The stratospheric ozone layer absorbs most of the harmful ultraviolet B (UVB) radiation (shortwave UV rays measuring 280-315 nanometers, or nm) emitted by the sun. In small amounts, UVB radiation is helpful to life (e.g., for the production of vitamin D). But what happens when the ozone layer is threatened?
That's what happened in the 1970s and 1980s. Concerns regarding the ozone layer began in 1970 with Paul Crutzen's study1 suggesting that nitrogen oxides could act as catalysts to destroy ozone. Then, Harold Johnston2suggested that nitrogen oxides from stratospheric supersonic transport aircraft (such as the Concorde) might deplete the ozone layer by 3 to 23 percent.
|The more damage there is to the ozone layer, the more UVB rays that reach our bodies.|
Next, Richard Stolarski and Ralph Cicerone3 identified a chlorine reaction that could do the same. And in 1974, Mario Molina and F. Sherwood Rowland4 suggested that CFCs (chlorofluorocarbons, certain inert gaseous compounds) could also accumulate in the atmosphere, releasing chlorine into the stratosphere that could deplete ozone; CFCs were widely used in spray cans, as refrigerants, and as industrial cleaning and drying agents. All of these experts' concerns proved prescient in 1985, when Joe Farman, Brian Gardiner, and Jon Shanklin published a paper5 describing the serious decline in ozone over Antarctica, the so-called Antarctic ozone hole. [Figure 1].
|FIGURE 1. Images of the Antarctic ozone hole in October, using artificial colors
to represent varying ozone layer thickness patterns. Total ozone is measured in
Dobson units (named after G. M. B. Dobson, an early leader of ozone observations).
The Dobson unit is a measure of ozone layer thickness. The natural Antarctic ozone
level in October is 300 Dobson units (green on our color scale). In the mid-latitudes,
ozone typically exceeded 450 Dobson units in the 1970's (red on our color scale).
October ozone levels over Antarctica now decrease to about 100 Dobson units
(purple on our color scale). Ozone levels outside of the ozone hole (30S-70S) have
also decreased; note the shift from the deep red colors in the early years to the
more faded orange colors in the later years.
The more damage there is to the ozone layer, the more UVB rays that reach our bodies. UVB rays (as well as the sun's longer-wave, UVA rays, from 315-400 nm) can produce DNA damage in the skin and mucous membranes, causing genetic mutations that can lead to skin cancer, premature skin aging, cataracts and other eye conditions. Excessive UV exposure can also weaken immune system functioning, reducing our ability to fight off skin cancers and other maladies. Therefore, ozone loss is a serious health threat.
Sounding the Alarm
In 1987, the international community responded to the ozone crisis with the Montreal Protocol. This treaty limited production and consumption of ozone-depleting substances, allowing for changes to the agreement as scientific understanding evolved. Since 1987, the Protocol has been updated seven times, and CFC production is now globally banned.
Now, more than 20 years after the Montreal Protocol was negotiated, CFC levels are finally declining in our atmosphere. It was first recognized in 1995 that CFC levels were no longer increasing in the troposphere, the portion of the atmosphere below the stratosphere. By 2000, satellite observations showed that chlorine levels in the stratosphere had stabilized, and today they are gradually decreasing. Change is slow because CFCs have very long lifetimes in our atmosphere; for example, CFC-12, a.k.a. Freon-12, will require 100 years to decrease to about one third of its current amount.
Between 1980 and 1995, ozone declined about four percent over the mid-latitudes of the Northern Hemisphere. But today, combined ground and satellite observation shows that ozone is only about three percent below the levels seen in the 1960s and 70s. [Figure 2]. This means that the ozone layer may be starting to repair itself.
|FIGURE 2. Percentage of reduction of Northern mid-latitude ozone (35-60N) from
satellite (red) and ground stations (black) from the 1960s to the 2000s. Between 1980
and 1995, ozone declined about four percent over the mid-latitudes of the Northern
Hemisphere. But today, combined ground and satellite observation shows that ozone
is only about three percent below the levels seen in the 1960s and 70s. The very
low values in the early 1990s resulted from the influence on stratospheric ozone
of particles emitted by the eruption of Mount Pinatubo, The Philippines, in 1991.
