Week 1 – Current and future Ozone holes

During September and October the large spring-time depletion of stratospheric ozone, the so-called ozone-hole, occurs over the Southern Hemisphere polar cap. Despite the success of the Montreal Protocol in restricting production and emission of ozone depleting substances, large concentrations remain in the stratosphere and continue to cause major depletion of ozone each spring. A summary of observations of stratospheric ozone throughout Antarctic winter and early spring can be found here. This year has been rather typical of those in the last decade, with a cold, stable stratospheric vortex forming and remaining over the pole during winter, shown in the image below:

Current ozone hole depth and extent (source: NASA Ozone Hole Watch)

You can also see the formation of the ozone hole through the early spring in this video (note: the video is produced using a combination of satellite monitoring and data assimilation and has some artifacts and inconsistencies)

The extent and depth of the ozone hole is fairly typical for events over the past ten years, with a peak area of 22 million square kilometers occuring on September 25th and a minimum average ozone column of 118 Dobson Units (DU) occuring on October 1st. Ozone is measured by considering the integrated amount throughout the depth of the atmosphere. One DU represents the equivalent depth of ozone (in units of 10 micrometers) present in the column if it were all brought to standard temperature and pressure. As a simple guide to a rather esoteric unit, the ozone contained in a typical column of 300 DU would be 3mm thick (or approximately the width of a one pound coin) if all the ozone were brought to the surface. The ozone hole is normally measured by considering columns of air south of 40S with ozone columns of less than 220DU (or approximately the width of a shiny new two-pence coin).

Ozone depletion peaks in spring because it requires both chemical processing of the ozone depleting substances on the surface of polar stratospheric clouds followed by input of sunlight to provide energy for the ozone depleting reactions (for more information about the mechanism of ozone depletion see twenty questions about the ozone layer.

Given the success of the Montreal Protocol in restricting emissions of ozone depleting substances, ozone holes will become smaller and less pronounced over the 21st century. However, predicting exactly how and when stratospheric ozone amounts will ‘recover’ and even agreeing on a definition of what recovery means is a difficult and interesting scientific challenge. One difficulty in predicting and understanding ozone in the 21st century is the interaction of ozone depletion and climate change. Increases in greenhouse gases have several competing influences on stratospheric ozone (changing chemical reaction rates, adding hydrogen and nitrogen containing compounds which change reaction cycles, influencing the formation of polar stratospheric clouds and stratospheric dynamics and transport in general) and predicting their influence requires complex climate models which explicitly represent the two-way coupling between stratospheric ozone chemistry and climate. These coupled chemistry-climate models (or CCMs) are somewhat unwieldy beasts and our understanding of how best to develop, calibrate and use them is only in its early stages. Recently members of the department have been involved in a major international experiment designed to improve our understanding of CCMs.

The results of this experiment have been one major piece of evidence used by the 2010 World Meteorological Organisation/United Nations Environment Program Scientific Assessment of Ozone Depletion. This assessment functions much like the IPCC by bringing together scientists from around the world to assess the scientific evidence about stratospheric ozone and produce major scientific reports (usually every four years) on the current state and future predictions of stratospheric ozone. The 2010 report will be published in January and much of its focus will be on the links between stratospheric ozone and climate.

In the Department, several members of staff have contibuted to the assessment. In particular, we have considered different sources of uncertainty in predictions of future stratospheric ozone amounts (full paper available here).

Prediction uncertainty for decadal-mean, global-mean total ozone column, orange shows internal variability, blue shows model uncertainty, green shows scenario uncertainty. Black dots show observed ozone column.

This figure shows predictions of the decadal-mean, global-mean ozone column from a small ensemble of chemistry-climate models. The mean date for the ‘return’ of ozone to 1980 values is around 2040 but there is significant uncertainty from several different sources (shown by the coloured wedges). In our analysis we divide the uncertainty into that due to the natural, internal variability of the chemistry-climate system (orange), that due to differences between the different chemistry-climate models (blue) and that due to differences in the projected concentrations of greenhouse-gasses used in different runs of the models (green). Our analysis shows that at least in the medium-term (upto and beyond the return date for global ozone) most of the uncertainty in ozone predictions is due to differences between the chemistry-climate models. In many ways this is exciting, since it means that (at least in theory) the uncertainty in predictions of ozone could be much reduced if we understood differences between the CCMs in more detail.

Finally, if you don’t have much interest in the stratosphere and don’t live in Antarctica you might wonder why stratospheric ozone is relevant to you. In recent years there have been many studies which have shown that the depletion of stratospheric ozone has had a significant influence on climate, potentially masking some of the climate-change signal. As we look to the future, recent studies suggest a competition between ozone recovery and greenhouse warming will determine a wide-range of climate variables in the southern hemisphere, including the position of the jet-stream, the width of the tropical Hadley cell and the position of the sub-tropical dry-zones. If you want to hear more about this topic then please come along to our strat-hour seminar by Julie Arblaster on Monday 18th October.

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