," Dr. Jacqueline E. Mohan of The Ecosystems Center at the Marine Biological Laboratory in Woods Hole, Massachusetts, the study's lead author, told Reuters Health.

Mohan and her team studied plots of a forested area surrounded by PVC pipes that pumped out carbon dioxide, allowing them evaluate the effects of greenhouse gas in a real-life forest environment. They compared poison ivy growth over a six-year period in three carbon dioxide-enriched areas and three areas with normal air.

The average carbon dioxide levels are now about 370 micrograms per liter. Carbon dioxide levels were increased to 570 micrograms per liter, the level that will be reached by mid-21st century if global warming is not reduced.

Under the high carbon dioxide conditions, the poison ivy plants grew 150 percent faster every year than the control plants. In addition, the plants contained 153 percent more of the urushiol compound that causes an allergic reaction.

Over the past two decades, Mohan noted, scientists have observed increased worldwide growth in vines, which is in some cases choking out the regrowth of trees. Vines benefit from extra carbon dioxide, she explained, because the gas fuels photosynthesis. Unlike trees, vines have to devote relatively little energy to growing wood, and can instead pump the extra photosynthesis energy into leaf production.

Even if people stopped producing greenhouse gases today, which is highly unlikely, the levels of carbon dioxide are expected to continue to rise for a few decades, Mohan added. "This is pretty much what we can expect to be at in 50 to 60 years."

"I'm telling people, every time you fill up the gas tank, think of poison ivy."