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Our close proximity to the Antarctic ozone hole and our high rate of skin cancer justify New Zealanders’ concern about ozone depletion and its effects on UV radiation. Richard McKenzie and Greg Bodeker report on the most recent assessments of the science of ozone depletion and its environmental effects.
Under the terms of the Montreal Protocol for the protection of the ozone layer, assessments of our state of knowledge on the subject are required by the signatory parties every four years. NIWA scientists based at Lauder, Central Otago, have been heavily involved with both the WMO Science Panel assessment, and the UNEP Effects Panel assessment, which are part of this process. The Science Panel reports on changes in our understanding of the atmospheric physics and chemistry relating to ozone depletion and UV increases, and the Effects Panel reports on the expected environmental effects of the changes in UV radiation. In each case, the panels review the published literature to provide an assessment of the current state of knowledge and how it differs from previous assessments, and updates predictions of ozone and UV levels. Here we summarise the major results of the most recent of these assessments, and how they affect New Zealand.
Ozone changes
Interactions between ozone depletion and climate change are a source of uncertainty in predictions of future changes in ozone, and have been a major focus of research in recent years. Although the issues are quite separate, there are important linkages. Previously, it was thought that these interactions could significantly delay the recovery of ozone. New results from more sophisticated models suggest that this is unlikely and we now expect that ozone at southern midlatitudes will recover from the effects of manmade ozonedepleting chemicals around the middle of the century. It will take the polar regions a decade or two longer to recover to 1980 ozone levels because of the slower turn-over time of air in those areas.
The first signs of recovery are already evident at midlatitudes, including New Zealand, where ozone in the upper stratosphere has levelled off, or even been slightly higher in recent years than around the turn of the century. These early signs of recovery are expected because the concentrations of ozone-depleting chemicals reached a maximum in the late 1990s, and have been declining since then. This is the most tangible evidence that the Montreal Protocol is working. These findings, and the outlook for the future, are summarised qualitatively in the illustration top right, which was published in the executive summary of the Science Panel’s assessment.
However, there is no room for complacency, for a number of reasons:
- Our understanding of atmospheric chemistry and dynamics is incomplete. Models have been unable to reproduce all of the observed variability in ozone, especially in the southern hemisphere.
- A large, explosive volcanic eruption in the next few years – while atmospheric chlorine and bromine remains elevated – could result in severe ozone depletion.
- We rely on continued political and economic will to ensure continued adherence to the terms of the Montreal Protocol (see ‘Methyl bromide and the Protocol’).
Ozone depletion vs global warming
| Ozone depletion | Global warming |
|---|---|
| Global problem | Global problem |
| Manmade (especially by the developed world)/td> | Manmade (especially by the developed world) |
| Some culprit gases in common (such as O3, H2O,CFCs) | Some culprit gases in common (such as O3, H2O,CFCs) |
| Caused by CFCs | Caused by CO2, CH4, N2O |
| Initial scepticism by big business | Initial scepticism by big business |
| Replacements available | No replacements available |
| International agreement (eventually) | Some countries not yet cooperating (USA, Australia) |
| Clouds influence | Clouds influence |
| Aerosols influence | Aerosols influence |
| Surface albedo influence | Surface albedo influence |
| Solar cycle influence | Solar cycle influence |
| Dynamical influence | Dynamical influence |
| International response: Montreal Protocol + amendments | International response: Kyoto Convention + ? |
| May be past the worst | Worst is still to come |
A significant part of the apparent recovery in ozone in recent years, especially at northern mid-latitudes, can be attributed to changes in atmospheric wind patterns rather than being solely due to changes in ozone-depleting chemicals. In the future, further changes in circulation patterns may be expected in response to climate change. Thus, while there is consensus that ozone will continue to increase, we cannot be certain on the details of how ozone will evolve in the years ahead.
Furthermore, there is uncertainty regarding whether ozone will revert to levels similar to those prior to the onset of ozone depletion in 1980, or whether interactions with climate change will result in stabilisation at higher or lower levels. The most recent evidence suggests that, because of interactions with climate change, there will be a super-recovery in ozone by the end of the century (that is, higher values of ozone than in 1980). Past experience tells us that ozone has variability on a wide range of time scales, and its global ‘equilibrium’ will always be dynamic.
In New Zealand we have reason to be concerned about the situation in Antarctica, since our summertime ozone is strongly correlated with springtime ozone depletion there.
Approximately half of the summer-time ozone decreases that have occurred in New Zealand since the late 1970s can be attributed to the export of ozone-poor air from the Antarctic ozone hole after its break-up in November or December. Although the ozone hole has been smaller in recent years, it reached near record proportions in the spring of 2006.
Ultraviolet radiation
Our concern about ozone depletion arose because of the protective shield it provides against solar UV radiation transmitted to the Earth’s surface. As ozone decreases, the intensity of UV increases, and vice versa. The graph right shows the changes in peak UV intensities that have occurred in New Zealand in recent years in response to changes in ozone. The highest UV intensities to date occurred in the summer of 1998–99, when ozone was at a minimum. Since then, the peak UV intensities have tended to be lower. Although not all of the observed decreases in UV can be attributed to ozone increases, similar recoveries in UV have also been seen at other unpolluted sites. There is greater uncertainty about future UV than ozone because it is influenced by several factors other than ozone, which will in turn be influenced by climate change. These include changes in cloud cover, water vapour, and atmospheric pollution (such as aerosols and tropospheric ozone, and other trace gases, such as NO2, SO2). The effects of the ozone hole are expected to continue to exert a significant but variable influence on summer ozone and UV radiation in New Zealand for several decades.
