Scientists from government and academia have confirmed that most of the gases responsible for stratospheric ozone depletion are produced by human activities and are not naturally occurring in the atmosphere. Measurements of air trapped in polar snowpack (called firn) in Antarctica and Greenland reveal for the first time that the major ozone-depleting gases were not present in detectable amounts in the atmosphere in the late 19th and early 20th centuries.
Ice cores have been analyzed previously for more abundant atmospheric gases such as carbon dioxide, but there is not enough air trapped in ice core samples to enable researchers to detect the part per trillion levels of the ozone-depleting halogen-gases.
The data, which were obtained at two sites in Antarctica and one site in Greenland from depths of up to 120 meters, are consistent with suggested anthropogenic (human-caused) emission histories of these gases. This indicates that human emissions can account entirely for their presence in the atmosphere, according to an article in the current issue of the Journal Nature.
Lead author James Butler, of the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., says that the data confirm that these major chlorofluorocarbons (CFC's), halons, and chlorinated solvents in the atmosphere are entirely produced from human emissions and that contributions from volcanoes or the biosphere are negligible or non-existent.
Suggestions that volcanoes, biota, or other natural sources could be contributing significantly to the atmospheric burden of these ozone-depleting gases were considered plausible by some, because measurements were not available to confirm the absence of these gases in the atmosphere before their anthropogenic emission began in the mid-20th century. Butler says that the measurements made by the team of researchers confirm that these compounds were not around before man began emitting them.
The CFC's, chlorinated solvents, and halons measured in this study are all listed as significant ozone-depleting substances in the amended and adjusted Montreal Protocol on Substances that Deplete the Ozone Layer. The Montreal Protocol is an international agreement to reduce the global production of ozone-depleting substances.
Two other compounds studied by the team, methyl chloride and methyl bromide, showed evidence of significant pre- 20th century sources. Whereas methyl chloride, which may have a natural component as large as 90 percent, is not listed in the Montreal Protocol, methyl bromide, a widely used fumigant, was added to the Protocol in 1992.
This study suggests that methyl bromide was present in the southern hemisphere at about 5.5 parts per trillion (ppt) in 1900, and at about 6.5 ppt before the onset of its use as a fumigant in the early 1960's. Since then, it appears to have increased to about 8 ppt in the southern hemisphere. Unfortunately, northern hemispheric results for methyl bromide were inconclusive: the authors in their analysis determined that this site was subject to other influences and was not representative for methyl bromide.
The team of scientists participating in the study includes Butler, Stephen Montzka, Andrew Clarke, and James Elkins at NOAA/CMDL; Mark Battle and Michael Bender, at Princeton University; Cara Sucher from the University of Rhode Island; Jeffrey Severinghaus from Scripps Institution of Oceanography; and Eric Saltzman, Rosential School of Marine and Atmospheric Sciences, University of Miami. This research was conducted in conjunction with the National Science Foundation's Office of Polar Programs, and was funded in part by NOAA's Climate and Global Change Program, the National Institute of Global Environmental Change, and the Methyl Bromide Global Coalition.
Butler says, "This study demonstrates that the pre-20th century atmosphere was essentially devoid of the long-lived gases currently depleting stratospheric ozone. It underscores the human contribution to these gases in the atmosphere and the need for compliance with current international agreements to bring the atmosphere back to pre-ozone hole conditions."
For more information on this paper, consult the CMDL website at:
www.cmdl.noaa.gov
or the Nature Magazine website at www.nature.com
Journal
Nature