Depletion of the ozone layer: causes, status and recovery
Ozone depletion and its causes
The atmosphere extends a few hundred kilometres above the Earth. It is made of layers that surround the Earth like rings. However, 99% of its total mass lies in two regions within the first 50 kilometres above the Earth's surface. These two regions are called the troposhpere and the stratosphere. The troposhere is closest to the Earth. It extends to about 6 to 17 kilometres above the Earth's surface and is thickest at the equator. The stratosphere extends out, beyond the troposhere, to about 50 kilometres above the Earth. The furthest layer, the mesosphere, is found roughly 50 km to 80 km above sea level.
Ozone depletion is the term commonly used to describe the thinning of the ozone layer in the stratosphere. Ozone depletion occurs when the natural balance between the production and destruction of ozone in the stratosphere is tipped in favour of destruction. Human activity is the major factor in tipping that natural balance, mostly from releasing artificial chemicals, known as ozone-depleting substances (ODS), to the atmosphere. These are stable substances that do not break down in the lower atmosphere and contain either/both chlorine and/or bromine.
The theory about ozone depletion was first put forward in 1974 by American scientists Mario Molina and F. Sherwood Rowland. They were concerned about the impact of CFCs on the ozone layer. Their hypothesis was met with a great deal of scepticism, but scientific work over the next 20 years proved them correct and prompted almost every country in the world to action. In 1995, Drs. Molina and Rowland were given a Nobel Prize in Chemistry, along with a third ozone researcher, Paul Crutzen from the Netherlands.
Ozone-depleting substances containing chlorine include chlorofluorocarbons (CFCs), carbon tetrachloride, methyl chloroform and hydrochlorofluorocarbons (HCFCs). Halons, methyl bromide and hydrobromofluorocarbons (HBFCs) are ODSs that contain bromine.
The best-known and most abundant of the ODS are the CFCs. A single atom of chlorine from a CFC can destroy 100,000 or more molecules of ozone. Ozone depletion only stops when the chlorine randomly reacts with another molecule to form a long-lived, stable substance. At that point it is no longer free to react with ozone.
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While it is true that volcanoes and oceans release large amounts of chlorine, the chlorine from these sources dissolves in water so it washes out of the lower atmosphere in rain. Volcanoes may, in worst case scenarios, cause temporary ozone loss.
Status of ozone depletion
Ozone measurements fluctuate from day to day, season to season and one year to the next. Ozone concentrations are normally higher in the spring and lowest in the fall. In spite of these fluctuations, scientists have determined, based on data collected since the 1950's, that ozone levels were relatively stable until the late 1970's. Observations of an Antarctic ozone "hole" and atmospheric records indicating seasonal declines in global ozone levels provide strong evidence that global ozone depletion is occurring.
Severe depletion over the Antarctic has been occurring since 1979 and a general downturn in global ozone levels has been observed since the early 1980's. The ozone hole over the Antarctic reached record proportions in the spring of 2000 at 28.3 million square kilometres and vertical profiles from stations near the South pole showed complete ozone destruction in the lower stratosphere. Ozone decreases of as much as 70% have been observed on a few days.
Severe ozone depletion was also measured over the Arctic. Lowest values over the Arctic occurred in 2000 north of Sweden, with about 60% depletion in some layers of the atmosphere. In addition to the Earth's poles, ozone depletion now affects almost all of North America, Europe, Russia, Australia, New Zealand, and a sizable part of South America. However, smaller decreases in stratospheric ozone have been observed in mid-latitude regions of the world.
The ozone layer over southern Canada has thinned by an average of about 7% since the 1980s. In the late 1990s, average ozone depletion in the summer over Canada was between 3% and 7%. Ozone depletion in Canada is usually greatest in the late winter and early spring. In 1993, for example, average ozone values over Canada were 14% below normal from January to April.
In their assessment of ozone depletion in 2006, the Scientific Assessment Panel, a group of experts established under the Montreal Protocol, made the following key findings:
- The total abundances of human-made ozone-depleting gases in the troposhere continue to decline from the peak values reached in the 1992-1994 time period.
- The total abundances of human-made ozone-depleting gases in the stratosphere show a downward trend from their peak values of the late 1990s.
- Large Antartic ozone holes continue to occur. The severity of Antartic ozone depletion has not continued to increase since the late 1990s and, since 2000, ozone levels have been higher than in some preceding years.
- Arctic ozone depletion shows large year-to-year variability, driven by meteorological conditions. Over the past four decades, these conditions contributed to severe ozone depletion.
- The decline in stratospheric ozone over mid-latitude (between 60°S and 60°N) seen in the 1990s has not continued.
Recovery of the ozone layer
No one knows for certain how much more ozone depletion will occur. There is a substantial time lag between the time when ODS emissions begin to decline and the point at which the ozone layer begins to recover. It takes years for CFCs and other ozone-depleting compounds to reach the stratosphere. Many of them can persist in the stratosphere for centuries; some have life spans of 25 to 400 years. Almost all of the CFCs and halons ever released are still in the atmosphere and will continue to destroy ozone for many years to come.
In spite of these uncertainties and substantial time lag, the natural balance between ozone creation and destruction can be restored if concentrations of ozone-destroying chemicals are reduced. However, this might require the complete elimination of ozone-destroying chemicals. In addition, there is some concern that the increase in greenhouse gas concentrations may result in delayed ozone layer recovery. Scientists estimate that they will not be able to measure any recovery until 2030.
It is important to note that scientific knowledge of the atmosphere and the processes that deplete the ozone layer is not complete. The sudden and unexpected appearance of the Antarctic ozone hole reveals that the ozone layer does not respond predictably to the quantities of industrial chemicals we are dumping into it.
 The term ozone "hole" refers to a large and rapid decrease in the abundance of ozone molecules, not the complete absence of them. The Antarctic ozone "hole" occurs during the southern spring between September and November.
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