Analysis of several different satellite records and surface monitoring instruments indicates that the ozone layer is no longer declining, according to a study by scientists working with the Center for Integrating Statistical and Environmental Science (CISES) at the University of Chicago.
In some parts of the world, the ozone layer has increased a small amount in the past few years, although it still well below normal levels.
The results will be published Aug. 31 in the Journal of Geophysical Research and follow 18 years after an international agreement, the Montreal Protocol, was established to limit the production of chemicals determined to be harmful to the atmosphere.
The work, funded by the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration and the Environmental Protection Agency, is a collaboration between atmospheric scientists and statisticians through CISES. "The work of this team of scientists and statisticians is widely recognized as some of the most authoritative in the statistical analysis of stratospheric ozone," said Michael Stein, director of CISES at the University of Chicago.
"These early signs indicate one of the strongest success stories of international cooperation in the face of an environmental threat," said retired Navy Vice Adm. Conrad C. Lautenbacher, Ph.D., undersecretary of commerce for oceans and atmosphere and NOAA administrator.
For the past few years, studies have focused on ozone declining in the topmost layer of the atmosphere where there is naturally very little ozone. However, this study addresses the total ozone column layer that has significant impact on how much ultraviolet radiation is coming through the atmosphere, said Betsy Weatherhead of the University of Colorado.
"Our work focuses on the thickness of the ozone layer and is therefore relevant to the amount of harmful ultraviolet (UV) radiation reaching the surface of the Earth," said Weatherhead, a co-author on the paper.
Overexposure to UV radiation can cause an increase in skin cancers and cataracts in the eyes. Scientists warn that skin and eye precautions, such as wearing sunscreen, wide-brimmed hats and sunglasses, still need to be taken.
"This news about the ozone layer is encouraging, but people should not get a false sense of security. Ultraviolet radiation is still dangerous and we urge people to be 'sun smart' when outdoors," said dermatologist Clay J. Cockerell, M.D., president of the American Academy of Dermatology.
Scientists say that ozone in some areas is still quite low compared to historical times and that the return of ozone to normal levels will be slow--likely taking several decades. The chemicals responsible for the ozone depletion can take years to filter up to the stratosphere, where most of the ozone is located, said Weatherhead.
"Some of these chemicals remain in the stratosphere for many decades, meaning that chemicals produced years ago will continue to be harmful for decades to come," said Sherwood Rowland, who, along with Mario Molina and Paul Crutzen, won the 1995 Nobel Prize in chemistry for their work identifying the threat to the ozone layer.
Other factors can affect the recovery process, such as changes in temperature, clouds, volcanic particles, water vapor, methane, and natural variability. Internationally, scientists continue to work to understand the recent changes and the likely future concentrations of ozone.
The lead author of the study, Greg Reinsel of the University of Wisconsin-Madison, was one of the first scientists to quantify the decline in ozone in research papers published more than 20 years ago.
He died unexpectedly after completing this study, the first to show the leveling off of the total ozone layer, Weatherhead said. "The finding of positive signs about the ozone layer represents a bittersweet culmination to over twenty years of effort by Greg and his colleagues," said Michael Stein.
Other co-authors are Alvin Miller, Lawrence Flynn, and Ron Nagatani from NOAA, George Tiao, Professor Emeritus in the Graduate School of Business at the University of Chicago, and Don Wuebbles of the University of Illinois.
The Center for Integrating Statistical and Environmental Science was established in 2002. Affiliated with a number of universities and federal agencies, the center develops multidisciplinary statistical methods for more precise environmental risk assessments. CISES is wholly funded through a Science to Achieve Results (STAR) grant from the United States EPA.
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