The Recovery of the Ozone Layer

by Evonne Marzouk


Recently, a University of Colorado at Boulder study has confirmed that the Earth’s ozone layer is slowly being replenished as a result of the 1987 Montreal Protocol.[1] The new study shows a larger than expected recovery of ozone in the northern mid-latitudes in recent years. While this is good news for the ozone layer and the international efforts to protect it, we still must be cautious to protect the ozone layer in the future, and to protect ourselves from its depletion in the short term.

Ozone is a gas that is naturally present in our atmosphere.[2] Each ozone molecule contains three atoms of oxygen (O3). Ninety percent of our ozone is present in Earth’s stratosphere (the upper part of our atmosphere, more than 10 kilometers or 6 miles above Earth’s surface).[3]
In this upper atmosphere, ultraviolet (UV) radiation from the sun affects regular oxygen (O2) molecules, dividing them into the two oxygen atoms which form them, and allowing them to form molecules of ozone (O3). Over time, these molecules settle into more stable O2 molecules, which are then divided and form as O3 again. Through this balance, a very thin layer of stratospheric ozone is created, which absorbs UV radiation. The UV radiation can be absorbed by essential molecules, such as DNA, in plants, animals and humans, and damage them. The ozone layer protects us from this radiation.[4]
In the mid-1970s, it was discovered that some human-produced chemicals, such as chlorofluorocarbons (CFCs), which were used for refrigeration and air conditioning, foam blowing, and industrial cleaning, could destroy ozone and deplete the ozone layer. By reducing the beneficial shield from UV sunlight, this could increase human incidence of skin cancer and eye cataracts. Observations of the ozone layer showed that depletion was occurring. The most severe ozone loss was discovered to be over Antarctica, commonly called the “ozone hole” because the ozone depletion there was so large and localized. Ozone layer thinning also has been observed over other regions of the globe, such as the Arctic and northern middle latitudes.[5] There has been a seasonal decline of up to 10 percent of ozone at mid-latitudes, the location of much of North America, South America and Europe.
Dr. Deneb Karentz, a Professor in the Departments of Biology and Environmental Science at the University of San Francisco, and her colleagues have determined that increased levels of UV-B radiation in the Antarctic can cause changes in marine organisms, including impairing metabolic processes, decreasing growth, reducing reproductive potential, and genetic damage. Because different plants and animals respond with different sensitivities to UV effects, the impacts will ultimately change the species composition in eco-systems in the Antarctic. Even small changes in the quantity or quality of food sources could ultimately affect animals at the top of the food chain, such as penguins, seals, and whales, and through their links to the rest of the world’s oceans, could affect changes in the rest of the world.[6]
The 1987 United Nations Montreal Protocol, a global agreement, was created to ban ozone-depleting substances and prevent destruction of the ozone layer. According to the United Nations Environment Programme (UNEP), with continued compliance, recovery of the ozone layer is expected in the late 21st century.
However, the recovery is occurring in a changing atmosphere and is unlikely to stabilize at pre-1980 levels, according to the new study. The unexpected increased recovery in recent years may be partially a result of natural variability, including shifts in air temperatures and atmospheric transport, the influences of the 11-year solar cycle and an absence of major volcanic activity on Earth.[7]
By the end of this century, ozone levels could be slightly higher or slightly lower than before 1980 because of high natural variability and human caused changes like warming temperatures. At higher latitudes, for example, warmer temperatures at Earth’s surface can trigger colder conditions in the lower stratosphere and promote the formation of polar stratospheric clouds, which can contribute to severe ozone depletion.
Future ozone levels will be affected by air temperature, atmospheric dynamics and the presence of other air pollutants. For example, nitrous oxide, or N2O — a result of fertilizer production and gas-driven vehicles on Earth — could lead to significant depletion of protective ozone molecules. As CFCs become a less dominant factor controlling ozone, it will be important to prevent other threats to ozone, such as greenhouse gases, N2O and methane.
The new study, which shows ozone levels have stabilized or increased slightly in the past 10 years, used data from satellites and ground stations to compare changes in the ozone layer to past depletion levels. The researchers used data from 14 modeling studies published by scientific groups from around the world for the study. The ozone data was collected by a suite of NASA and NOAA satellites and ground stations. The study follows a 2005 study indicating the ozone layer was no longer in decline after nearly two decades of depletion from harmful chemicals.

The full recovery of the ozone layer is still decades in the future, meaning that the amount of UV radiation reaching Earth will continue to be higher than normal for many years. As a result, it is still necessary for people to take precautions, such as sunscreen, protective clothing, sunglasses, and hats to protect themselves from UV radiation.



[1]      See The study was coordinated by Betsy Weatherhead, a researcher with the Cooperative Institute for Research in Environmental Sciences, a joint institute of CU-Boulder and the National Oceanic and Atmospheric Administration. A paper by Weatherhead and Signe Bech Andersen of the Danish Meteorological Institute in Copenhagen was featured on the cover of the May 4 issue of Nature.
[2]      Canfei Nesharim featured a two-part series on ozone in 2003. For more information on the chemistry of ozone and ozone depletion, see the article in environmantal issues titled “The Ozone Layer.”
[4]      Global Change Research Information Office (GCRIO), see 5.
[7]      For example, the 1993 eruption of Mt. Pinatubo in the Philippines caused ozone levels to backslide for several years.

Originally posted in “On Eagles’ Wings” June 20, 2006.