Climate Engineering by Arctic Winter Cirrus Thinning: Risks and Feasibility (AWiCiT)

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Climate Engineering (CE) methods are discussed as possible instruments to counteract negative climate change impacts. In addition to carbon dioxide removal and solar radiation management, a method that suggests increases in the outgoing longwave radiation by reducing the warming effect of cirrus clouds has been proposed and that we plan to follow up in this proposal. In order to maximize the focus on longwave radiation, we focus on thinning of Arctic cirrus in winter with the goal to answer the following question: Is Arctic winter cirrus thinning (AWiCiT) feasible and what is the maximum extent of cooling that could be achieved? The risks and side effects of AWiCiT will be studied regionally in terms of possible changes in the Arctic stratosphere including possible modifications of the ozone layer and lower-lying clouds by applying the ICON-ART weather-chemistry forecast model. Possible effects on the global circulation, ocean currents and sea ice coverage will be addressed using the coupled aerosol-atmosphere-ocean global climate model MPI-ESM-HAM. In order to address these questions, we need to validate if the current models are good enough to tackle the questions formulated above. In particular, can they reproduce the observed extent and vertical distribution of cirrus in the Arctic winter? What are the transport pathways of natural and seeding ice nucleating particles (INP) under the dynamic conditions of the Arctic winter? They need to be studied in order to estimate the lifetimes of seeding particles in the desired region. Are the altitudes and routes of the high-flying commercial aircraft sufficient to seed an appreciable fraction of Arctic cirrus or should the seeding area be extended to mid-latitudes? Is under these conditions Bismuth-Tri-iodide (BiJ3), the currently proposed seeing aerosol particle the best seeding agent? Cirrus thinning is only effective if the natural cirrus form predominantly by homogeneous freezing of solution droplets. If they mainly form by heterogeneous freezing on INP, seeding could cause an overseeding leading to a warming instead of a cooling. Therefore the cirrus properties, especially in terms of the contribution of heterogeneous freezing to their formation in the present-day climate, need to be assessed.