Even in the event of a full recovery of ozone, UV intensities in New Zealand will remain much higher than at corresponding latitudes in the northern hemisphere. Ground-based measurements have shown that the peak intensity of sunburning UV radiation is about 40% greater in New Zealand than at corresponding latitudes in the northern hemisphere (although it is still much lower than the peak global UV intensities at high altitudes near the equator). Global estimates of UV from satellite-based instruments show much smaller contrasts between the northern and southern hemispheres because the sensors do not adequately probe the lowermost regions of the atmosphere, where UV is most severely affected by pollutants.
Because of the success of the Montreal Protocol, decreases in ozone and the attendant environmental effects of increases in UV have been much smaller than anticipated. Outside regions directly influenced by the Antarctic ozone hole, increases in UV irradiances have been modest. At mid southern latitudes, such as New Zealand’s, ozone is currently about 6% lower than in the 1970s, and the corresponding increases in UV radiation over the intervening period have been of comparable magnitude.
Interactions with climate change
Models show that we should expect to see interactions between ozone depletion and climate change. These interactions can be in both senses: climate changes can influence ozone depletion, and ozone depletion can affect climate. For example, the accumulation of greenhouse gases that increase temperatures at the Earth’s surface lead to decreased temperatures in the stratosphere where most of the ozone is found. These in turn lead to slower ozone depletion rates at mid-latitudes, but more efficient ozone depletion on ice crystals that form at cold temperatures in the polar stratosphere.
Some of these linkages are illustrated in the ‘feedback’ graphic. In some cases, these interactions exacerbate the environmental impacts. For example, studies have shown that there is increased susceptibility of plastic building materials to UV damage, and increased skin sensitivity to UV radiation under warmer conditions. In other cases, there is reduced sensitivity. For example, in plants, increases in UV can alter plants in ways which reduce their susceptibility to heat and drought stress. Changes at the ecosystem level can be complex and can act in both directions through changes in radiation shielding (for example, by forest canopies, water, ice, snow, or clouds) and surface reflectivity (from ice and snow). Complex interactions between UV and air quality may also be influenced by climate change, through changes in temperature and humidity.
Special New Zealand perspectives
A new development in the ozone/UV story has emerged with a large body of literature on the beneficial effects of UV radiation. This is important for New Zealand: although we are subjected to extreme skin-damaging UV in the summer, our wintertime UV, which is essential for vitamin D production, is low. Consequently, a significant fraction of our population is deficient in vitamin D, especially in the winter months. The health costs of these deficiencies are potentially far greater than the costs of skin damage attributable to high levels of UV.
These assessments show that a battle against CFCs has been successful, but the war against ozone depletion and UV has yet to be won. We are still in the period when UV risks are close to their maximum, so we must continue to be vigilant to minimise our summertime UV exposure. In the years ahead, we will be monitoring the situation carefully and we’ll use observations in conjunction with models to improve our ability to understand and predict future changes.
Methyl bromide and the Protocol
The continued success of the Montreal Protocol requires that the global community continues to honour its terms. Significant reductions in the risk can also be achieved through policy decisions which lead to accelerations in the phase-out rate of chemicals that are harmful to ozone. One example in this category is methyl bromide.
Tropospheric methyl bromide concentrations decreased globally by 14% from 1997 to 2004. However, over the same period, methyl bromide use in New Zealand increased from 160 tonnes/year to 243 tonnes/year. It is used as a fumigant to treat imported and exported fruit, vegetables, and timber.
Under the Montreal Protocol, New Zealand must apply annually for a critical-use exemption for methyl bromide (in addition to the use allowed for quarantine and pre-shipment). Current critical-use exemptions are all for strawberry growers.
The New Zealand government has determined that 2007 will be the last year that an application will be made for the critical use of methyl bromide by the strawberry industry.
The Montreal Protocol is working
- The concentrations of ozone-depleting chemicals are now decreasing.
- The decline in ozone in the 1980s and 1990s has not continued in recent years.
- Ozone is expected to recover slowly over the decades ahead.
- UV intensities in NZ will remain high in summer (and low in winter).
Further reading and useful links
UNEP. (2007). Environmental effects of ozone depletion and its interactions with climate change: 2006 assessment. United Nations Environment Programme. (The report will be published in a special issue of Photochemical & Photobiological Sciences, March 2007: www.rsc.org/publishing/journals/PP/)
WMO. (2007). Scientific Assessment of Ozone Depletion: 2006. Global Ozone Research and Monitoring Project Report No. 50. World Meteorological Organization, Geneva.
Both reports are available on the website of the UNEP Ozone Secretariat: www.unep.ch/ozone/Assessment_Panels/
Wright, C.; Bodeker, G.; Reeder, T. (2005). How much UV are New Zealand schoolchildren getting? Water & Atmosphere 13(2): 10–11.
UV Radiation and its Effects: an update, Papers and posters from the NIWA UV Workshop, Dunedin, 2006 www.niwa.co.nz/rc/atmos/uvconference/2006/papers
Dr Richard McKenzie and Dr Greg Bodeker study atmospheric processes at NIWA in Lauder, Central Otago. Much of their work focuses on atmospheric chemistry, ozone depletion, ultraviolet radiation, and health effects of UV exposure.
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