Scientific Assessment of Ozone Depletion: 2006

Executive Summary

2006 Ozone Assessment Executive Summary cover
Executive Summary

The provisions of the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer include the requirement that the Parties to the Protocol base their future decisions on the updated scientific, environmental, technical, and economic information that is assessed through Panels drawn from the worldwide expert communities. To provide that input to the decision-making process, advances in scientific understanding were assessed by the Scientific Assessment Panel in 1989, 1991, 1994, 1998, and 2002. This information helped support discussions among the Parties that led to the subsequent Amendments and Adjustments of the 1987 Protocol. The 2006 Scientific Assessment summarized here is the sixth in that series. The information contained in this Executive Summary is based on the eight detailed chapters of the full report.

The previous Assessment presented evidence that the tropospheric abundances of most ozone-depleting substances, as well as of stratospheric chlorine, were stable or decreasing due to actions taken under the Montreal Protocol (see schematic Figure 1a, b), with the stratospheric abundances showing a time lag due to the time for surface emissions to reach the stratosphere. Based on these facts, it was stated that "The Montreal Protocol is working, and the ozone-layer depletion from the Protocol's controlled substances is expected to begin to ameliorate within the next decade or so."


An important next step is to ask whether stratospheric ozone and surface ultraviolet (UV) radiation are responding as expected to the controls on ozone-depleting substances imposed by the Protocol (see schematic Figure 1c, d). In addressing this question, it is necessary to consider factors other than ozone-depleting substances that also influence ozone and UV radiation. These factors include natural dynamical variability, volcanic eruptions, solar variations, aerosols (airborne fine particles), and climate change.

Pre-1980 values are often used as a benchmark for ozone and UV recovery. However, because of the abovementioned factors, if and when ozone and UV radiation return to their pre-1980 values would not be associated solely with the return of ozone-depleting substances to their pre-1980 values (see schematic Figure 1c, d).

We address the behavior of ozone-depleting substances, the response of stratospheric ozone and UV radiation to ozone-depleting substances and other factors to date, and the future evolution of ozone and UV radiation. We first present the main findings, then the detailed supporting evidence, and finally the implications for policy formulation.

Recent Major Findings and Current Scientific Understanding

Since the Scientific Assessment of Ozone Depletion: 2002, numerous laboratory investigations, atmospheric observations, and theoretical and modeling studies have produced new key findings and have strengthened the overall understanding of the ozone layer and its effect on UV radiation. These advances are highlighted in the following summary of the current understanding of the impact of human activities and natural phenomena on the ozone layer, as well as the coupling between climate change and stratospheric ozone depletion.

Ozone-Depleting Substances

The previous Assessment noted that the tropospheric abundances of ozone-depleting gases were declining, whereas the stratospheric abundances were at or near their peak. Now we ask: Are the tropospheric and stratospheric abundances of ozone-depleting gases showing further changes that can be linked to policy actions, and how are they expected to evolve in the future?

Ozone, Ultraviolet Radiation, and Other Related Changes to Date

The previous Assessment noted that stratospheric ozone depletion was expected to begin to reverse within the next decade or so. Now we ask: Are there any signs of this beginning? Have other factors influenced the behavior of ozone during this period? Has surface UV radiation responded as anticipated?


The Future of the Ozone Layer and Surface UV Radiation

The previous (2002) Assessment noted that climate change would influence the future of the ozone layer. Now we ask: How has our understanding of this issue progressed? How has our increased understanding led to an improved estimate of the state of the future ozone layer?

Previous Assessments have relied heavily on two-dimensional models for projections of future ozone, and these models are used again here. The 2002 Assessment noted the emerging use of three-dimensional Chemistry-Climate Models; we have now used these models extensively in this current 2006 Assessment.

Additional Scientific Evidence and Related Information

For the purpose of this report, ozone-depleting substances are considered as being either long-lived (more than 6 months in the atmosphere) or very short-lived (less than 6 months in the atmosphere), and these are considered in the next two sections. The atmospheric lifetime determines the likelihood of halogens emitted in the troposphere reaching the stratosphere, and so being able to deplete stratospheric ozone. The cumulative effective abundance of halogens in the stratosphere is quantified as equivalent effective stratospheric chlorine (EESC).1

1. Equivalent effective stratospheric chlorine (EESC), a metric noted in the previous Assessment, has found widespread use. EESC is a gauge of the overall stratospheric burden of ozone-depleting halogen. It is derived from ground-based measurements of ozone-depleting substances, with consideration given to the number of chlorine and bromine atoms in ozone-depleting substances, the rates at which different ozone-depleting substances release their halogen once they reach the stratosphere, and the higher per-atom efficiency for bromine relative to chlorine in destroying ozone.

Long-Lived Ozone-Depleting Substances


Halogenated Very Short-Lived Substances

Global Ozone (60°S-60°N)


Polar Ozone


Impact of Climate Change

The Ozone Layer in the 21st Century

UV Radiation and Its Changes

Ozone Depletion Potentials and Global Warming Potentials

Implications for Policy Formulation

The results from over three decades of research have provided a progressively better understanding of the interaction of human activity and the ozone layer. New policy-relevant insights into the roles of ozone-depleting gases havebeen conveyed to decisionmakers through a series of international state-of-understanding assessment reports. The research findings in the Scientific Assessment of Ozone Depletion: 2006 that are given above and are summarized here provide direct current scientific input to governmental, industrial, and other policy decisions associated with protection ofthe ozone layer: