Concern over stratospheric ozone depletion has restricted or eliminated production of many halocarbons. The phase-out of human-produced halocarbons was the result of the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. As a result of these efforts, mixing ratios of many ozone-depleting gases have been declining at the earth's surface in recent years; this decline continued in 2005. Reports from many laboratories around the world that perform measurements of halocarbons show that tropospheric mixing ratios of CFC-12, the longest-lived and most abundant human-made ozone-depleting gas in the atmosphere, peaked within the last few years. These measurements also show that mixing ratios of some halogenated gases continue to increase globally. The most rapid increases are in HCFCs and HFCs, which are chemicals commonly used as replacements for chlorofluorocarbons (CFCs), halons, and other ozone-depleting gases. Although HCFCs contain chlorine (Cl) and deplete O3 with a reduced efficiency compared to CFCs, HFCs do not participate in O3 destroying reactions.
Changes in the direct radiative influence of long-lived halocarbons can be estimated from observed changes in atmospheric mixing ratios with knowledge of trace gas radiative efficiencies. Such an analysis suggests that the direct radiative forcing of these gases was still increasing in 2005, though at a much slower rate than observed from 1970 to 1990.
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