Twenty five years ago, scientists and politicians unveiled the Montreal Protocol on Substances that Deplete the Ozone Layer, regarded by the United Nations (UN) as “the most successful treaty in UN history.” On 16 September 1987, 24 nations signed on to the protocol; 173 more have signed on in the subsequent years. The international agreement likely solved a global environmental crisis, while presenting a model of developing and employing environmental policy.
Urged by data from the laboratory, aircraft, and satellites, the Montreal Protocol first reduced and then banned the chlorine- and bromine-based chemicals (particularly chlorofluorocarbons, or CFCs) that destroy atmospheric ozone. The destruction of the ozone layer allows more of the Sun’s ultraviolet radiation to reach the ground, increasing the threats of sunburns, skin cancer, and eye damage. The most obvious indication of the depletion is the annual “ozone hole” that forms around the South Pole.
The four images of Fig. 1 show maps of the ozone hole on 16 September (the International Day for the Preservation of the Ozone Layer) in the years 1979, 1987, 2006, and 2011. The first two maps have been created using data from the Nimbus-7 satellite. The other two maps were produced with data acquired by the Aura satellite. Though taken by different instruments, the data sets have all been examined and reanalyzed by scientific models.
Ozone in the stratosphere is usually measured in Dobson Units (DU), which is the number of molecules required to create a layer of pure ozone 0.01 mm thick at a temperature of 0 degrees Celsius and an air pressure of 1 atmosphere (the pressure at the surface of the Earth). The average amount of ozone in the atmosphere is 300 DU, equivalent to a layer 3 mm thick.
In 1979, when scientists were just coming to realize that atmospheric ozone could be depleted, the area of ozone depletion over Antarctica grew to 1.1 million km2, with a minimum ozone concentration of 194 DU. In 1987, as the Montreal Protocol was being signed, the area of the hole reached 22.4 million km2, and ozone concentrations decreased to 109 DU. By 2006, the ozone depletion measurements were 29.6 million km2 and just 84 DU. By 2011, the data were 26 million km2, and 95 DU.
According to NASA scientist Pawan Bhartia, “The Antarctic hole is stabilizing and may be slowly recovering. Our focus now is to make sure that it is healing as expected.” The amount of ozone-depleting substances (ODS) in the atmosphere has ceased to rise in recent years, and may actually be decreasing. The annual ozone hole should remain for a while, though, as CFCs and other ODSs can persist for decades in the atmosphere. Scientists found in a 2009 study that without the Montreal Protocol, global ozone depletion (not just Antarctic) would be at least 10 times greater than current levels by 2050.
“Changes in the ozone hole now are not significantly driven by changes in CFCs, but instead driven by year-to-year changes in weather in the stratospheresphere; said Bhartia, who in 1985 was the first researcher to present satellite data showing the Antarctic ozone hole. “Like two snowflakes, two ozone holes are never alike.”
References
NASA’s Earth Observatory
http://earthobservatory.nasa.gov/IOTD/view.php?id=79198