However, the Antarctic ozone hole hasn't gone away. Each year severe ozone losses between August and September culminate in a broad region of stratospheric air, at an altitude of 12 miles, that has nearly zero ozone. Over the past 15 years, the ozone-hole area has grown to about 25 million square kilometers (9.7 million square miles) in spring - an area larger than the continent of North America. In Antarctica during the dark winter months, special types of clouds called polar stratospheric clouds (PSCs) are formed in the extremely cold stratospheric air (less than 109 degrees F). These PSCs convert stable chlorine molecular species into radical forms that can destroy ozone molecules while regenerating themselves (a catalytic cycle). As the sun rises over Antarctica in the southern hemisphere spring, the sunlight provides the energy necessary for the ozone destruction. This results in accelerated destruction of the ozone layer in the Antarctic spring period compared to other regions further to the north. In addition, a 'polar vortex' wind pattern keeps ozone from elsewhere from filling in the depleted section, leaving the vast hole to develop during the spring period. Fortunately, the size of the hole is not likely to increase further because chlorine and bromine compounds are no longer increasing.
The same processes that have caused the ozone hole over the Antarctic have similarly affected the region over the North Pole. While less stable stratospheric weather patterns limit ozone destruction over the Arctic, early spring Arctic ozone levels are typically 10 percent lower than levels seen in the 1970s.
Ozone and UV
Reduced stratospheric ozone leads to increases in surface ultraviolet (UV) radiation. UV light is divided into three bands: UVC (100-280 nanometers), UVB, and UVA. UVC is the most dangerous, but is mostly absorbed by oxygen and ozone molecules in the stratosphere and does not reach the earth's surface. When UV radiation passes through the earth's atmosphere, some of it collides with other molecules and particles, causing it to bounce around and change direction. This scattered UV radiation can increase sunburn risk even when you are in the shade.
UVB is partly scattered back to space by the atmosphere and absorbed by ozone, limiting how much reaches the earth throughout the year, while UVA is absorbed less by ozone and scattered less by the atmosphere; thus a larger percentage of UVA reaches the earth year-round. UVB strikes the earth more intensely during summer months when the sun is higher in the sky, so that the radiation travels on a shorter path through the atmosphere. Ozone and clouds are the most important factors in limiting UVB penetration. How much UV reaches the earth's surface depends on the amount of ozone overhead, clouds, small particles or aerosols, and pollution.
|As much as 80 percent of UVR can pass through thin clouds that appear to block the sun, so that you can sunburn even on an apparently cloudy day.|
Measuring long-term changes in UV penetration is quite difficult. First, the amount of ozone overhead varies daily because of weather patterns, and to a lesser extent, due to ozone destruction by CFCs. Secondly, cloud systems change rapidly, and can cause the amount of surface UV to change by 10 to 50 percent (occasionally more) within minutes: While thick, uniform cloud layers allow little UV to arrive at the earth's surface, side reflections from tall, puffy clouds can actually enhance penetration of UV radiation by a few percent when compared with clear skies. As much as 80 percent of UV Radiation can pass through thin clouds that appear to block the sun, so that you can sunburn even on an apparently cloudy day. Since 1979, we have been able to estimate this large variation in UV levels on the ground by using satellites that measure ozone and cloud amounts over the entire earth, and then calculating the UV penetration to the ground. The calculations are validated by ground-based instruments.
Because of ozone depletion, by the mid-1990s average clear sky erythemal (skin reddening/sunburn-inducing) radiation had increased by about seven percent from levels in the 1970s at middle latitudes. This has fallen off somewhat and is now only about four percent higher than in the 1970s. [Figure 3].
|FIGURE 3. The calculated percent change in UV irradiance caused by percent
changes in ozone over the continental United States. The ozone change is estimated
from satellite measurements over the United States.
