The Impact of Short-Lived Halocarbons on Ozone and Climate (ISHOC): An International Multi-Model Intercomparison

Depletion of the stratospheric ozone layer has been at the forefront of environmental concern over the last 40 years. The layer shields Earth's surface from certain wavelengths of harmful ultraviolet (UV) radiation that would otherwise be detrimental to human and plant health. Ozone also absorbs terrestrial infra-red (IR) radiation meaning it is a greenhouse gas, and changes in its abundance can therefore impact climate. The primary cause of ozone depletion is the release of halogens (chlorine and bromine) from long-lived anthropogenic compounds, such as chlorofluorocarbons (CFCs) and halons. Production of these ozone-depleting compounds is now controlled by the UN Montreal Protocol, but they were once widely used in refrigeration and fire suppression units, among other applications. Due to the success of the Protocol, the stratospheric abundance of chlorine and bromine is now declining, albeit slowly, and the ozone layer is widely expected to 'recover' to levels observed pre-1980 in the middle to latter half of this century. However, a key uncertainty, highlighted in the WMO/UNEP 2014 Assessment of Stratospheric Ozone Depletion, is the increasing emissions of uncontrolled chlorine-containing Very Short-Lived Substances (Cl-VSLS) which can also reach the stratosphere and cause ozone loss.

The most abundant Cl-VSLS is dichloromethane (CH2Cl2), whose tropospheric abundance has increased by >60% over the last decade. CH2Cl2 is human-produced and in the Northern Hemisphere, close to industrial sources, long-term observations show a mean CH2Cl2 growth rate of ~8%/year. The precise cause of these increases is unknown. However, emissions of CH2Cl2 (and other Cl-VSLS) are known to be relatively large over Asia, and in the absence of policy controls on production, atmospheric concentrations are expected to continue to increase in coming years. Our recent modelling work has shown (i) that the contribution of Cl-VSLS to stratospheric chlorine has already doubled in the last decade alone, and (ii) that sustained CH2Cl2 growth could delay the recovery of the Antarctic Ozone Hole by up to several decades. This would significantly offset some of the gains achieved by the Montreal Protocol, and because the Ozone Hole influences surface climate of the Southern Hemisphere in several ways, could affect forward predictions of climate change.

This project (ISHOC) establishes a new task force comprised of world-leading chemistry-climate modelling groups. We will perform the first concerted multi-model assessment of the threat posed to stratospheric ozone from CH2Cl2 growth. Lancaster University will lead the model intercomparison in collaboration with the University of Cambridge, and an international consortium of 9 partners. We will develop a series of growth scenarios describing possible future trajectories of CH2Cl2 in the atmosphere. Each of the models in our consortium will perform forward simulations considering these scenarios and the output will be analysed to determine (a) the expected delay to ozone recovery in different regions of the stratosphere due to CH2Cl2 growth and (b) the subsequent implications for climate and surface UV. The results from ISHOC will provide powerful new insight into the role of compounds not controlled by the Montreal Protocol in ozone depletion, which will be highly relevant to future international assessments of ozone and climate change (e.g. WMO/UNEP and IPCC reports). While the focus of ISHOC is on CH2Cl2, the task force will remain active beyond the project to examine future threats to ozone from other uncontrolled Cl-VSLS (e.g. CHCl3, C2H4Cl2) as they emerge. Indeed, our ongoing work suggests that emissions of these Cl-VSLS are also increasing.

A multi-model framework is the best approach to explore future projections of ozone for policy purposes, as it allows exploration of model uncertainty and a large sample size to (a) identify long-term trends and (b) differentiate those from inter-annual variability. Results from ISHOC have the potential to generate major impact in the area of science policy and potentially also in the commercial sector.

(1.) Science Policy - The project will provide powerful new insight into the threat posed to stratospheric ozone from uncontrolled emissions of chlorinated Very Short-Lived Substances (Cl-VSLS). Scientific interest in Cl-VSLS has dramatically increased since the discovery that (a) the atmospheric concentration of CH2Cl2 has increased rapidly over the last decade (WMO, 2014) and (b) that Cl-VSLS could delay recovery of Earth's ozone layer by up to several decades (Hossaini et al., 2017). As CH2Cl2 has anthropogenic sources and because its production is not controlled by the UN Montreal Protocol, there is a strong policy relevance to this emerging topic. It is likely that policy-makers will face pressure to consider adding CH2Cl2, and potentially other Cl-VSLS, to the list of compounds controlled by the Montreal Protocol in the future. As such, our timely results will prove highly informative to such decisions and through contributing to evidence-based policy assessment in this way (e.g. should CH2Cl2 production be controlled?) significant impact can be achieved.

The findings of ISHOC will be of international significance and we anticipate will feature heavily in future WMO/UNEP Stratospheric Ozone Assessments (next due in 2018 and 2022). Although written primarily by scientists, these international reports are aimed at policy makers and, to a lesser extent, the public through (a) co-publication of additional material such as the 'Summary for Policy Makers' and (b) their publication in the public domain (online and in print). Both Hossaini and Young are co-authors of the upcoming 2018 Ozone Assessment and both previously co-authored the report in 2014. On this basis, we are well placed to ensure our results are appropriately disseminated and that impact is realised through this channel. Additionally, we note that our project team works closely with Professor John Pyle (University of Cambridge) and Professor Nigel Paul (Lancaster University) who are co-chairs of the Scientific Assessment Panel and the Environmental Effects Assessment Panel, respectively, of the WMO/UNEP ozone assessment framework.

(2.) Commercial - Given the potential for Cl-VSLS to be included in the Montreal Protocol, there will very likely be strong interest in the findings of ISHOC from the chlorocarbon production industry, which operates globally. With this in mind, we have identified industry representatives with which we will engage to ensure our findings are disseminated effectively and for knowledge exchange. This includes David Sherry, a consultant with specific experience and expertise in the chlorocarbon industry (http://nolansherry.com/). We will inform such parties of our findings concerning CH2Cl2 and its role in ozone depletion, and in exchange will seek bottom-up information on the distribution/magnitude of CH2Cl2 emissions (and other Cl-VSLS) from Asia, where data within academia is currently very sparse. In addition, we note that there is extensive and ongoing research and development in both the academic and commercial sectors into producing alternative 'green solvents' (i.e. to replace compounds such as CH2Cl2). For example, this a major research theme in the Chemistry Department at the University of York (where Hossaini was previously a member of staff) which works closely with industry (https://www.york.ac.uk/chemistry/research/green/industry/). We will engage our York colleagues with our findings on CH2Cl2 to help maximise impact at the academic/industry interface and drive research priorities concerning Cl-VSLS in both sectors.