Attribution of observed changes in stratospheric ozone and temperature

Abstract

Abstract. Three recently-completed sets of simulations of multiple chemistry-climate models with greenhouse gases only, with all anthropogenic forcings, and with anthropogenic and natural forcings, allow the causes of observed stratospheric changes to be quantitatively assessed using detection and attribution techniques. The total column ozone response to halogenated ozone depleting substances and to natural forcings is detectable in observations, but the total column ozone response to greenhouse gas changes is not separately detectable. In the middle and upper stratosphere, simulated and observed SBUV/SAGE ozone changes are broadly consistent, and separate anthropogenic and natural responses are detectable in observations. The influence of ozone depleting substances and natural forcings can also be detected separately in observed lower stratospheric temperature, and the magnitudes of the simulated and observed responses to these forcings and to greenhouse gas changes are found to be consistent. In the mid and upper stratosphere the simulated natural and combined anthropogenic responses are detectable and consistent with observations, but the influences of greenhouse gases and ozone-depleting substances could not be separately detected in our analysis.

Detectability of the impacts of ozone-depleting substances and greenhouse gases upon stratospheric ozone accounting for nonlinearities in historical forcings

“…Previous studies have found that a strengthening of the BDC results in lower ozone concentrations in the tropical lower stratosphere and an increase in concentrations in the extratropics, and modeling studies have suggested that increases in GHG concentrations lead to a strengthening of the BDC Fleming et al, 2011;Garcia et al, 2008;Gillett et al, 2011;Oman et al, 2010). In contrast, a recent study by Polvani et al (2017) found that trends in ODSs, and not in GHG levels, have been the primary driver of trends in tropical upwelling.…”

“…Formal identification of an anthropogenic climate change "fingerprint" has been successfully achieved with observations of atmosphere and ocean temperatures, sea level, ocean acidity, various components of the water cycle and the cryosphere, and certain climate extremes (Bindoff et al, 2013). To date, how-ever, few formal D&A methods have been applied in studies involving stratospheric ozone (see Gillett et al, 2011, for one exception to this). There is evidence that stratospheric ozone is transitioning from an era of widespread and readily detectable depletion (linked to changes in anthropogenic chlorofluorocarbons) to an era characterized by early signs of recovery or healing (Solomon et al, 2016).…”

“…To the best of the authors' knowledge, only one previous study has considered the formal attribution of anthropogenic emissions to observed stratospheric ozone changes: Gillett et al (2011). Their investigation examined zonal-mean solar backscatter ultraviolet (SBUV) ozone measurements (Mclinden et al, 2009) over the 27-year period from 1979 to 2005 in the middle and upper stratosphere (50-1 hPa).…”

“…The ozone response to changes in ODSs and the response to changes in natural external forcings were estimated through subtraction of simulations. Gillett et al (2011) used a standard space-time optimal regression methodology for signal detection and attribution (see, e.g., Allen and Tett, 1999). In this approach, the observations are modeled as a linear sum of simulated responses (fingerprints) to individual forcings, with each response scaled by an estimated regression coefficient (expressing the strength of the space-time response pattern in observations).…”

Trends in stratospheric ozone profiles using functional mixed models

“…Furthermore, part of the observed recovery in total ozone column levels may not be due to the Montreal Protocol restrictions on the production of chlorofluorocarbons (CFCs), but rather due to an increase in greenhouse gases (GHGs), which warm the troposphere, but increase stratospheric cooling that in turn may slow ozone depletion. Chemistry-climate models do not yet simulate these interactions well, or do it with large uncertainties, and some joint effort by the CCMVal and CCMVal-2 projects focuses on intercomparisons of such models (Gillett et al, 2011). Having good estimates of trends from the lower to the upper stratosphere can potentially help disentangle this issue and improve numerical modeling.…”