Ozone depletion
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Ozone depletion refers to reductions in the amount of ozone in the stratosphere. There was a reduction of approximately 5% detected from 1979 to 1990.
The ozone hole refers to annual, temporary reductions in the polar regions, where much larger losses in ozone occur each spring - up to 70% over Antarctica or 30% over the Arctic -- followed by recovery in the summer. This reduction is held by a wide scientific consensus to be due primarily to an increase in concentrations of stratospheric chlorine from breakdown of human manufactured CFC emissions. Ozone depletion varies geographically and by season.
Since the ozone layer prevents most harmful wavelengths of ultraviolet light from passing through the earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern and led to speedy adoption of the Montreal Protocol banning CFC use.
In public policy discussions, the term ozone layer depletion is considered synonymous with the theory that a trend of global ozone depletion, which is caused by CFC emissions, is subsequently allowing more ultraviolet radiation to reach the earth's surface. It is suspected that a variety of biological consequences, including, for example, increases in melanoma and the destruction of plankton populations in the ocean's photic zone, may result from this increased UV exposure.
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History of the research
In 1970 Prof. Paul Crutzen pointed out the possibility that nitrogen oxides from fertilizers and supersonic aircraft might deplete the ozone layer. In 1974 Frank Sherwood Rowland and Mario J. Molina realised that when CFCs finally break apart in the atmosphere and release chlorine atoms they cause ozone depletion. These three scientists received the Nobel Prize in Chemistry in 1995 for this work. They calculated that if CFC production continued to increase at the going rate of 10%/year until 1990, then remain steady, CFCs would cause a global 5 to 7 percent ozone loss by 1995 and 30-50% loss by 2050. However, the discovery of the Antarctic "ozone hole" by Farman, Gardiner and Shanklin (announced in a paper in Nature in May 1985) was a surprise - chemical reactions on PSCs in the cold Antarctic stratosphere caused faster depletion than expected - and caused worldwide publicity. Satellite measurements showing massive depletion of ozone around the south pole were becoming available at the same time. However, these were initially rejected as unreasonable by data quality control algorithms; the ozone hole was only detected in satellite data when the raw data was reprocessed following evidence of an ozone hole in in situ observations.
Ozone depletion has been observed all over the globe but is greatest at high latitudes (that is, near the poles). The best known example is the annual thinning of the ozone layer over Antarctica during the polar spring (see ozone hole section below).
Since 1981 the UNEP has sponsored a series of reports on scientific assessment of ozone depletion. The most recent is from 2002.
General
Releasing CFCs into the atmosphere has caused ozone depletion. Since the ozone layer absorbs UV, this would be expected to increase surface UV levels, which could lead to damage, including increases in skin cancer. This is the reason for the Montreal Protocol on Substances that Deplete the Ozone Layer. Although decreases in stratospheric ozone are well-tied to CFC's, and there are good theoretical reasons to believe that decreases in ozone will lead to increases in surface UV, there is not much direct observational evidence linking ozone depletion to additional skin cancer in human beings.
Ozone in the Earth's atmosphere is generally created by ultraviolet light striking oxygen molecules, which consist of two oxygen atoms (O2), creating two single oxygen atoms, known as atomic oxygen. The atomic oxygen then combines with a molecule of O2 to create ozone, O3. The ozone molecule is also unstable and when hit by ultraviolet light it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle. Ozone can be destroyed by atomic chlorine, fluorine or bromine in the atmosphere. These elements are found in certain stable compounds, especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere and there be liberated by the action of ultraviolet light. Most importantly, the chlorine atoms so generated destroy ozone molecules in a catalytic cycle. In this cycle a single chlorine atom would keep on destroying ozone forever, were it not for reactions that remove chlorine atoms from this cycle by forming reservoir species such as hydrochloric acid and chlorine nitrate. The reactivation of atomic chlorine from these reservoir species is normally slow, but is enhanced by the presence of polar stratospheric clouds which appear during Antarctic winters, leading to a strong seasonal cycle in ozone hole formation.
The importance of the reaction on polar stratospheric cloud is many-fold. The PSC's only form in extreme cold; the Antarctic stratosphere is colder than the Arctic; which is why ozone holes first formed over Antarctica and why the Arctic ones are not as deep. The original predictions failed to take into account PCS's, which is why a sudden Antarctic ozone hole, rather than gradual global depletion, was such a surprise.
Ozone depletion also explains much of the observed reduction in stratospheric and upper tropospheric temperatures [1] [2]. This is because the reason for the warmth of the stratosphere is absorption of UV radiation by ozone, hence reduced ozone leads to cooling. Some stratospheric cooling is also predicted from increases in greenhouse gases such as CO2; however the ozone-induced cooling is found to be probably dominant.
