Man-made climate change has been a global concern for several years, but as industrial emissions of some greenhouse and ozone-depleting gases drop, scientists are finding new sources to worry about. Among them is rice, the worlds most important wetland food crop. At first glance, rice production--a practice that is several thousand years old--might seem environmentally sound. But the world population is growing fast, and the rice fields needed to feed it emit so-called hydrocarbons, which are partly responsible for global warming and expanding the ozone hole. According to a new study in today's Science, rice cultivation may begin to take a serious toll on the worlds climate.

The study, led by Ralph J. Cicerone and Kelly R. Redeker of the University of California, Irvine, examines the emissions of hydrocarbons in California rice paddies over several seasons. In particular, they monitored methane and methyl halides, including methyl iodide, methyl bromide and methyl chloride. While it has long been known that rice fields emit significant amounts of methane, this study is the first to look at methyl halide emissions there. Although all of the substances the team studied work as catalysts--facilitating the breakdown of ozone into oxygen--the methyl halides are most effective, meaning that even in comparatively small quantities, they pose a significant threat to the ozone layer. Indeed, their increased production as the result of human activity, along with increased emissions of other ozone-depleting substances (ODS), is tipping the balance: ozone molecules in the stratosphere are being broken up faster than they are formed.

What the California researchers found was that the methyl halides apparently form in different ways, independent from methane production. (Methane forms as a by-product of anaerobic bacterial decomposition of organic matter in the soil and reaches the atmosphere through the roots and stems of the rice plants.) Like methane emissions, methyl bromide and methyl iodide are affected by growth stages, the organic content of the soil and flooding events. It is unclear, though, whether the plants or the soil are the source. In contrast, methyl chloride emission levels didnt fluctuate, leading the researchers to believe that the paddy environment--and not necessarily the rice--released the gases. Based on the data, the scientists conclude that "worldwide rice production is responsible for [about] 1 percent of atmospheric methyl bromide and 4 percent of atmospheric methyl iodide" and that "methyl iodide emissions from rice paddies provide a sizable terrestrial source to the global budget."