Methyl bromide ozone threat persists

Methyl bromide has been re-confirmed as a significant ozone depleting compound by the 1994 UNEP Scientific Assessment of ozone depletion(1). In May 1995, several governments formally proposed dates for reducing use and phasing-out the fumigant the preparatory meeting of the Montreal Protocol, an agreement to protect the ozone layer. Melanie Miller reports on developments.

UNEP's Scientific Assessment was written and reviewed by many of the world's leading experts on ozone issues. It builds on UNEP's 1992 report on methyl bromide(2), and is the most authoritative review available to date. The Assessment states that action on methyl bromide would have a rapid impact on the extent of ozone loss. It concludes that eliminating methyl bromide emissions from agriculture and industry by the year 2001 represents the greatest single step that governments can take to further protect the ozone layer.
    Methyl bromide's potency (Ozone Depletion Potential or ODP) is likely to be between 30% and 80% of the potency of CFC-11, the benchmark substance. The Assessment took account of remaining uncertainties in the data, and concluded it was improbable that methyl bromide's ODP would be lower than 0.3 or higher than 0.8 (compared to 1.0 for CFC-11 when measured over hundreds of years). If the improbable occurred and its ODP were reduced to 0.2, it would still be higher than methyl chloroform which has an ODP of 0.1 and will be phased out in 1996. HCFCs have ODPs of 0.01 to 0.04.
    Methyl bromide's actual lifetime in the atmosphere is very brief, so calculating the ODP over a shorter time provides a more realistic indicator of ozone damage. Its lifetime ODP is calculated to be more than 12.0, which means its potency is at least 12 times greater than CFC-11. Atom for atom, bromine from methyl bromide is estimated to be about 50 times more destructive to ozone than chlorine from CFCs.
    On the basis of current estimates, about 44% of methyl bromide may come from human sources. The percentage may be revised upwards in future because new research suggests the oceans contribute a smaller net quantity than previously thought. Scientists believe the oceans have emitted methyl bromide and methyl chloride for centuries, and are part of a balanced system of natural ozone break-up and replenishment. The additional bromine and chlorine from human sources create problems for the ozone layer-they break up ozone faster than it can be replenished, disrupting the natural balance. In effect, this means precise quantification of natural emissions is not necessary; scientists have sufficient data to conclude that the extra human sources need to be controlled.
    Another detailed UNEP report has identified technically feasible alternatives, either currently available or at an advanced stage of development, for about 90% of methyl bromide use(3). When controls were agreed for CFCs and halons in 1987 few potential alternatives had been identified, so methyl bromide users are in a significantly better position at this stage. The Montreal Protocol's Assessment Panel points out that users and producers of methyl bromide have been reluctant to commercialise and implement alternatives because they believe methyl bromide uses are more important and more difficult to eliminate than CFCs and halons. While the users of methyl bromide face substantial challenges, the Panel says they are not more difficult than the problems already overcome by users of CFCs and halons (4). The Panel believes controlling methyl bromide would be more cost-effective than further controls on other ozone depleting chemicals.
    The Netherlands has taken the lead in the Montreal Protocol by proposing a 50% cut in use by 1998 and phase out by 2001 in industrialised countries. The US, the world's major manufacturer and user pressed for global phase out by 2001. Other major users called for a 50% cut: Japan by 2001 and Italy by 2005. Some countries called for extra time for developing countries to comply. A number of countries want quarantine and pre-shipment uses to remain exempt from controls, while others proposed some limits in future. A decision will be taken by governments in December.

Melanie Miller is an environmental policy analyst, PO Box 665, Napier, New Zealand, Tel/fax +646 835 3501 Email  m-miller@ramhb.co.nz.

References
1. World Meteorological Organisation 'Scientific Assessment of Ozone Depletion' WMO report No. 37, UNEP, 1994. Available from WMO, GO3OS, PO box 2300, 1211-Geneva, Switzerland.
2. UNEP 'Methyl Bromide: Its Atmospheric Science, Technology and Economics' Montreal Protocol Assessment Supplement, UNEP, Nairobi, 1992.
3. MBTOC 'Report of the Methyl Bromide Technical Options Committee' UNEP, 1994. Available from: The Librarian, Stored Grain Research Lab, GPO box 1700, Canberra, ACT 2601, Australia. Fax  +616 246 4202. US$27 plus $11 freight.
4. Technology and Economic Assessment Panel, 'Supplement to the 1994 Assessments' Montreal Protocol Assessment, UNEP, 1995. Available from: Ozone Secretariat, UNEP, PO Box 30552, Nairobi, Kenya.

[This article first appeared in Pesticides News No. 28, June 1995, page 4]