"This is the beginning of a recovery of the ozone layer," said professor Michael Newchurch of the University of Alabama in Huntsville (UAH), the scientist who led the ozone trend-analysis research team. "We had a monumental problem of global scale that we have started to solve."
Using data from three NASA satellites and three international ground stations, the team found that ozone depletion in the upper stratosphere -- the layer of the atmosphere between 35 and 45 km [22-28 miles] above the ground -- has slowed since 1997. "We are extremely pleased to have the highly calibrated, long term satellite and ground-based data records necessary to observe these small, but important changes in the ozone layer," said Newchurch. The results of this work have been accepted for publication in the American Geophysical Union's Journal of Geophysical Research - Atmospheres.
Ozone is a damaging pollutant in the lower atmosphere near the ground, but in the stratosphere, it shields the Earth from harmful ultraviolet solar radiation. Almost 30 years ago, scientists Mario Molina, F. Sherwood Rowland, and Paul Crutzen showed that chlorine released into the stratosphere from chlorofluorocarbons (CFC), chemicals used as refrigerants and aerosol propellants, was destroying the protective ozone layer. This discovery led to an international ban on CFC-based products and to the 1995 Nobel Prize in chemistry for the three scientists.
"There have been several amendments to that ban, each of which tightened restrictions on CFCs and other halogenated hydrocarbons," said Newchurch, an associate professor of atmospheric science at UAH. "We are now at the point where the restrictions are tight enough to result in measured turnaround of CFC amounts at the surface. Now, we can say that what we're doing is working, and we should continue the ban.
"We're not gaining ozone, we're just losing it less quickly. But the trend line is flattening. And the amount of chlorine in that layer of the stratosphere has not yet peaked, but has slowed down significantly, so we should start to see some ozone improvement in the coming years," he said.
The slowing of ozone destruction is seen only in the upper stratosphere, where ozone depletion is due primarily to chlorine pollution, Newchurch said. "But there's not much ozone up there, and it has a small effect on the total ozone column. We don't see compelling evidence that the destruction of ozone is slowing in the lower stratosphere, where 80 percent of the protective ozone layer exists." Many factors, including chlorine levels, influence ozone depletion in the lower stratosphere, the layer of atmosphere between about 20 and 35 kilometers [16-22 miles] up.
"Fixing the chlorine problem is never, by itself, going to solve the lower stratosphere problem," Newchurch said. "There are many things that push the lower stratosphere." One of those things is the concentration of greenhouse gases, such as carbon dioxide and methane. While these gases warm the lower atmosphere, they cool the stratosphere by radiating heat out to space.
Cooling the stratosphere has both good and bad effects on ozone destruction, Newchurch said. Cooling the air in the upper stratosphere slows the rate of chemical destruction reactions, thereby increasing the ozone amounts. In the lower stratosphere, cooling also changes wind and air mixing patterns in a way that can increase ozone depletion, especially in high latitudes.
Newchurch's co-investigators included Eun-Su Yang, now at the Georgia Institute of Technology, Derek M. Cunnold at Georgia Institute of Technology, Gregory Reinsel at the University of Wisconsin, Joseph M. Zawodny at NASA's Langley Research Center, and James Russell at Hampton University. They analyzed satellite measurements of ozone, hydrogen chloride, and greenhouse gases, along with ground-based measurements of ozone and solar activity.
"As the satellites orbit Earth, they see a sunset and a sunrise once every 90-minute orbit," Newchurch said. "The instruments look at the Sun as it sets or rises, as the sunlight is filtered through the atmosphere. Because ozone and other constituents absorb light at known wavelengths, we can measure how much light at those wavelengths is coming through the atmosphere and calculate from that the amount of ozone and other gases." The team used data from three NASA Earth-observing satellites: SAGE 1, which operated from 1979 through 1981; and SAGE II, which went into orbit in October 1994, and the HALOE instrument aboard NASA's UARS satellite, launched in 1991, which are still operating.
"SAGE and HALOE instruments both look at the atmosphere, but they have very different techniques and are on different satellites," said Newchurch. "The fact that they both see the same trend in the ozone and see the same trend as the ground-based instruments is very compelling evidence that they are both right."
Released into the atmosphere, CFC molecules will take several years to "percolate" upward into the stratosphere. As they rise, ultraviolet light them breaks up, releasing the chlorine. This free chlorine reacts with ozone, converting two ozone molecules into three oxygen molecules. Eventually, most of the chlorine bonds with hydrogen atoms to form nearly inert hydrogen chloride, which over a period of years drifts into the lower atmosphere. There, it dissolves into water vapor and is rained out of the atmosphere. The entire process takes decades," Newchurch said.