October 17th, 2007 10:31 EST
Effect of Climate Change on Recovery of Ozone Hole Remains Unclear
This is the first article in a two-part series about the Montreal Protocol and stratospheric ozone.
Washington -- For 20 years -- since September 16, 1987, when representatives of 24 nations signed the first accord -- the Montreal Protocol on Substances that Deplete the Ozone Layer increasingly has limited the production and use of nearly 100 substances that destroy stratospheric ozone.
The agreement, ratified by 191 countries, has helped cut production of ozone-depleting chemicals from more than 1.8 million metric tons in 1987 to 83,000 metric tons at the end of 2005.
Because of broad international compliance with the protocol and development by chemical manufacturers of less-harmful substitutes for the controlled substances, the ozone layer -- Earth's protection in the stratosphere against the sun's harmful ultraviolet radiation -- has not grown thinner since 1998 over most of the globe and is projected to return to pre-1980 levels by 2050 to 2075.
The projected recovery, especially of the severe and recurring ozone depletion in springtime over Antarctica, called the ozone hole, depends on continued compliance with the protocol's provisions and, scientists are discovering, the ultimate effects of climate change.
"When these predictions were made, the issue of climate was not so much on the table," Guy Brasseur, associate director of the U.S. National Center for Atmospheric Research in the Earth and Sun Systems Laboratory, told USINFO.
"We now know that as climate is changing, we have a change of temperature in the stratosphere, which is cooling while the surface is warming," he added. "The amount of water in the stratosphere could change, and so the link between the ozone problem and the climate problem becomes important and could accelerate or delay the recovery of ozone."
INTO THE STRATOSPHERE
Ozone (O3), discovered in laboratory experiments in the mid-1800s, is a gas that is naturally present in the atmosphere. About 10 percent of ozone is in the troposphere, from the Earth's surface upward 10 kilometers to 16 kilometers. The rest is in the stratosphere, from the top of the troposphere to an altitude of 50 kilometers.
Ozone helps life on Earth or hurts it, depending on its location in the atmosphere. Near the surface, in the troposphere, ozone levels increased by human-made pollutants can warm Earth's surface, limit crop yields and forest growth and reduce human lung capacity, causing chest pains, throat irritation and coughing.
Above the troposphere, stratospheric ozone absorbs some of the sun's biologically harmful ultraviolet (UV-B) radiation; protects people from increased risk of skin cancer, cataracts and suppressed immune systems; and protects animal and plant life from a range of damage.
Ozone forms throughout the atmosphere in a series of chemical steps involving sunlight.
In the stratosphere, sunlight breaks apart an oxygen molecule (O2), producing two oxygen (O) atoms. Each oxygen atom combines with an oxygen molecule to produce ozone (O + O2 = O3). The reactions occur wherever sunlight is present in the stratosphere, and ozone production is balanced with its destruction by natural and human-made gases in the stratosphere.
Gases that are most damaging to stratospheric ozone contain the chemicals chlorine and bromine, called halogens, and various source gases transport both into the stratosphere. Some of the most destructive chlorine source gases are human-made chlorofluorocarbons (CFCs).
CFCs were developed in the 1930s and were used in industrial, commercial and household applications because they are nontoxic, nonflammable and do not react with other chemical compounds, at least not at the Earth's surface.
CFCs are used in coolants for commercial and home refrigeration units, aerosol propellants, electronic cleaning solvents, foam-blowing agents and other products.
"These were great compounds when they were first invented," Anne Douglass, deputy project scientist for NASA's Aura spacecraft, told USINFO. Aura’s instruments monitor the atmosphere's chemical composition and gather data that help researchers better understand ozone chemistry through computer models.
Before CFCs, she added, "electric refrigerators were dangerous because of the gases they had in them -- including ammonia -- and they weren’t safe. The thing that makes these compounds dangerous to ozone, which is that they don't get broken apart unless they're at 30 kilometers [altitude], is the exact thing that makes them safe for people."
In 2006, some 310 international scientific experts released the Scientific Assessment of Ozone Depletion: 2006. The study was sponsored by the World Meteorological Organization (WMO)/U.N. Environment Programme (UNEP)
"As a result of the Montreal Protocol," the report reads, "the total abundance of ozone-depleting gases in the atmosphere has begun to decrease in recent years. If the nations of the world continue to follow the provisions of the Montreal Protocol, the decrease will continue throughout the 21st century."
So the protocol is working as intended in terms of atmospheric abundances of ozone-depleting substances, but questions remain.
"Right now, we are in the accountability phase of the protocol," A.R. Ravishankara, director of the Chemical Sciences Division of the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Colorado, told USINFO. "In other words, did it work as intended and are there things we don't know, are there things we need to know, and are there other angles we should be thinking about?"
Added Ravishankara, a co-chair of the WMO-UNEP Scientific Assessment Panel, "The citizens of the world came together through their representatives and made the protocol. Now, I think it's perfectly appropriate for them to ask, did it work as intended?"
More information about the Montreal Protocol is available at the UNEP Web site.
The full text of Scientific Assessment of Ozone Depletion: 2006 is available at the NOAA Web site.
(USINFO is produced by the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)
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