Researchers have been successful in identifying the relationship between a reduction of arctic ozone and climate changes. Current observations show that the arctic ozone layer reacts much more sensitively to climate changes than predicted from earlier model calculations. A study published in “Geophysical Research Letters” (see below) demonstrates that, within the last 40 years, climate conditions in the stratosphere have facilitated the ozone decline in the Arctic. After analysis of ozone measurements from the past 12 years, researchers were able to determine precisely the impacts of temperature variations on arctic ozone losses: “For each degree Celsius of cooling, we must expect an additional ozone loss of 15 Dobson units. This is three times more than in current model calculations”, explains Dr Markus Rex, leader of the study from the Potsdam Research Unit of the Alfred-Wegener-Institute of Polar and Marine Research (AWI).
Climate conditions in the stratosphere facilitate ozone reduction
Over the next 50 years, the slow decay of previously liberated chlorofluorocarbons (CFCs) and halons will be a source of ozone destroying chlorine and bromine, despite the fact that the production of these substances is now strictly regulated worldwide and, for the most part, prohibited. Usually, liberated chlorine atoms are chemically bound in substances that do not damage the ozone layer. During the polar winter, however, when temperatures in the stratosphere at 15 to 25 km altitude fall to extremely low values, so-called 'polar stratospheric clouds' (PSC) are formed. These clouds cause the conversion of passive chlorine compounds into highly reactive radicals, which are extremely effective in destroying ozone as soon as the sun returns to polar regions after the polar night.
Cold Winters- indicators for ozone losses
An evaluation of meteorological data from the last 40 years demonstrates that arctic winters have become considerably colder. These cold winters determine the extent of ozone losses in the arctic stratosphere. Winter cooling has increased the maximum dimensions of PSCs threefold over the last 40 years. Researchers attribute extensive ozone losses during several winters of the 1990s to this climate change: “If we still had the stratospheric climate conditions that prevailed in the 1960s, today's arctic ozone losses would not be of such concern, despite of CFC emissions”, says Rex and with his statement stresses the importance of climate changes for ozone depletion. The cooling of the stratosphere is an expected side effect of the anthropogenic greenhouse phenomenon: warm radiation from the earth’s surface is retained in the lower atmospheric layers, resulting in a cooling of the overlying stratosphere. In addition, internal variability of the climate system and other, as yet unknown, factors can contribute to the observed cooling.
Over the next several decades, any further trend in ozone loss will depend on the relationship between the slow decrease of CFC concentrations on the one hand, and a potential cooling of the stratosphere caused by rising concentrations of greenhouse gasses on the other. “If, in the future, cooling were to exceed the levels observed to date by several degrees Celsius, we will have to expect strong additional ozone losses,” says Markus Rex and adds “at present we need not fear that conditions in the Arctic will be as bad as in the Antarctic where, on account of much lower temperatures, a gaping hole develops in the ozone layer every year.”
(„Arctic ozone loss and climate change“, M. Rex (AWI), R.J.Salawitch (Jet Propulsion Laboratory, California Institute of Technoloy, NASA, USA), P. von der Gathen (AWI), N.R.P. Harris, (European Ozone Research Coordinating Unit, Cambridge, UK), M.P. Chipperfield (Univ. Leeds, UK), B. Naujokat (Freie Universitaet Berlin), volume 31, doi:10.1029/2003GL018844, March 1,
2004.Bremerhaven, April 28 2004