The Bright Side
Overall, the news is good: The Montreal Protocol has had a dramatic impact in reducing ozone-depleting substances. The ozone layer is no longer declining, and there are signs of improvement. The Montreal Protocol has also helped to slow dangerous climatic changes such as global warming by reducing CFCs and other ozonedepleting substances, powerful greenhouse gases that prevent infrared radiation from escaping the atmosphere, reflecting it back towards earth and thus causing the earth to warm. While the yet-to-be-signed Kyoto Agreement on greenhouse gases would have reduced carbon dioxide (CO2) emissions by two billion tons by 2010, the Montreal Protocol's reduction of ozone-depleting substances already reduced greenhouse gases emissions by the equivalent of more than 10 billion tons of CO2 at the end of 2008.
When can we expect ozone levels to return to those seen in the 1970s? With CFC production curtailed, scientists have estimated release rates of existing CFC stocks to project future levels. These projections are fed into complex computer models of the chemistry, radiation, and dynamics of our atmosphere. The models project that ozone levels in the northern mid-latitudes will recover by about 2050, while polar levels (the Antarctic ozone hole) will recover by approximately 2065. A future epidemic of UV-related skin cancers may have been avoided.
Worst Case Scenario
|When can we expect ozone levels to return to thoes seen in the 1970s?|
What might have happened if we had done nothing about CFCs? In the 1970s, prior to discovery of the ozone problem, CFC production was increasing 7-10 percent per year. Using the same computer models that predict the future recovery, we estimated that CFC emissions would have increased by three percent per year after 1974. By 2060, the levels of stratospheric chlorine would have been 16 times above 1980 levels. Average global ozone levels would have decreased by two thirds. The UV index in the northern mid-latitudes would have increased to a value near 30 for midsummer noon conditions. The average mid-summer UV index value now is about 10 in these regions. Typically, it takes about 15 minutes for a fair-skinned person to develop perceptible sunburn in mid-summer. In this theoretical world ("world avoided") it would have taken less than five minutes to develop a perceptible burn. [Figure 4]
|FIGURE 4. UV index from 1980 to 2060 for the WORLD AVOIDED (black) vs. our
currently predicted future (red). The UV index is calculated for early July local
noon conditions. The standard UV index "risk" scale is also superimposed on the
But thanks to guidance by scientists and prompt intervention by policy makers, industry leaders, and diplomats around the world, CFC-related ozone depletion did not become an environmental catastrophe. The process was not easy, but we now know that the price for doing nothing would have been very high.
Dr. Herman is an atmospheric physicist at NASA's Goddard Space Flight Center. He performs research in atmospheric radiation and in the development of instrumentation for measuring UV and visible radiation as well as atmospheric trace gas amounts.
Dr. Newman is an atmospheric physicist at the NASA Goddard Space Flight Center. He is the co-chair of the Scientific Assessment Panel for the international Montreal Protocol agreement that regulates ozone-depleting substances.
1. Crutzen, PJ. The Influence of nitrogen oxides on the atmospheric ozone content. Quart J of the Royal Met Soc 1970; 96:320-325.
2. Johnston H. Reduction of stratospheric ozone by nitrogen oxide catalysts from supersonic transport exhaust. Science 1971 Aug 6; 173(3996):517-522.
3. Stolarski RS, Cicerone RJ. Stratospheric chlorine: a possible sink of ozone. Canad J Chem 1974; 52:1610-1615.
4. Molina MJ, Rowland FS. Stratospheric sink for chlorofluoromethanes: chlorine atom-catalysed destruction of ozone. Nature 1974; 249:810-2.
5. Farman JC, Gardiner BG, Shanklin, JD. Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction. Nature 1985; 315:207-10. Ravishankara AR, Kurylo MJ, Ennis CA (eds.) Trends in emissions of ozonedepleting substances, ozone layer recovery, and implications for ultraviolet radiation exposure. A report by the US Climate Change Science Program and the Subcommittee on Global Change Research. Department of Commerce, NOAA's National Climatic Data Center, 2008 Nov 13; http://www.climatescience.gov/ Library/sap/sap2-4/final-report/