Climate for students - UV & ozone

Frequently asked questions about climate, UV and the ozone layer.

What is the state of the global ozone layer now?

Global ozone levels remain significantly suppressed below the levels experienced before 1980. The global decreases in ozone have been driven primarily by extensive spring-time ozone depletion over the Antarctic and the mixing of this ozone depleted air to lower latitudes following the breakup of the polar vortex (strong westerly winds which form in early winter) in November or December of each year. In recent years, Arctic ozone depletion has also contributed to global ozone declines. In addition to polar ozone depletion, in situ ozone depletion also occurs at midlatitudes on sulphate aerosols. When sulphate aerosol levels were increased after the Mt. Pinatubo eruption in June 1991, midlatitude ozone depletion, particularly in the northern hemisphere, was much stronger.

Figure 1 shows the global mass of ozone for each day from November 1978 to the end of 1998. These calculations were made by scientists at NIWA using satellite-based measurements of total column ozone made by the NASA Total Ozone Mapping Spectrometer (TOMS) instruments and the ESA Global Ozone Monitoring Experiment (GOME). Note that the units for the mass of ozone are Mt which are millions of tons (109 kg) of ozone. Clearly, generating ozone at the surface to supplement stratospheric ozone levels is completely impractical.

Ozone depletion over the Antarctic remains as severe as ever. Figure 2 shows the annual mean mass of ozone depleted over Antarctica (the red line and symbols) together with the increase in the amount of chlorine in the stratosphere (the region of the atmosphere between approximately 10 and 50 km where most of the ozone resides). In addition to some inter-annual variability, there has been a steady increase in the severity of Antarctic ozone depletion from 1979 to the present.

The severe ozone depletion occurring over the Antarctic continent during the spring of each year, the ‘ozone hole’, is confined to the Antarctic and never extends over New Zealand.

Figure 3 shows the southern hemisphere total column ozone distribution on 6 October 2001 which was a typical day during the 2001 Antarctic ozone hole period. The blue colours show low ozone values less than 200 DU (pre-1980 values of ozone over the Antarctic were always greater than 220DU). Note that New Zealand is far from the Antarctic ozone hole and during October finds itself under a ridge of high ozone. In fact, New Zealand experiences its highest ozone levels during October, the time of the Antarctic ozone hole.

Has the Montreal Protocol been effective in slowing global ozone depletion?

The Montreal Protocol for the protection of the ozone layer, and its amendments (London 1990, Copenhagen 1992, Vienna 1995, Montreal 1997, Beijing 1999) has significantly reduced the input of chlorine and bromine containing gases to the atmosphere. New Zealand is a signatory to the Montreal Protocol.

The international protocol, signed on 16 September 1987, was agreed upon only 2 years after the discovery of the Antarctic ozone hole in 1985. The interaction between scientists and policy-makers to achieve this international agreement in such a short time, provides a model for other, similar processes. Figure 4 shows projections of future chlorine and bromine levels in the stratosphere for three different scenarios. Global measurements of the primary chlorine containing compounds e.g. CFC-11, indicate that levels near the surface reached their maximum in the early 1990s. Stratospheric levels lag tropospheric levels by approximately 5 years and therefore stratospheric chlorine levels are expected to have reached their maximum in the late 1990s. Continued measurements are needed to verify this turn around as there is considerable variability in the measurements.

CFCs have a very long life-time in the stratosphere (approximately 100 years) and therefore chlorine levels in the stratosphere will decrease only slowly (see Figure 4). As chlorine levels decrease, ozone depletion should slow, although there are interactions with climate change which could affect the recovery of the ozone layer, as detailed below.

How have ozone levels changed in the past over New Zealand?

To understand how ozone and UV levels in New Zealand will change in the future, it is first necessary to understand why ozone levels have changed in the past. Figure 5 shows the annual mean total column ozone over Lauder (representative of New Zealand) from 1979 to 2000. It is clear that the long-term change has not been a smooth, linear, decrease but has consisted of a number of downward steps. Ozone was relatively constant from 1979 to 1984, after which there was a downward step of approximately 20DU. Levels were then fairly constant from 1985 to 1996, after which there was another downward step of 16DU. Ozone levels from 1997 to 2000 showed a quasi-biennial pattern with even numbered years having approximately 10DU more ozone than odd numbered years. There are two key questions related to past ozone changes over New Zealand that remain unanswered, viz.:

  1. What caused the sudden decrease in ozone from 1984 to 1985?
  2. The Mt. Pinatubo volcanic eruption in June 1991 caused significant ozone decreases at northern hemisphere mid-latitudes in the following 2 or 3 years. Why were no such changes observed over New Zealand?

