Executive Summary, Scientific Assessment of Ozone Depletion: 2002 31 July 2002Released by WMO/UNEP on 23 August 2002EXECUTIVE SUMMARYFINALUNEP/WMO"SCIENTIFIC ASSESSMENT OF OZONEDEPLETION: 2002"PREPARED BY THESCIENTIFIC ASSESSMENT PANELOF THEMONTREAL PROTOCOL ON SUBSTANCESTHAT DEPLETE THE OZONE LAYERExecutive Summary, Scientific Assessment of Ozone Depletion: 2002 31 July 20021Executive SummaryThe provisions of the 1987 Montreal Protocol on Substances that Deplete the OzoneLayer include the requirement that the Parties to the Protocol base their future decisions on thecurrent scientific, environmental, technical, and economic information that is assessed throughpanels drawn from the worldwide expert communities. To provide that input to the decision-making process, advances in understanding on these topics were assessed in 1989, 1991, 1994,and 1998. This information helped support discussions among the Parties that led to thesubsequent Amendments and Adjustments of the 1987 Protocol. The 2002 ScientificAssessment summarized here is the fifth in that series.Recent Major Findings and Current Scientific UnderstandingSince the Scientific Assessment of Ozone Depletion: 1998, numerous laboratoryinvestigations, atmospheric observations, and theoretical and modeling studies have producednew key findings and have strengthened overall understanding of the ozone layer and its effecton ultraviolet (UV) radiation. These advances are highlighted in the following summary of thecurrent understanding of the impact of human activities and natural phenomena on the ozonelayer and the coupling of the ozone layer and the climate system.Changes in ozone-depleting compounds• In the troposphere (i.e., lower atmosphere), observations show that the total combinedeffective abundance of ozone-depleting compounds continues to decline slowly from thepeak that occurred in 1992-1994. Total chlorine is declining, while bromine fromindustrial halons is still increasing, albeit at a slower rate than was occurring previously(and as reported in the 1998 Assessment). Total tropospheric chlorine from the long- andshort-lived chlorocarbons was about 5% lower in 2000 than that observed at its peak in 1992-1994, and the rate of change in 2000 was about −22 parts per trillion per year (−0.6% peryear). The once-dominant influence of methyl chloroform (CH3CCl3) on this total decline isdiminishing because the atmospheric abundance of methyl chloroform is sharply decreasing.Total chlorine from the major chlorofluorocarbons (CFCs) is no longer increasing, in contrastto the slight increase that was occurring at the time of the 1998 Assessment. Specifically, in2000, the atmospheric abundances of CFC-11 and CFC-113 continue to decrease, while therate of increase of CFC-12 has slowed. Total tropospheric bromine from halons continues toincrease at about 3% per year, which is about two-thirds of the rate for 1996 reported in the1998 Assessment. The observed abundances of CFCs, hydrochlorofluorocarbons (HCFCs),and methyl chloroform in the lower atmosphere continue to be consistent with reportedproduction and estimated emissions.Executive Summary, Scientific Assessment of Ozone Depletion: 2002 31 July 20022• Analyses of air trapped in snow since the late 19th century have confirmed that non-industrial sources of the CFCs, halons, and major chlorocarbons were insignificant.Since the last assessment, analyses of "firn air" (i.e., air trapped in snow above glaciers) haverevealed the abundance of long-lived atmospheric species at the time the air became trapped.As a result, trends in the atmospheric abundance for many ozone-depleting substances havebeen traced over the past century, to well before significant industrial sources of thecompounds existed. These records show that the mixing ratios of the CFCs, halons, carbontetrachloride (CCl4), methyl chloroform, and HCFCs in the oldest air sampled are negligiblecompared to the amounts measured in today's background atmosphere. Further, the deduced20th century records for these compounds are broadly consistent with calculated historiesbased on records of industrial production. The data suggest that substantial natural sourcesexist for atmospheric methyl bromide. They also show increases throughout the 20th century,but these increases do not allow unambiguous quantification of the industrial fraction ofmethyl bromide emissions in recent years. The estimate of this fraction, based on anassessment of understanding of the budget of this gas, remains at 10-40%, as given in the1998 Assessment.• The abundances of HCFCs in the lower atmosphere continue to increase. HCFCs areamong the gases used as transition substitutes for CFCs, halons, and chlorinated solvents. Inthe year 2000, HCFCs represented 6% of total chlorine abundance from anthropogenic gasesin the lower atmosphere. The rate of increase in chlorine from HCFCs was constant at 10parts per trillion per year from 1996 to 2000.• Observations in the stratosphere indicate that the total chlorine abundance is at or neara peak, while bromine abundances are probably still increasing. The sum of hydrogenchloride (HCl) and chlorine nitrate (ClONO2) is an effective surrogate for the abundance ofstratospheric chlorine. An extended time series of ground-based measurements shows thatthe total stratospheric column amounts of these species, which have grown steadily fordecades, have plateaued in recent years. Further, space-based measurements of HCl in theupper stratosphere indicate a broadly similar behavior. There are indications that bromineabundances in the stratosphere increased during the 1990s, but changes in stratosphericbromine are not as well characterized as those of stratospheric chlorine. These stratosphericchanges are consistent with expectations based on the understanding of trace-gas trends inthe troposphere, stratospheric chemistry, and atmospheric transport from the troposphere tothe stratosphere.• Very short-lived organic chlorine-, bromine-, and iodine-containing source gases havethe potential to deplete stratospheric ozone, but quantitative estimation of theirpotentials is more challenging than for longer-lived species like CFCs. The very short-lived compounds reside in the atmosphere for a few months or less because they are rapidlydecomposed chemically in the troposphere. Yet, a fraction of their emissions and theproducts from their tropospheric destruction can potentially reach the stratosphere.