On the attribution of stratospheric ozone and temperature changes to changes in ozone-depleting substances and well-mixed greenhouse gases T. G. Shepherd and A. I. Jonsson Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 1A7, Canada
Abstract. The vertical profile of global-mean stratospheric temperature changes has
traditionally represented an important diagnostic for the attribution of the
cooling effects of stratospheric ozone depletion and CO2 increases.
However, CO2-induced cooling alters ozone abundance by perturbing ozone
chemistry, thereby coupling the stratospheric ozone and temperature responses to
changes in CO2 and ozone-depleting substances (ODSs). Here we untangle
the ozone-temperature coupling and show that the attribution of global-mean
stratospheric temperature changes to CO2 and ODS changes (which are the
true anthropogenic forcing agents) can be quite different from the
traditional attribution to CO2 and ozone changes. The significance of
these effects is quantified empirically using simulations from a
three-dimensional chemistry-climate model. The results confirm the essential
validity of the traditional approach in attributing changes during the past
period of rapid ODS increases, although we find that about 10% of the
upper stratospheric ozone decrease from ODS increases over the period
1975–1995 was offset by the increase in CO2, and the CO2-induced
cooling in the upper stratosphere has been somewhat overestimated. When
considering ozone recovery, however, the ozone-temperature coupling is a
first-order effect; fully 2/5 of the upper stratospheric ozone increase
projected to occur from 2010–2040 is attributable to CO2 increases.
Thus, it has now become necessary to base attribution of global-mean
stratospheric temperature changes on CO2 and ODS changes rather than on
CO2 and ozone changes.
Citation: Shepherd, T. G. and Jonsson, A. I.: On the attribution of stratospheric ozone and temperature changes to changes in ozone-depleting substances and well-mixed greenhouse gases, Atmos. Chem. Phys., 8, 1435-1444, doi:10.5194/acp-8-1435-2008, 2008.