While the ozone hole never comes over New Zealand, past ozone changes over New Zealand are expected to have been strongly influenced by Antarctic ozone depletion. The effect of Antarctic ozone depletion of New Zealand ozone levels is the focus of current research at NIWA.

How can we expect ozone and UV levels in New Zealand to change in the future?

As chlorine levels in the stratosphere begin to decline, we expect that ozone levels over New Zealand will return to their 1970s values. As ozone levels increase, so UV levels will decrease. However, because UV levels over New Zealand are naturally higher than those at similar northern hemisphere latitudes (primarily because of the pristine cleanliness of New Zealand’s air), care will always need to be taken to avoid excessive UV exposure.

Figure 6 shows expected changes in ozone (left panel) and UV right panel at 45oS and 45oN until the end of this century. Note that ozone levels at 45oS are naturally lower than those at 45oN and that changes in UV are significantly higher at southern hemisphere midlatitudes than at northern hemisphere midlatitude.

What other factors can be expected to affect how ozone and UV will change in the future?

A number of important assumptions are made when predictions are made as to how ozone, and hence UV, will change in the coming years. Some of these assumptions are:

  1. No volcanoes: Volcanoes, such as the Mt. Pinatubo eruption in June 1991, can cause large perturbations to the chemistry of the atmosphere. If they are sufficiently powerful they can inject sulphuric acid directly into the stratosphere. The sulphuric acid combines with water to form sulphate aerosols. These small droplets promote ozone depleting reactions by providing surface on which the reactions take place.
  2. Adherence to the Montreal Protocol: Only if countries strictly adhere to the Montreal Protocol will chlorine levels in the stratosphere decrease and ozone recover. Some countries are not signatories to the Montreal Protocol, and illegal trade of CFCs between these and highly industrialized countries could result in increases in emissions of CFCs. International policing of trade in industrial chemicals is needed to ensure that this doesn’t occur.
  3. No new chemicals: It is possible that new chemicals with new industrial uses will be produced and that emissions of these chemicals could harm the ozone layer. Research into the potential adverse affects of new chemicals on the chemistry of the atmosphere needs to be done before these chemicals are approved for industrial scale use.

What changes do we expect in human health as a result of future UV changes?

Potential adverse affects on human health as a result of future UV changes are very difficult to assess. It appears that adverse affects of UV exposure are cumulative with UV irradiance received during childhood being more important than later in life. There have been significant changes in lifestyle over the past 50 years – people now spend more time working indoors with occasional holidays in sunny locations e.g. seaside resorts. These short episodes of anomalously high UV exposure are significantly more damaging than lower levels of sustained UV exposure throughout the year. Higher cases of skin cancer in recent years are as likely to be a factor of lifestyle changes over the past 50 years than from increases in UV as a result of ozone depletion. Separating the effects of UV irradiance increases and lifestyle changes on New Zealand skin cancer rates is very complicated and requires much more research.

Is climate change going to affect the recovery of the ozone hole?

While greenhouse gases warm the troposphere, they cool the stratosphere. A colder stratosphere leads to higher probability for the formation of polar stratospheric clouds, a key prerequisite for ozone depletion. There are a number of other important processes which link climate change to ozone depletion. Most of these processes exacerbate ozone depletion when tropospheric greenhouse gases are increasing (Figure 7).

Recent research has shown that viewing the stratosphere in isolation severely compromises our ability to accurately forecast changes. It is necessary to view the atmosphere as a whole, including the many processes coupling tropospheric changes to stratospheric changes, and atmospheric changes to oceanic changes. One of the best ways to do such research is using models known as coupled chemistry-climate models. These are standard climate models to which atmospheric chemistry (both tropospheric and stratospheric) has been added to model the atmosphere holistically. However such models are extremely computationally demanding. It has only been since the acquisition of the cray supercomputer that NIWA has been able to run such models. Our ability to do this research, which is still in its infancy, places NIWA at the forefront of international research into the coupling between climate change and ozone depletion.

UV radiation sensors on the roof of the optics building at Lauder. [NIWA]
Ozone hole 12 September 2012, 26 Mio km2, NASA image, Ozone Hole Watch
Figure 1: The global mass of ozone in Mt (1Mt = 109 kg). [NIWA]
Figure 2: The change in the size of the Antarctic ozone hole and the increase in stratospheric chlorine. [NIWA]
Figure 3: The 2001 Antarctic ozone hole. [NIWA]
Figure 4: Projections of future chlorine/bromine loading of the stratosphere. [NIWA]
Figure 5: Annual average total column ozone over Lauder, New Zealand. [NIWA]
Figure 6: Expected changes in ozone and UV at 45oS and 45oN. [NIWA]
Figure 7: Potential delay in the recovery of the ozone layer over New Zealand as a result of greenhouse gas increases. [NIWA]