Low stratospheric temperatures at the altitude of the ozone layer are a prerequisite for Arctic ozone depletion. Temperatures inside the Arctic polar vortex show a large variability between different years. The polar vortex is a low-pressure system in the stratosphere at an altitude of 15 to 50 kilometres that forms over the Arctic every autumn and may persist until spring. Chlorine, which is normally present in chemically inactive reservoir gases, can be released from these gases at the surface of polar stratospheric clouds at low temperatures. Subsequently, chlorine species in combination with bromine destroy ozone when exposed to sunlight.
The relation between column ozone loss accumulated during a winter/spring season and the area of the vortex below the formation temperature of polar stratospheric clouds is linear to a good approximation (Rex et al., 2004; 2006; Tilmes et al., 2006; Pommereau et al., 2018). The Match campaigns (see preceding segment) provide the amount of ozone loss during many Arctic winter/spring seasons in the past for this study. As expected lower temperatures lead to more ozone losses. Moreover, Rex et al. (2004, 2006) showed that there is a tendency for cold stratospheric Arctic winters to get colder. This trend was confirmed by von der Gathen et al., (2021) with the help of several state-of-the-art meteorological data sets, i.e. ERA5, MERRA-2, JRA-55, and CFSR/CFSv2.
Von der Gathen et al., (2021) investigated the future stratospheric temperature trend in the polar vortex in the output from 53 computer models of the international “Coupled Model Intercomparison Project Phases 5 and 6” (CMIP5, CMIP6). Depending on the greenhouse gas emission scenarios used in these models the trends project into the future with different magnitude, with larger temperature trends related to stronger greenhouse gas emissions. That finding clearly shows that the stratospheric temperature trend in the polar vortex is part of climate change and therefore the product of global greenhouse-gas emissions.
Despite the production ban issued in the 1987 Montreal Protocol, substances like chlorofluorocarbons (CFCs) and halons, which contain ozone-destroying chlorine and bromine atoms, are still abundant in the atmosphere, because of their long lifetimes. The concentrations of these substances in the polar vortex continued to rise until the year 2000. Since then, they have been on decline and are currently (2021) at roughly 90 percent of the maximum. Only by the end of the century they will reach values below 50 percent of the maximum (WMO, 2018). These reductions will eventually lead to a recovery of the global ozone layer and even to a super-recovery in mid-latitudes, where gas-phase catalytic destruction cycles are prevalent. I.e., in the global mean, there will be more ozone in the stratosphere 2100 compared to the years before 1980. However, the situation inside the Arctic polar vortex can be different due to the strength of the climate change and the related temperature trend inside the vortex. In an extreme emission scenario the amount of ozone losses can even increase until the end of the century (von der Gathen et al., 2021).
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