New Directions: Stratospheric ozone recovery in a changing atmosphere

About 90% of the ozone in the Earth’s atmosphere resides in the stratosphere, forming the ‘‘ozone layer’’ which shields life on Earth from harmful ultraviolet radiation. In the mid-1970s, it was recognized that anthropogenic chlorofluorocarbons (CFCs) could deplete the ozone layer. Observations of the ozone layer indicated that depletion was occurring due to the buildup of CFCs and Halons in the stratosphere. The most severe depletion was discovered over the Antarctic in the mid-1980s, which is commonly referred to as the ‘‘ozone hole’’. To protect the ozone layer, the Montreal Protocol was crafted in 1987 by the United Nations to address this global problem. Fig. 1 shows the predicted trends in the stratospheric chlorine concentration as specified by the Montreal Protocol and its subsequent amendments. Many observations in the stratosphere indicate that the total chlorine abundance is at or near a peak, in agreement with the predictions shown in Fig. 1. Future ozone recovery is primarily linked to halogen loading in the stratosphere. Assuming global compliance with the Montreal Protocol, the ozone layer is expected to recover by the middle of the 21st century (Fig. 2). Time-dependent data and model simulations of ozone are shown in Fig. 2. The range of model predictions come from various models that utilize different assumptions about the future climate and composition of the atmosphere. These assumptions account for estimated differences in atmospheric composition between 1980, before the ozone hole began, and 2050. The 1992 Pinatubo volcanic aerosols caused the drop in the measured and modeled ozone concentration seen in Fig. 2. Cloud and aerosol surfaces increase the ozone depletion potential of reactive halogen and nitrogen gases through heterogeneous chemistry. An outcome not included in models is the occurrence of one or more large eruptions in the coming decades. Such events can delay ozone recovery for several years following each eruption.