Predictions of ozone remains a difficult science. The World Meteorological Organization Global Ozone Research and Monitoring Project - Report No. 44, which on balance comes out strongly in favour of the Montreal protocol, nonetheless notes that projections of ozone loss for the 1994-1997 period made in the UNEP 1994 Assessment had been an overestimate.
Observations
The most pronounced decrease in ozone has been in the lower stratosphere. However, the ozone hole is most usually measured not in terms of ozone concentrations at these levels (which are typically of a few parts per million) but by reduction in the total column ozone, above a point on the earth's surface, which is normally expressed in Dobson units. Marked decreases in column ozone in the antarctic spring and early summer compared to the early 1970s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer (TOMS) [3].
Substantial reductions of up to 70% in the ozone column observed in the austral (i.e. southern hemispheric) spring over Antarctica and first reported in 1985 (Farman et al 1985) are continuing [4]. Through the 1990's, total column ozone in September and October have continued to be 40-50% lower than pre-ozone-hole values. In the arctic, declines are greatest in winter/spring and the amount is more variable year-to-year than in the Antarctic: when the stratosphere is colder the losses are greater, up to 30%
In mid latitudes it is probably preferable to speak of ozone depletion rather than holes; declines are about 3% below pre-1980 values for 35-60N and about 6% for 35-60S. In the tropics, there are no significant trends.
The ozone hole and its causes
The cause of the ozone holes is generally agreed to be CFC (Chlorofluorocarbon) compounds which break down (due to ultraviolet light) and become free radicals containing chlorine high in the Earth's atmosphere. These radicals then break down the ozone catalytically. Ozone destruction due to chlorine radicals from CFCs can take place in the gas phase, but occurs particularly rapidly on the surface of polar stratospheric clouds (PSC), which form over the poles (particularly the south pole) during winter. A rise in CFC production has accompanied the ozone depletion and a plausible chemical mechanism for CFC's role in ozone depletion has been proposed. As a result, a worldwide ban on most uses of CFCs, the Montreal Protocol, was signed and entered into force in 1989.
The photochemical processes involved are complex but well understood, with UV radiation being involved in both the natural production and destruction of ozone, as well as the breakdown of CFCs into free radicals and the destruction of ozone by chlorine radicals. The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring; during winter, even though PSCs are at their most abundant, there is no light over the pole to drive the chemical reactions.
CFCs are a byproduct of some chemical processes, and were also used in air conditioning/cooling units. They were also widely used as aerosol propellants prior to the 1980s. What makes CFCs so effective in breaking down ozone is that one CFC radical acts as a catalyst and can break down many ozone molecules. Furthermore, these radicals stay in the atmosphere for a very long time.
Scientists have increasingly been able to attribute the observed ozone depletion to the increase of anthropogenic halogen compounds from CFCs, by the use of complex chemical transport models and their validation against observational data (e.g. SLIMCAT). These models work by combining satellite measurements of chemical concentrations and meteorological fields with chemical reaction rate constants obtained in lab experiments, and are able to identify not only the key chemical reactions but also the transport processes which bring CFC photolysis products into contact with ozone.
Increased UV due to the ozone hole
Although ozone, O3, is a minority constituent in the earth's atmosphere, it is responsible for most of the main absorption of ultraviolet (UV) radiation in the atmosphere. Correspondingly, a significant decrease in atmospheric ozone could be expected to give rise to significantly increased levels of UV near the surface.
Increases in surface UV due to the ozone hole can be partially inferred by radiative transfer model calculations, but cannot be calculated from direct measurements because of the lack of reliable historical (pre-ozone-hole) surface UV data, although more recent surface UV observation measurement programmes exist (e.g. at Lauder, New Zealand [5]).
Because it is this same UV radiation that creates the ozone in the ozone layer from O2 (regular oxygen) in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical production of ozone at lower levels (in the troposphere), although the overall observed trends in total column ozone are still a decrease, largely because ozone produced lower down has a naturally shorter photochemical lifetime, so it is destroyed before the concentrations could reach a level which would compensate for the ozone reduction higher up.
Biological effects of increased UV
The main public concern regarding the ozone hole has been the effects of surface UV on human health. As the ozone hole over Antarctica has in some instances grown so large as to reach southern parts of Australia and New Zealand, environmentalists have been concerned that the increase in surface UV could be significant.
UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to malignant melanoma (skin cancer) -- for example one study showed that a 10% increase in the UVB was associated with a 19% increase in melanomas for men and 16% for women (Fears et al, Cancer Res. 2002, 62(14):3992-6).
