Stratospheric Ozone Loss in Mid-latitudes in Summer - a Potential Risk of Climate Engineering? (CE-O3)
Final Report Abstract
The world’s climate is changing caused by industrial emitted greenhouse gases (GHG). Increasing atmospheric GHG concentrations result in a global warming, which changes habitats all around the globe. For limiting the risks of climate change, a supporting option could be the application of sulfate geoengineering. The concept of sulfate geoengineering is to inject sulfate aerosols into the stratosphere extenuating the sunlight, which reaches and thus warms the Earth surface. In this way, the surface temperature would be kept at today’s level avoiding negative consequences of GHG induced global warming. However, sulfate geoengineering is not free of risks and these potential risks have to be explored before applying geoengineering. The project CE-O3 was aimed to assess the risk of catalytic ozone loss known from polar winter to occur in the mid-latitude lowermost stratosphere in summertime as a potential side effect of sulfate geoengineering. This process was further proposed to potentially occur in the mid-latitudes for today’s conditions in combination with convective overshooting events transporting water vapor into the lower stratosphere. The chemical ozone loss mechanism and its sensitivity to a variety of conditions is analyzed by conducting box-model simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS), which are chemically initialized based on aircraft measurements from the lowermost stratosphere above central North America. This analysis comprises heterogeneous chlorine activation (the key step of the mechanism), chlorine-catalyzed ozone loss cycles and the maintenance of activated chlorine. Focus sing on a realistic trajectory a threshold in water vapor has to be exceeded and maintained for stratospheric ozone loss to occur. This water vapor threshold is found to be mainly determined by the temperature and sulfate content. However, a simulation based on observed conditions most likely for this ozone loss process did not show significant chlorine activation. Further, the likelihood for this ozone loss process to occur and its impact on ozone today or in future scenarios considering both climate change and an additional application of sulfate geoengineering is determined. Therefore, chlorine activation thresholds depending on water vapor and temperature are calculated based on CLaMS box-model simulations. These thresholds are compared with conditions found in the lower stratosphere in results of climate simulations using the Geoengineering Large Ensemble Simulations (GLENS). Through extensive sensitivity tests within the project CE-O3 it could be shown that the application of geoengineering leads to a 2–3 times higher probability than for today that the investigated ozone depletion mechanism occurs. However, the probability of chlorine activation over Central North America using sulfate geoengineering is very low (3.3%). In all cases considered for today and in future, less than 0.4% of ozone in the lower stratosphere are destroyed caused by heterogeneous chlorine activation causing an upper limit for total ozone column reduction of 0.11 DU. In summary, the project CE-O3 showed the first time that the risk of increased ozone depletion in the lower stratosphere in summer over North America is negligible for today’s as well as for possible future conditions such as climate change and sulfate geoengineering. This is an important contribution to future reports to assess the risks of solar radiation management by the injection of sulfate aerosol into the atmosphere e.g. by the World Meteorological Organization (WMO).
Publications
- The maintenance of elevated active chlorine levels in the Antarctic lower stratosphere through HCl null cycles, Atmos. Chem. Phys., 18, 2985–2997, 2018
Müller, R., Grooß, J.-U., Zafar, A. M., Robrecht, S., and Lehmann, R.
(See online at https://doi.org/10.5194/acp-18-2985-2018) - The relevance of reactions of the methyl peroxy radical (CH3O2) and methylhypochlorite (CH3OCl) for Antarctic chlorine activation and ozone loss, Tellus B: Chemical and Physical Meteorology, 70, 1–18
Zafar, A. M., Müller, R., Grooß, J.-U., Robrecht, S., Vogel, B., and Lehmann, R.
(See online at https://doi.org/10.1080/16000889.2018.1507391) - Mechanism of ozone loss under enhanced water vapour conditions in the mid-latitude lower stratosphere in summer, Atmos. Chem. Phys., 19, 5805–5833
Robrecht, S., Vogel, B., Grooß, J.-U., Rosenlof, K., Thornberry, T., Rollins, A., Krämer, M., Christensen, L., and Müller, R.
(See online at https://doi.org/10.5194/acp-19-5805-2019) - Potential of future stratospheric ozone loss in the mid-latitudes under climate change and sulfate geoengineering, Atmospheric Chemistry and Physics Discussions, 2020, 1–40
Robrecht, S., Vogel, B., Tilmes, S., and Müller, R.
(See online at https://doi.org/10.5194/acp-2020-747)
