As a consequence of the Montreal Protocol and several international amendments on the ban of industrially produced CFCs the concentrations of ozone destroying halogens are expected to decrease significantly early next century, diminishing the threat of an Arctic ozone hole. In the issue of 26 March 1999 of the scientific journal SCIENCE an article has been published by a consortium of German, Swiss and British scientists, under the leadership of the Max Planck Institute for Chemistry in Mainz, which raises questions about a rapid future mitigation of ozone losses in the Arctic stratosphere. Rather, based on a modeling study in combination with balloon and aircraft-borne observations the scientists call attention to the possibility that future climate changes might counterbalance the desired positive effect on ozone due to reduced stratospheric chlorine loadings.
While future climate change is expected to heat up the lower atmosphere, it is likely to cool the air at the altitude of the ozone layer in the stratosphere. As a consequence, this might increase the occurrence of polar stratospheric clouds, which are believed to play an essential role in ozone hole chemistry. In particular, the cloud particles may sediment to lower atmospheric layers, thereby removing trace gases which under normal circumstances lead to a deactivation of ozone-destroying chlorine. In other words, in a colder stratosphere the sedimenting particles may inhibit an important mechanism protecting ozone.
The scientists find this sedimentation process to be very sensitive to temperature changes. This process has been known for years to lead to massive chemical changes in the trace gas distribution of the Antarctic stratosphere, which is about 10 degrees colder than the Arctic. In their present study the researchers see first signals for such sedimentation processes also in recent winters in the Arctic, which have been particulary cold. They conclude that conditions in the Arctic are presently at a threshold for vertical redistribution of ozone-protecting trace gases due to the precipitation of the cloud particles. In the future, due to greenhouse gas emissions these processes might become strongly enhanced and postpone the anticipated healing of the ozone layer over the northern polar region to late in the next century.
Published: 22-3-99
Contact: Thomas Peter
Atmospheric Sciences (LAPETH),
Zürich/Switzerland
Phone: 41-1-633-2755
Fax: 41-1-633-1058
Journal
Science