Quantifying the contributions to stratospheric ozone changes from ozone depleting substances and greenhouse gases

Abstract

Abstract.A state-of-the-art chemistry climate model coupled to a three-dimensional ocean model is used to produce three experiments, all seamlessly covering the period 1950-2100, forced by different combinations of long-lived Greenhouse Gases (GHGs) and Ozone Depleting Substances (ODSs). The experiments are designed to quantify the separate effects of GHGs and ODSs on the evolution of ozone, as well as the extent to which these effects are independent of each other, by alternately holding one set of these two forcings constant in combination with a third experiment where both ODSs and GHGs vary. We estimate that up to the year 2000 the net decrease in the column amount of ozone above 20 hPa is approximately 75% of the decrease that can be attributed to ODSs due to the offsetting effects of cooling by increased CO 2 . Over the 21st century, as ODSs decrease, continued cooling from CO 2 is projected to account for more than 50% of the projected increase in ozone above 20 hPa. Changes in ozone below 20 hPa show a redistribution of ozone from tropical to extra-tropical latitudes with an increase in the Brewer-Dobson circulation. In addition to a latitudinal redistribution of ozone, we find that the globally averaged column amount of ozone below 20 hPa decreases over the 21st century, which significantly mitigates the effect of upper stratospheric cooling on total column ozone. Analysis by linear regression shows that the recovery of ozone from the effects of ODSs generally follows the decline in reactive chlorine and bromine levels, with the exception of the lower polar stratosphere where recovery of ozone in the second half of the 21st century is slower than would be indicated by the decline in reactive chlorine and bromine concentraCorrespondence to: D. A. Plummer (david.plummer@ec.gc.ca) tions. These results also reveal the degree to which GHGrelated effects mute the chemical effects of N 2 O on ozone in the standard future scenario used for the WMO Ozone Assessment. Increases in the residual circulation of the atmosphere and chemical effects from CO 2 cooling more than halve the increase in reactive nitrogen in the mid to upper stratosphere that results from the specified increase in N 2 O between 1950 and 2100.

Drivers of hemispheric differences in return dates of mid-latitude stratospheric ozone to historical levels

“…Above 30 hPa, trends decrease towards zero and become hemispherically symmetric in the multi-model mean. The structure of the trends agrees well with the differences in cumulative ozone columns from the mid-20th to the end of the 21st century shown by Plummer et al (2010). They used sensitivity simulations to attribute these trends almost fully to changes in GHG concentrations, thus validating our method of separating the effects of ODSs and GHGs through linear regression.…”

“…Enhanced efficiencies are found in the SH, where an expansion or shift of the polar vortex provides the conditions for heterogeneous ozone depletion in midwinter north of 60 • S. This effect was noted earlier for southern polar ozone, for example by Plummer et al (2010), who found a nonlinearity of the response of ozone to ODSs in the Antarctic lower stratosphere under changing GHG concentrations. Similarly, Revell et al (2012a) reported an increase of Antarctic lower stratospheric ozone in a scenario with lower GHG concentration increases as compared to the standard REF-B2 simulation.…”

The sensitivity of stratospheric ozone changes through the 21st century to N<sub>2</sub>O and CH<sub>4</sub>

“…More recently, Plummer et al (2010) found that nitrogen species induced large stratospheric ozone losses once the effects of CO 2 -induced stratospheric cooling were removed. In addition, increasing sea-surface temperatures (SSTs) are projected to strengthen the Brewer-Dobson circulation, resulting in a faster removal rate of reservoir nitrogen species from the stratosphere (Cook and Roscoe, 2012).…”

The decrease in mid-stratospheric tropical ozone since 1991

“…In mixing ratio terms, this gives a rate of ∼ 5-7 ppbv yr −1 . Plummer et al (2010) studied O 3 changes in a model including GHGs (greenhouse gases) and ODSs (ozone-depleting substances). They ran two experiments with a faster Brewer-Dobson circulation, and these two experiments showed, at 10 hPa in the tropics, a decrease in reactive nitrogen and an increase in both O 3 and N 2 O at 10 hPa relative to the experiments with a slower circulation.…”