Since it was first reported in 1985, scientists have watched the ozone hole form over Antarctica every spring in the Southern Hemisphere. The hole’s size is determined by both temperature and the amount of chlorine in the atmosphere. Chlorine atoms hitch a ride to the stratosphere on chlorofluorocarbons (CFCs) that humans pumped into the air via aerosols, solvents, and refrigerants. When CFCs reach the stratosphere, their chlorine atoms are freed by the Sun’s ultraviolet light and eat up ozone—a molecule with three oxygen atoms that shields Earth from harmful radiation.
In 1987, the nations of the world negotiated the Montreal Protocol to stop the harmful emissions of CFCs. The hole quit growing by the mid-1990s, but recovery of the protective shield has been slow because many ozone-depleting substances can last in the atmosphere for 50 and even 100 years. Scientists project that the hole will not vanish until around 2070. However, conclusively tying the international regulations to ozone recovery is not straightforward.
While CFCs are no longer emitted, scientists calculate that the slow destruction of CFCs already in the atmosphere causes chlorine to decline each year by roughly 20 parts per trillion (ppt), or about 0.5%. However, Earth-orbiting satellites indicate that the natural variability of chlorine levels over Antarctica is up to 10 times larger than the expected annual decline. In some years the chlorine level declined by as much as 200 ppt, but in other years measurements indicate it increased by as much as 150 ppt. According to Strahan et al., it will take a decade of chlorine declines to be certain the Montreal Protocol has caused the ozone hole to shrink.
Low temperatures in the Antarctic stratosphere currently play the largest role in determining the size of the ozone hole. In 2006, the combination of very low temperatures and high chlorine levels produced the largest ozone hole ever. By 2040, the authors expect that chlorine levels will have fallen enough that the ozone hole will be smaller than it is today even during a very cold year. The authors say it is unlikely any future ozone hole will grow to 2006 levels.
Still, the team cautions that natural fluctuations mean it will take years of watching both temperature and chlorine levels before ozone trends can be attributed to the Montreal Protocol. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2014JD022295, 2014)
—Eric Betz, Freelance Writer
Citation: Betz, E. (2015), Ozone hole to remain large during cold years, Eos, 96, doi:10.1029/2015EO028937. Published on 28 April 2015.