Plants have long been seen as organisms that help to rid the globe of harmful carbon dioxide and, in turn, reduce the effects of cars, trucks and industrial plants. But all may not be quite like that.

Scientists at Germany's Max-Planck-Institute in Heidelberg, Germany, have found out that plants produce methane, considered one of the earth's major greenhouse gases, which lock the sun's energy inside the earth's atmosphere.

"We have made the discovery that plants under normal environmental conditions ... produce methane and emit it into the atmosphere," said Frank Keppler, who directed the research team's laboratory studies. "We have done rough calculations on a global scale and have estimated that it could be a really major source of atmospheric methane."

It has been widely held that methane gas is produced primarily in areas of low oxygen levels such as swamps and rice paddies where bacteria act upon vegetable matter. Farm animals like cows and sheep release gas as part of their digestive processes.

A threat to carbon trading?