The hole in the ozone and what we’re doing about it
The ozone hole is more than a physical concept. It's a powerful symbol of how humans can wreak damage to the planet in ways we never dreamed. Be that as it may, the risks posed by the ozone hole appear to be on the decrease, and in many ways they pale next to the dangers connected to global warming.
In 1985 a team of British researchers was shocked to discover that levels of ozone in the stratosphere above Hailey Bay, on the Antarctic coast, had plummeted in recent years. Up until then, hints from ground and satellite data had been either overlooked or discounted, largely because nobody expected to find such a dramatic change.
Ozone is a pollutant at ground level, harmful when we breathe it However, the ozone layer that sits within the lower stratosphere (focused at about 25-40km/15-25 miles high) is a godsend. Even though it's a tiny fraction of the stratospheric air, it intercepts much of the ultraviolet light that can produce sunburns and skin cancer, damage our eyes, and cause all kinds of ill-health. Stratospheric ozone is measured from ground by a light beam that bounces up, hits the ozone and returns. By monitoring the strength of this signal, scientists can calculate the amount of ozone in an imaginary column that stretches above the measuring site. In 1985, the British team found that ozone had diminished from a near-normal level of 300 dobson units to about 200. Since then, values below 100 have been found.
A flurry of measurements from both ground instruments and satellites over the next few years clarified what was happening. Three factors conspire to form this patch of depletion – the ozone hole – that lasts for a few weeks during each Southern Hemisphere spring, from about September to November. The first ingredient is a special type of cloud, a polar stratospheric cloud, that only forms as winter temperatures fall below about -80°C/-112°F at high altitudes and latitudes. These clouds represent the stage for ozone depletion. The main players on that stage are the industrial chemicals known as chlorofluorocarbons (CFCs). Starting in the 1920s, when they were developed for refrigeration, CFCs have been used in spray cans, air conditioners, computer-manufacturing plants and many other places. CFCs are versatile in part because they're extremely stable compounds, but that very stability means that they remain in the air for many years before breaking down. Although CFCs are heavier than clean air, they mix easily through the atmosphere; once lofted into the stratosphere, they can easily remain there long enough to do damage. Their risk was recognized as early as the 1970s, when the US banned CFCs in spray cans. At that time nobody anticipated the peculiar drama about to unfold in the Antarctic.
Once CFCs are on that stage with the polar stratospheric clouds, all is in place for the arrival of the final protagonist: sunlight. As the sue-month Antarctic night comes to an end each September, round-the-clock sunshine helps break down the CFCs. This releases chlorine, and the chlorine uses the surface of the polar stratospheric cloud to break down ozone into oxygen. A single molecule of chlorine can destroy many ozone molecules over a few weeks. By November of each year, the stratosphere has warmed up, the clouds disappear, and the ozone hole closes once more.
If the ozone hole covered the whole planet, we'd have to wear sunscreen and hats for weeks at a time. Fortunately, the hole has never extended much beyond Antarctica, although it has encroached on southern Chile. Southern Australia and New Zealand, while outside the hole per se, have seen ozone reductions of more than 10 percent at times.
Why isn't there an Arctic ozone hole?
In fact, there is, although it's not as powerful and reliable as its southern counterpart. For a variety of reasons, including the effect that extensive mountains across North America and Eurasia have on air circulation, the wintertime vortex that forms over the Arctic is less stable than its Antarctic counterpart. This limits the growth of polar stratospheric clouds and helps keep a bona fide ozone hole from forming. Still, the springtime depletion over the Arctic grew to levels of 60 percent by 2000 (comparable to the
Antarctic's percentage drop), thanks largely to increasingly colder Arctic temperatures.
A related concern – distinct from the ozone hole – is the weaker but broader and more persistent ozone depletion of some 5 to 10 percent across much of the globe compared to the temporary drops of 50 percent or greater in the ozone hole itself. Some of this worldwide depletion is likely due to the yearly dispersal of the ozone hole and the mixing of that ozone-depleted air around the globe.
Before long, the ozone hole should be on the mend. The 1987 Montreal Protocol, orchestrated by the United Nations and ratified with amazing speed, called for a gradual rampdown in CFC manufacture, with a virtually complete ban by 1995. In place of CFCs are substitutes such as halochlorofluorocarbons, which are far less likely to break down and release chlorine. There is a black market for CFCs, which will help keep some emissions leaking into the air, but global CFC emissions have dropped significantly since the late 1990s, and chlorine concentrations in the stratosphere have also begun to show signs of a decrease.
Then there's global cooling…
One puzzle remains. Scientists thought wed see recovery in ozone levels by the year 2000, yet that year's hole was the strongest yet, and through 2005 the ozone hole's extent varied little from its 1990s average. The problem, oddly enough, seems to be global cooling – in the stratosphere, that is. Even as the global average of surface temperature has hit record highs here at ground level, it's been dropping to record lows in the stratosphere. This is because ozone itself is a greenhouse gas. By removing it from the stratosphere, we've been helping that layer to cool down dramatically plus allowing more energy to get down to the lower atmosphere, where we live. Another cause of the chill is – ironically – the increase in carbon dioxide, which acts as a cooling agent high in the stratosphere. These cold temperatures have induced polar stratospheric clouds to form more readily and last longer. This, in turn, provides a sturdier stage for CFCs to continue causing damage – in the Arctic as well as the Antarctic – even as the culprits go down in numbers.
Thus, although we're doing the right things in the long run, nature has added a delay of what could be several decades to the ozone layer's recovery. It's likely to be 2050 or beyond before ozone levels are back near normal, and with global warming set to produce major changes in the global atmosphere, it's possible that the ozone layer won't completely return to its pre-1980s state. For now, a small but significant amount of extra ultraviolet light (a few percent's worth) will continue to reach the parts of our planet where most people live. That could trigger a measurable rise in skin cancer rates, as well as some harder-to-predict impacts on other species.