By Cheryl Pellerin
Washington File Staff Writer

Washington - After 17 years of work to phase out the production and use of chlorofluorocarbons (CFCs), chemicals that harm the Earth’s protective ozone layer, ozone depletion has stopped, scientists say, but has not yet begun to reverse.

Experts predict that the ozone layer could be completely restored by 2060-2065 if releases are halted of manmade substances containing the chemicals chlorine (such as CFCs) and bromine, and if Earth’s surface has not warmed by then – a condition that could increase the damaging effect of CFCs on ozone.

“If you look at the ozone records from all over the world,” said David Hofmann, a scientist from the U.S. National Oceanic and Atmospheric Administration (NOAA), during an August 22 press briefing, “the data indicates that the reduction in ozone has stopped.”

He added, “We have not seen what you might call the second stage of ozone recovery, where ozone begins increasing again in Antarctica.”

A British Antarctic survey team discovered a hole in the ozone layer over Antarctica in 1985. Theories about the cause included solar activity that affected the magnetic field, atmospheric motions and chemical reactions involving CFCs, which had been used since the 1930s as refrigerants and propellants, such as aerosol sprays.

Hofmann, now director of the Global Monitoring Division, and NOAA senior scientist Susan Solomon were members of the first team, called the National Ozone Expedition, to arrive in Antarctica in August 1986 to determine the cause of a hole in the Antarctic ozone layer.

AFFECTING THE CLIMATE SYSTEM

The expedition, funded by NASA, NOAA and the U.S. National Science Foundation, which operates McMurdo Station at the South Pole, consisted of four teams of scientists from NOAA, NASA and two U.S. universities - the State University of New York at Stoneybrook and the University of Wyoming.

Their observations – the first definitive demonstration that people and their activities could affect the planet’s climate system – helped determine the chemical basis for the ozone loss and formed the scientific underpinning for the Montreal Protocol on Substances that Deplete the Ozone Layer.

The protocol is a 160-nation international treaty that entered into force in 1989 and phased out the production of ozone-depleting CFCs and other substances in industrialized countries and led to the current decline in these ozone-depleting gases. Since 1989, it has been revised five times – in 1990 (London), 1992 (Copenhagen), 1995 (Vienna), 1997 (Montreal) and 1999 (Beijing) – to speed up the phase out of CFCs and other compounds.

“These gases live for 50 to 100 years in the atmosphere,” said Solomon, winner of the 2004 Blue Planet Award and the 1999 National Medal of Science for her work. “So they’re going to remain present, depleting ozone long after we stop emitting them.”

The observation that ozone destruction has stopped, she added, “certainly shows that the Montreal Protocol has been effective at starting us on the path to ozone recovery, so it’s an unprecedented global environmental success story."

PROTECTIVE OZONE

Ozone is a relatively unstable molecule made up of three oxygen atoms (O₃), rather than the two oxygen atoms that make up the breathable oxygen (O₂) in the atmosphere.

Ozone is generated from some kinds of pollution and from natural sources. Depending on where ozone is in the atmosphere, it can protect or harm life on Earth.

Closer to Earth, in the troposphere - the layer of the atmosphere from the surface to about 10 kilometers up - ozone is a harmful pollutant that damages human health, vegetation and many common materials. It is a key ingredient of urban smog.

In the stratosphere, between 16 kilometers and 48 kilometers above the planet, a 24-kilometer-thick ozone layer acts as a shield to protect Earth's people, animals and plant life from the sun's harmful ultraviolet (UV) radiation.

The protective role of the ozone layer in the stratosphere is so important that scientists believe life on land probably would not have evolved - and could not exist today - without it.

OZONE AND CHLORINE

The chemical chlorine is the main problem in ozone depletion, and most chlorine in the stratosphere comes from human activity, specifically the release of CFCs.

Because of their low toxicity, valuable physical properties and chemical stability in the lower atmosphere, CFCs have been used heavily since the 1960s as refrigerants, industrial cleaning solvents, propellants in aerosol spray cans and in Styrofoam manufacture.

Once in the atmosphere, though, CFCs drift slowly upward - it takes five years or six years for gases released on the surface to reach the stratosphere - where they are broken up by UV radiation, releasing the chlorine that chemically destroys ozone.

First, UV radiation breaks off a chlorine atom from a CFC molecule. The chlorine atom attacks an ozone molecule (O₃), breaking it apart and destroying the ozone. The result is an ordinary oxygen molecule (O₂) and a chlorine monoxide molecule (Cl+O).

Then, a single oxygen atom (O) attacks the chlorine monoxide molecule, releasing the chlorine atom and forming an ordinary oxygen molecule (O₂). The chlorine atom now is free to attack and destroy another ozone molecule (O₃). One chlorine atom can repeat this destructive cycle thousands of times.

OZONE AND GLOBAL WARMING

Ozone depletion and global warming are not directly related – increasing concentrations of carbon dioxide in the atmosphere are the main cause of climate change, and chlorine, bromine and other ozone-depleting chemicals in the upper atmosphere deplete ozone.

“What the two problems have in common, it’s fair to say,” Solomon said, “is that both are linked to the question of the long lifetimes of gases in the atmosphere.”

CFCs last from 50 years to 100 years in the atmosphere, so it will take that much longer for the ozone layer to recover even after decreasing CFC emissions. The same is true for carbon dioxide.

“The carbon dioxide that’s in our atmosphere today – even if we were to stop emitting it tomorrow – would live for many decades, centuries and beyond,” Solomon said. “A fraction of the carbon dioxide that we’ve put into the atmosphere today due to human activity would still be there in 1,000 years.”

To monitor ozone-depleting gases in the atmosphere, NOAA has developed a new Ozone Depleting Gas Index (ODGI) - a number based on NOAA's measurements of all the ozone-depleting gases that indicates the atmosphere’s recovery toward pre-ozone-hole conditions.

Hofmann said the ODGI indicates that the ozone-depleting potential of the gases already has declined, in agreement with the early signs of leveling off of ozone loss and the success of the Montreal Protocol.

“I’m very optimistic that we will have a normal ozone layer sometime,” Hofmann said, “not in my lifetime, but perhaps in yours.”

More information about the ozone layer and the ozone hole are available on the NOAA Web site. A history of the ozone hole discovery is available on the National Science Foundation Web site.