Montreal Protocol and ODS
The Montreal Protocol controls the production and consumption of the major long-lived chlorine and bromine-containing Ozone-Depleting Substances (ODSs) such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), solvents (e.g. CCl4) and halons. It is rightly considered one of the most successful global environmental treaties. However, it is important to remain vigilant about the state of the ozone layer.
Renewed CFC-11 Production
Through action taken under the Montreal Protocol, atmospheric chlorine peaked in the 1990s and has been declining since – pointing to the overall success of the Protocol. This decline was achieved by countries following the agree reductions in production of compounds such as CFCs. Nevertheless, comprehensive atmospheric monitoring and detailed modelling are imperative to ensure that the atmosphere is behaving as expected.
By 2018 Steve Montzka (NOAA, USA) and co-workers noticed that the atmospheric decline of CFC-11 was slowing down, disagreeing with expectations. This was based on a network of surface observations. Converting these observations to a surface emission flux requires careful removal of observed variability due to atmospheric transport. In a follow-up study (Montzka et al., 2021) we used atmospheric tracers from TOMCAT to estimate this transport variability, thereby allowing more accurate quantification of the CFC-11 surface emissions. These updated and improved estimates showed that renewed CFC-11 production had clearly occurred but also that the alarm raised by the initial NOAA observations had largely led to a halt in this activity. Overall this episode was a success for the Protocol and the atmospheric observations and modelling that underpin it.
Global CFC-11 emissions derived from a combined NOAA and AGAGE mole fraction history and a 3-box model with constant dynamics (air mixing times between boxes; black points and lines) and with consideration of time-varying dynamics estimated from the TOMCAT 3D model (red points and thick red lines) (points represent calendar year means). Uncertainties on annual values were derived considering only measured atmospheric variability and network representation errors. Also shown are multiple estimates of expected emissions in the absence of unreported production after 2010. These expectations are from: (1) an extrapolation of the observationally derived emissions decline during 2002 to 2012 (orange dotted line), (2) results from a Bayesian probabilistic model analysis of banks and emissions that incorporates observational and inventory data (grey-green shaded area), (3) multiple representations of the TEAP inventory model (blue-shaded region), and (4) a modification of TEAP’s ‘most likely’ scenario (blue-dashed line; TEAP*). Observationally derived emissions are offset slightly because they were derived with mean CFC-11 lifetime (54 yr) inherent to TOMCAT. The influence of uncertainty in the CFC-11 lifetime on observationally derived emissions was calculated for lifetimes of 50 yr and 60 yr (thin red lines). Adapted from Montzka et al. (2021).
World Avoided
The Montreal Protocol has successfully prevented severe, global ozone depletion. It is important to be able to demonstrate this to policy makers and the public. To do this we can run ‘world avoided’ simulations where models are forced by halocarbon scenarios which would have been followed in the absence of the Protocol.
We used the TOMCAT/SLIMCAT CTM to demonstrate the benefits already achieved by the Montreal Protocol (Chipperfield et al., 2015). This paper highlighted the ‘Arctic ozone holes’ that we could already be experiencing by around 2010 without the Protocol, with resultant impacts on surface UV radiation. Results from this study were used in the Executive Summary of the WMO/UNEP 2018 Ozone Assessment (see figure) and our paper was included as one of the EOS Distinguished Dozen for ozone layer research.
Observed and modelled column ozone in the Arctic. Without the success of the Montreal Protocol, a deep ozone hole could have formed in the Arctic in 2011, and smaller Arctic ozone holes would have become a regular occurrence. The 2010/2011 Arctic winter had unusually persistent low temperatures in the stratosphere that led to strong chemical ozone destruction. Satellite observations from the Ozone Monitoring Instrument (OMI) in March 2011 show a region of low column ozone surrounded by regions of higher ozone (panel (a)). The March observations are well simulated by a TOMCAT simulation with observed abundances of ODSs, as seen by comparing the maps of panels (a) and (b) as well as the black and blue curves in panel (d), which show the measured and modelled timelines for the mid-2010 to mid-2011 period of daily minimum column-ozone values in the Arctic region (latitudes greater than 45°N). If TOMCAT is run with projected ODS abundances in the absence of Montreal Protocol controls, a much more severe and prolonged Arctic ozone hole is seen (panel (c) and the red curve in panel (d)). (Executive Summary Figure ES-6 from WMO/UNEP (2018)).
Similarly, we used the TOMCAT CTM to show the impact of continued renewed production of CFC-11 that was highlighted above (Dhomse et al., 2019). Our simulations showed the dangerous ozone depletion (delay to recovery) that would have occurred in these emissions had continued.
TOMCAT References
Chipperfield, M.P., S.S. Dhomse, W. Feng, R.L. McKenzie, G. Velders and J.A. Pyle, Quantifying the ozone and UV benefits already achieved by the Montreal Protocol, Nature Communications, 6, 7233, doi:10.1038/ncomms8233, 2015.
Dhomse, S.S., W. Feng, S.A. Montzka, R. Hossaini, J. Keeble, J.A. Pyle, J.S. Daniel and M.P. Chipperfield, Delay in recovery of the Antarctic ozone hole from unexpected CFC-11 emissions, Nature Communications, 10, 5781, doi:10.1038/s41467-019-13717-x, 2019.
Montzka, S.A. G.S. Dutton, R.W. Portmann, M.P. Chipperfield, S. Davis, W. Feng, A.J. Manning, E. Ray, M. Rigby, B.D. Hall, C. Siso, J.D. Nance, P.B. Krummel, J. Muhle, D. Young, S. O’Doherty, P.K. Salameh, C.M. Harth, R.G. Prinn, R.F. Weiss, J.W. Elkins, H. Walter-Terrinoni and C. Theodoridi, A decline in global CFC-11 emissions during 2018−2019, Nature, 590, 428-432, doi:10.1038/s41586-021-03260-5, 2021.