So far, ozone depletion in most locations has been typically a few percent. Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the effects could be substantially more dramatic. For example, recent research [6] has analyzed a widespread extinction of plankton 2 million years ago that coincided with a nearby supernova. Researchers speculate that the extinction was caused by a significant weakening of the ozone layer at that time when the radiation from the supernova produced nitrogen oxides that catalyzed the destruction of ozone (plankton are particularly susceptible to effects of UV light, and are vitally important to marine food-webs).
Aside from the direct effect of ultraviolet radiation on organisms, increased surface UV leads to increased tropospheric ozone, as noted above. Paradoxically, at ground-level increased ozone is generally recognised to be a health risk, as ozone is toxic due to its strong oxidant properties.
Public policy in response to the ozone hole
Environmentalists assert that the CFCs have caused so much damage to the ozone layer that the use of CFCs should be banned. The full extent of this damage CFCs have caused is not known and will not be known for decades; however marked decreases in column ozone have already been observed (see above).
In 1987, the Montreal Protocol was signed, controlling the emissions of CFCs. To some extent, their role has been replaced by the less damaging hydro-chloro-fluoro-carbons (HCFCs), although concerns remain regarding HCFCs also.
The future of ozone depletion
Since the adoption and strengthening of the Montreal Protocol has led to reductions in the emissions of CFC's, etc, the atmospheric concentrations of these compounds has begun to stabilize and in some cases decline. In the long term these substance will be gradually removed from the atmosphere and by 2050 ozone holes should have declined.
There is a slight caveat to this, however. Global warming from CO2 radiative forcing is expected (perhaps somewhat surprisingly) to cool the stratosphere. This, in turn, would lead to a relative increase in ozone depletion and the frequency of ozone holes. The effect may not be linear: ozone holes form because of polar stratospheric cloud; the formation of polar stratospheric clouds has a temperature threshold above which they will not form; cooling of the Arctic stratosphere might lead to Antarctic-ozone-hole-like conditions. But at the moment this is not clear.
Controversy regarding ozone science and policy
Any counter-measures which have a negative economic impact will remain a controversial issue due to the strong economic interests involved, with key questions regarding whether the scientific understanding is strong enough to warrant the proposed countermeasures. In this context it is worth noting that it is commonly believed that one reason for the relative ease of introduction of the Montreal protocol was the availability of CFC replacements at little extra cost.
The consensus amongst most atmospheric physicists and chemists is that the scientific understanding has now reached a level where countermeasures to control CFC emissions are justified, although the decision is ultimately one for policy-makers and society.
Some atmospheric scientists (for instance Fred Singer, founder of SEPP and also a global warming skeptic) and industry-sponsored advocacy groups question or completely deny a link between CFCs and ozone depletion [7]. It is common to see completely nonsensical arguments put forward to prove that CFCs cannot cause ozone depletion - for example, that they are heavier than air and so cannot reach the stratosphere [8]. CFCs are heavier than air, but just like argon, krypton and other heavy gases with a long lifetime they are uniformly distributed throughout the turbosphere and reach the upper atmosphere [9].
Despite this general consensus, the science behind ozone depletion remains complex, and some who oppose the enforcement of countermeasures point to some of the difficulties experienced in these studies. For example:
- Although increased UVB has been shown to constitute a melanoma risk (see above), it has been difficult for statistical studies to establish a direct link between ozone depletion and increased rates of melanoma. Although melanomas did increase significantly during the period 1970-1990, it is difficult to separate reliably the effect of ozone depletion from the effect of changes in lifestyle factors (e.g. time spent outdoors).
Related articles
- ozone
- ozone layer
- ozone-oxygen cycle
- Montreal Protocol
- Scientific assessment of ozone depletion
- CFC
- melanoma, skin cancer
- greenhouse gas
- ultraviolet
External links
- NOAA Stratospheric Ozone Webpage
- Stratospheric ozone depletion (Antarctic, Arctic, and global) at NOAA
- Ozone Depletion And Global Environmental Change at Columbia University
- Scientific Assessment of Ozone Depletion: 1998 - by WMO, UNEP, NOAA
- Ozone FAQ
- Methyl Bromide Blues
- Five Scientific Questions On The Cfc-Ozone Issue by S. Fred Singer
- A Critique of the UN Scientific Assessment of Ozone Depletion: 1994 by S. Fred Singer
- The Ozone-CFC Debacle: Hasty Action, Shaky Science by S. Fred Singer
- The Skeptics vs. the Ozone Hole
- WMO/UNEP Scientific Assessment of Ozone Depletion: 1998
- The Ozone Tour at the Centre for Atmospheric Science, University of Cambridge
- The Ozone FAQ - a superb resource (but last updated in 1997)
- The Politics of Methyl Bromide
- Jon Shanklin's Ozone pages, including a pamphlet.
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