During the Airborne Arctic Stratospheric Expedition in 1989, scientists found high levels of chlorine compounds in the Arctic stratosphere. They observed a 17% decrease in ozone levels on two flights, caused by chlorine compounds and polar stratospheric clouds. Models predict further ozone depletion in late March 1989. Ground-based measurements showed ozone variations and a strong increase, likely due to stratospheric warming. The impact of chlorine on ozone levels was not clearly seen due to these variations.