URGENT PHASE-OUT OF RAPID OZONE DESTROYERS NEEDED

The relatively short lifetimes of HCFCs and methyl bromide, mean that substances released in the early to mid-nineties will inflict their damage upon the ozone layer between now and 2015. ²

The World Meteorological Organization's (WMO)1991 Scientific Assessment noted that: "steady-state [long term ] ODPs represent the best current understanding of the long-term impacts of such [ODS] releases, but they do not describe short-term (decades or less) responses...³ When specific short-term time horizons are considered, time-dependent ODPs provide a more appropriate measure of impacts."⁴

Table 1: Short-term and medium-term ODPs for HCFC-22, HCFC-141b and methyl bromide (best estimates) ⁵
ODS	ODP at 1 year	ODP at 2 year	ODP at 5 year	ODP at 10 year	ODP at 20 year	ODP at 30 year	ODP at 50 year
CFC-11	1.0	1.0	1.0	1.0	1.0	1.0	1.0
HCFC-141b	>0.5	0.5	>0.4	>0.3	>0.2	>0.2	>0.1
HCFC-22	>0.1	>0.1	>0.1	>0.1	>0.1	>0.08	>0.07
methyl bromide	>10.0	>10.0	>7.0	>4.0	>2.0	>1.0	>1.0
Ozone Depletion Potentials (1-50 years after substances reach the stratosphere)

The 1994 WMO Scientific Assessment noted that time-dependent ODPs can be used to provide insight on "the short-term future of the ozone layer, (e.g. the next few decades, when peak chlorine and bromine loading are expected to occur)..."⁶ "Since the ozone layer is believed to respond relatively rapidly to changes in chlorine and/or bromine loading (time scale of about 3 - 5 years or less), time-dependent Ozone Depletion Potentials provide an appropriate measure of the expected ozone response to changing inputs of source gases relative to the reference molecule."⁷

Looking at a 500-year period HCFCs, for example, have ODPs of 0.01-0.1.⁸ They appear to have only 1% - 10% of the destructive capability of CFCs, or stated another way, "to be 90-99% better for the ozone layer than CFCs". However, as the WMO Scientific Assessment in 1991 noted: "The HCFCs have relatively short lifetimes, and generally display small ODPs over long time horizons, but exhibit ODPs over short time scales that can be as much as 3 to 10 times greater than their steady-state ODPs...⁹[and] While the ODPs for some short-lived compounds, including the HCFCs, may suggest only a modest impact on the chronic response by ozone, the acute response over the next several years will be more substantial."¹⁰

It is therefore prudent to focus on ODPs measured over the period of time when depletion will be worst: i.e.. the next 15 - 20 years. For example, the short term ozone depleting potential HCFC-141b is 40 to 50 percent that of CFC-11 for the first five years after reaching the atmosphere. Consequently, one kilo of HCFC-141b reaching the stratosphere this month will have destroyed half as much ozone as one kilo of CFC-11, the substance it is to replace, when measured one year from now.

Likewise, short and medium term ODPs reveal methyl bromide's high destructive capability. For two years after methyl bromide has reached the stratosphere, methyl bromide's ODP will remain greater than 10.0, i.e.. over ten times more damaging than an equal amount of CFC-11. After 20 years, methyl bromide's ODP will still be greater than the ODP of CFC-11, using most conservative estimates. Methyl bromide has a short lifetime of less than one year, so most of its damage is inflicted quickly.

The next 20 years are crucial. Given the alarming state of the ozone layer it is essential that in addition to early phase out dates, the Parties agree to very substantial and immediate reductions in consumption of rapid ozone destroyers (methyl bromide and HCFCs) at this meeting. By making rapid cuts in rapid ozone destroyers we can reduce the period of high UV exposure.

The significance of these Rapid Ozone Destroyers is amplified by the very real possibility that a significant number (perhaps a majority) of Article 5 countries will not be meeting the 1999 CFC freeze, which could then result in a net increase from current levels in global ODS consumption.

² 2 Methyl Bromide's total lifetime is approximately one year (new estimates are less than one year). Total lifetime for HCFC-141b is 9.4 years, while other HCFCs are 1.4-19.5 yr. In contrast, CFC-11 is about 50 yr . [Source: WMO 1994, p.13.6, Table 13-1.] [return]

³ WMO, 1990; SORG 1990 [return]

⁴ WMO 1991, page 6.4 [return]

⁵ Source: Derived from WMO Scientific Assesment, 1994, p.13.19, Figure 13-6 which presents time-dependent best estimate ODPs. [return]

⁶ WMO 1994, p.13.18 [return]

⁷ WMO 1994, p.13.18 [return]

⁸ WMO 1994 p.13.17 [return]

⁹ WMO 1991, p.6.17 [return]

¹⁰ WMO 1991, p.6.17 [return]