Depletion of the ozone layer in the 21st century.

Today the topics “ozone layer” and “ozone hole” have been placed on the back burner in public discussions of climate change. Since the mid-1980s it has been known that primarily chlorofluorocarbons (CFCs) and also halocarbons (compounds where carbon atoms are linked to fluorine, chlorine, bromine, or iodine, but also to hydrogen) are mainly responsible for the destruction of the ozone layer in the stratosphere (atmospheric layer at an altitude of 12 to 50 km). Hence the production and usage of these substances were nearly fully prohibited in the Montreal Protocol (1987) and in subsequent agreements. As a consequence of these international treaties the strong increase of CFC concentrations in the troposphere (atmospheric layer up to an altitude of about 12 km) has halted (Figure 1). Since the mid-1990s the amount of CFCs in the troposphere has been decreasing. As a result, a reduction in the stratospheric chlorine concentration has been observed in recent years. Therefore it is expected that the ozone layer will increase in thickness and that the ozone hole over the Antarctic will disappear. Owing to the long lifetimes of CFCs in the atmosphere, it will take until about the middle of this century before the stratospheric chlorine level returns to the values measured in the 1960s. One could conclude that the ozone layer will fully recover by the middle of this century. However, the reconstruction of the ozone layer also depends on other atmospheric processes which complicate a reliable assessment of the future evolution. As a result of increased concentrations of well-mixed greenhouse gases in the atmosphere (e.g. CO2, CH4, and N2O), the troposphere warms (greenhouse effect) and the stratosphere cools (enhanced emission of long-wavelength thermal radiation). A multitude of chemical reactions in the atmosphere depend on the predominating temperature. For example, the ozone content in the middle and upper stratosphere increases with decreasing temperature since the most important ozone-destroying reactions (homogeneous gasphase reactions) slow down. In contrast, lower temperatures in the lower stratosphere over the polar regions cause stronger ozone depletion as a result of heterogeneous chemical reactions on very cold cloud particles. As a consequence of changes in the thermal structure of the atmosphere, dynamic processes in the atmosphere are changing and impacting the distribution of the trace gases that have longer lifetimes. So far, studies with numerical atmospheric models, so-called climate–chemistry models (CCMs), have not provided a consistent picture with regard to the speed of ozone recovery. Results derived from model simulations agree on the point that the regeneration of the ozone layer will develop with regional differences. Model calculations indicate that, overall, processes related to climate change will cause an accelerated recovery of the ozone layer. Most CCMs predict a return to the ozone levels observed in the 1960s clearly before the middle of this century (Figure 2). Figure 1. Hemispheric monthly mean values of tropospheric mixing ratios (in ppt=parts per trillion=10 ) of the most important CFCs (CFC-12=CF2Cl2, CFC-11=CFCl3, and CFC-113=Cl2FC-CClF2). Crosses indicate measured values for the northern hemisphere, triangles for the southern hemisphere. Recent results are shown in the insets. (Taken from Figure 1-1 in Ref. [1].) AGAGE=Advanced Global Atmospheric Gases Experiment, ESRL=Earth System Research Laboratory, UCI=University of California, Irvine.