Atmospheric Impact of Agricultural Use of MeBr

To protect the earth from the detrimental effects of ozone depletion, an international treaty—The Montreal Protocol—was developed in the late 1980s. Since that time, it has been controlling the production and trade of ozone-depleting substances on a global basis and has been signed by more than 160 nations. The treaty phases out chlorofluorocarbons and other ozone-depleting compounds, including methyl bromide. In 1995, Montreal Protocol signatory countries agreed to freeze production of methyl bromide at 1991 levels for developed countries. Total phaseout for developed countries will occur January 1, 2005, except for quarantine, critical, and emergency exemptions. Developing countries will be allowed to use methyl bromide for several years past the developed-countries phaseout date.

Methyl bromide was designated an ozone-depleting substance in 1992, with an estimated ozone-depleting potential (ODP) of 0.7. The ODP is the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11, which is designated by the Montreal Protocol to have an ODP of 1.0. ODPs for ozone-depleting substances range from 0.01 up to 10.0. For example, carbon tetrachloride and methyl chloroform have ODPs of 1.2 and 0.11, respectively. In 1998, after further study, the Montreal Protocol reduced its ODP estimate of methyl bromide to 0.4.

While research to find alternatives to methyl bromide has continued, other scientists are examining the contribution the fumigant makes in depleting the ozone layer. Methyl bromide also is liberated into the atmosphere from natural sources, and efforts are being made to determine the ratio of atmospheric methyl bromide from natural and anthropogenic (man-made) sources, specifically uses related to agriculture. This will allow a better estimate of ozone layer improvement to be expected after the phasing out of anthropogenic sources.

Of Sources and Sinks

Examination of the intricate balance between where methyl bromide comes from, where it goes, and what happens to it in the troposphere is needed, but this cycle gets complicated. "The problem with methyl bromide is that it is not entirely man-made," says James Butler, who is with the National Oceanic and Atmospheric Administration's (NOAA) Climate Monitoring and Diagnostics Laboratory. Natural sources include oceans, biomass burning, wetlands, crops, and forests.

The ocean is the largest source of methyl bromide (emitting about 56 Gg y^-1) and the second largest sink (taking in about 77 Gg y^-1). "Gg y^-1" is gigagram per year, equivalent to 1,000 metric tons per year. According to research by Shari Yvon-Lewis of the NOAA Atlantic Oceanographic and Meteorological Laboratory, the ocean actually acts as a net sink, with a possible measurement range from –3,000 to –32,000 metric tons per year, meaning it takes more methyl bromide from the atmosphere than it emits to the atmosphere. However, there are many factors that influence the amount of methyl bromide emitted and absorbed by the Earth's oceans. The circulation patterns of the ocean, amount of precipitation, and water temperature all affect the delicate balance. By comparison, the contribution to the atmosphere by fumigation use of methyl bromide is reported to be about 60,000 metric tons per year, with 26,000 metric tons per year coming from soil fumigation.

The magnitude of sinks is not completely understood. "There are four ways to lose methyl bromide: removal by oceans, destruction in the stratosphere by ultraviolet radiation and OH reactions, removal by soils, and by plants," says Butler. Yvon-Lewis estimates that oceans take in 77,000 metric tons per year, as mentioned before, OH reactions and ultraviolet radiation destroy about 86,000 metric tons per year, and soils take up almost 47,000 metric tons per year. Scientists have as yet not been able to balance the equation consisting of known sources, known sinks, and measured methyl bromide in the atmosphere. "The reason the budget as we calculate it is imbalanced is because we have identified stronger sinks than we have sources. It is quite possible that we have not identified all the sources." A big unknown at this point is how much methyl bromide plants produce and how much they take up and destroy.

There are some theories about the unknown sources, namely plants. "Plants appear to produce and take in methyl bromide. There are some indications that salt marshes and other plants in the biosphere contribute to the budget," says Butler. Some research indicates that the global emission rate of the rapeseed plant is 7,000 metric tons per year. Wetlands contribute an additional 4,500 metric tons per year to the total global emission budget. Research investigating plants as sources and sinks of methyl bromide continues.

How Bad Is Methyl Bromide?

While the lowered ODP rate may seem encouraging, methyl bromide does more damage in the ozone layer than most other ODP substances, partly because of the high mixing rate in the atmosphere. It also escapes easily into the atmosphere where it contributes to the depletion of the ozone layer. Bill Thomas, of the U.S. Environmental Protection Agency, states, "While methyl bromide's ODP has fallen, it is unlikely it will fall below the 0.2 threshold. But if it did, it would be reclassified as a Class II substance, only altering the phaseout time line—not eliminating it." The present best guess is that emissions of methyl bromide from agricultural uses account for 20 to 30 percent of global methyl bromide sources and are thought to be responsible for 3 to 10 percent of the stratospheric ozone depletion, according to NOAA researchers.

Last Updated: September 5, 2001