Does the ACIA overstate the problem of ozone depletion? The overview report states that the “stratospheric ozone layer over the Arctic is not expected to improve significantly for at least a few decades”. This is partly because CFC concentrations (that enhance stratospheric ozone destruction) are only expected to decrease slowly as a function of restrictions imposed by the Montreal Protocol and subsequent amendments. Another factor is the fact that stratospheric temperatures are generally cooling as greenhouse gases increase (see MSU Temperature Record, also Why does the stratosphere cool when the troposphere warms?). Due to the temperature dependence on the rates of chemical reactions involving ozone, cooler temperatures also lead to more ozone destruction. Stratospheric temperatures, particularly near the pole are also significantly influenced by dynamical changes, and in particular, the strength of the polar vortex that is set up every winter at the edge of the polar night. Note as well that the strength of the vortex itself depends on the temperatures, which in turn depend on the ozone levels. Thus the future of Arctic ozone levels (and hence UV levels at the surface) depends on how these three complex elements interact.
Different model simulations of these processes going into the future do indeed show varying results. One, by Shindell and colleagues, indicated that the changes in the polar vortex might enhance polar cooling and lead to increased Arctic ozone loss. Another by Nagashima et al, indicated that the polar vortex changes would be minimal. Further studies with different levels of complexity show similarly varied results as summarized in the review by Austin et al., 2003.
To be clear, this is an important problem, and the uncertainties inherent in modelling a complex interacting system of chemistry and dynamics are legion. In particular, since the sensitivity of these processes depends enormously on the current state, getting the present day stratospheric temperatures correct near the pole is crucial. Future changes in the polar vortex appear to be quite model dependent, and so predictions of this aspect of polar change are highly uncertain. Changes to date in the Antarctic though do appear to require some amount of change in the polar vortex to explain, in particular, the large amount of springtime cooling in the polar vortex there (Shindell, 2003). Thus while the dynamic feedback finds some support in the data and in models, it is not well quantified.
However, none of the above-mentioned studies show any signficant improvements in the next few decades, and thus all actually support the measured ACIA statement. This can’t therefore be claimed as evidence for unjustified hype.
References:
Arctic Climate Impact Assessment (ACIA) Overview Report, 2004. Impacts of a Warming Arctic: Arctic Climate Impact Assessment (Cambridge University Press).
Austin, J., D. Shindell, S.R. Beagley, C. Br?hl, M. Dameris, E. Manzini, T. Nagashima, P. Newman, S. Pawson, G. Pitari, E. Rozanov, C. Schnadt, and T.G. Shepherd, 2003. Uncertainties and assessments of chemistry-climate models of the stratosphere. Atmospheric Chemistry and Physics, 3, 1-27.
Nagashima et al., 2002. Future development of the ozone layer calculated by a general circulation model with fully interactive chemistry. Geophysical Research Letters, 29, 10.1029/2001GL014026.
Shindell D., D. Rind and P. Lonergan, 1998. Increased polar stratospheric ozone losses and delayed eventual recovery owing to increasing greenhouse-gas concentration. Nature, 392, 589-592.
Shindell, D. 2003. Perspective: Whither Arctic climate?. Science 299, 215-216.
