Stratospheric Ozone Depletion - The Changing Arctic Environment: The Arctic Messenger - page 110

ozone formation and affects its transport into the stratosphere. In addition, increased tropo-
sphericlevelsofmethane(CH 4 )willresultinmoremethanetransportedtothestratosphere.
Here, methane interacts with the chlorine that is destroying ozone and converts it into an
inactive form.
Therefore, although the WMO 2010 report projected that global ozone levels will in-
crease roughly as the levels of depleting substances decline, the increase is expected to be
accelerated by the greenhouse gas-induced cooling of the upper stratosphere and changes
in stratospheric circulation. This suggests that a return to pre-1980 ozone levels at Northern
Hemisphere mid-latitudes could occur by (or before) 2050 and even rise above 1980 levels
by the latter part of the present century. The return of the lower-stratospheric ozone layer
over the Arctic to pre-1980 levels is also expected to be accelerated by the strengthening of
the stratospheric circulation and by stratospheric cooling. Ozone dynamics in the Antarctic
are thought to be less sensitive to global warming because changes in stratospheric circu-
lation are expected to be less pronounced. The return of ozone levels to pre-1980 levels
over southern mid-latitudes is not anticipated until the second half of the present century
because of the slower recovery expected over Antarctica.