Scientific Summary Chapter 3: Polar Ozone: Past, Present, and Future
Polar Ozone Changes
As stated in the previous Assessments, ozone-depleting substance (ODS) levels reached a maximum in the polar regions around the beginning of this century and have been slowly decreasing since then, consistent with the expectations based on compliance with the Montreal Protocol and its Amendments and adjustments. Considering the current elevated levels of ODSs, and their slow rate of decrease, changes in the size and depth of the Antarctic ozone hole and in the magnitude of the Arctic ozone depletion since 2000 have been mainly controlled by variations in temperature and dynamical processes.
Over the 2010–2013 period, the Antarctic ozone hole continued to appear each spring. The continued occurrence of an Antarctic ozone hole is expected because ODS levels have declined by only about 10% from the peak values reached at the beginning of this century.
Larger year-to-year variability of Antarctic springtime total ozone was observed over the last decade compared to the 1990s. The main driver of this pronounced variability has been variations in meteorological processes, notably the occurrence of dynamically induced disturbances of the Antarctic polar vortex.
A small increase of about 10–25 Dobson units (DU) in springtime Antarctic total ozone since 2000 can be derived by subtracting an estimate of the natural variability from the total ozone time series. However, uncertainties in this estimate and in the total ozone measurements preclude definitive attribution of this increase to the reduction of ODSs over this period.
Exceptionally low ozone abundances in the Arctic were observed in spring of 2011. These low ozone levels were due to anomalously persistent low temperatures and a strong, isolated polar vortex in the lower stratosphere that led to a large extent of halogen-induced chemical ozone depletion, and also to atypically weak transport of ozone-rich air into the vortex from lower latitudes. State-of-the-art chemical transport models (CTMs), which use observed winds and temperatures in the stratosphere together with known chemical processes, successfully reproduce the observed ozone concentrations.
Understanding of Polar Ozone Processes
Since the last Assessment, new laboratory measurements have strengthened our knowledge of polar ozone loss processes. Simulations using updated and improved models have been tested using the wealth of currently available measurements from satellites, ground-based networks, and dedicated campaigns.
CTMs are generally able to reproduce the observed polar chlorine activation by stratospheric particles and the rate of the resulting photochemical ozone loss. Since the last Assessment, better constraint of a key photochemical parameter based on recent laboratory measurements, i.e., the ClOOCl (ClO dimer) photolysis cross section, has increased confidence in our ability to quantitatively model polar ozone loss processes in CTMs.
Chemistry-climate models (CCMs), which calculate their own temperature and wind fields, do not fully reproduce the range of polar ozone variability. Most CCMs have limitations in simulating the temperature variability in polar regions in winter and spring, as well as the temporal and spatial variation of the polar vortex.
Future Changes in Polar Ozone
Projections of future ozone levels in this Assessment are mainly based on the CCM simulations used in the last Assessment. Individual studies using results from climate models provide new insights into the effects of carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) on future polar ozone levels by the end of this century.
Arctic and Antarctic ozone abundances are predicted to increase as a result of the expected reduction of ODSs. A return to values of ozone in high latitudes similar to those of the 1980s is likely during this century, with polar ozone predicted by CCMs to recover about 20 years earlier in the Arctic (2025–2035) than in the Antarctic (2045–2060). Updated ODS lifetimes have no significant effect on these estimated return dates to 1980 values.
During the next few decades, while stratospheric halogens remain elevated, large Arctic ozone loss events similar to that observed in spring 2011 would occur again under similar long- lasting cold stratospheric conditions. SCCM simulations indicate that dynamic variability will lead to occasional cold Arctic winters in the stratosphere but show no indication for enhanced frequency of their occurrence.
Climate change will be an especially important driver for polar ozone change in the second half of the 21st century. Increases in CO2 concentrations will lead to a cooling of the stratosphere and increases in all greenhouse gases are projected to strengthen the transport of ozone-rich air to higher latitudes. Under conditions of low halogen loading both of these changes are anticipated to increase polar ozone amounts. The changes are expected to have a larger impact on ozone in the Arctic than in the Antarctic due to a larger sensitivity of dynamical processes in the Northern Hemisphere to climate change. Polar ozone levels at the end of the century might be affected by changing concentrations of N2O and CH4 through their direct impact on atmospheric chemistry. The atmospheric concentrations of both of these gases are projected to increase in most future scenarios, but these projections are very uncertain.
Substantial polar ozone depletion could result from enhancements of sulfuric aerosols in the stratosphere during the next few decades when stratospheric halogen levels remain high. Such enhancements could result from major volcanic eruptions in the tropics or deliberate "geoengineering" efforts. The surface area and number density of aerosol in polar regions are important parameters for heterogeneous chemistry and chlorine activation. The impact of sulfur dioxide (SO2) injection of either natural or anthropogenic origin on polar ozone depends on the halogen loading. In the next several decades, enhanced amounts of sulfuric acid aerosols would increase polar ozone depletion.