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

讨论了气候工程（CE）方法作为抵消气候变化负面影响的可能手段。除了二氧化碳去除和太阳辐射管理之外，还提出了一种通过减少卷云的变暖效应来增加外传长波辐射的方法，我们计划在该建议中进行后续研究。为了最大限度地关注长波辐射，我们将重点放在冬季北极卷云的减薄上，目的是回答以下问题：北极冬季卷云减薄是否可行，以及可以实现的最大降温程度是多少？应用ICON-ART天气化学预报模式，就北极平流层可能发生的变化，包括臭氧层和低空云层可能发生的变化，进行区域性的风险和副作用研究。对全球环流、洋流和海冰覆盖可能产生的影响将使用气溶胶-大气-海洋全球气候模式MPI-ESM-HAM进行分析。为了解决这些问题，我们需要验证当前的模型是否足以解决上述问题。特别是，他们能否再现在北极冬季观测到的卷云的范围和垂直分布？在北极冬季的动力条件下，天然冰核粒子和人工冰核粒子（INP）的输送途径是什么？需要对它们进行研究，以便估计在期望区域内播种粒子的寿命。高空飞行的商用飞机的高度和航线是否足以播种相当一部分北极卷云，还是应该将播种区域扩展到中纬度地区？在这些条件下，目前提出的悬浮粒子BiJ3是最好的播种剂吗？只有当自然卷云主要由溶液液滴均匀冻结形成时，卷云减薄才有效。如果它们主要是由INP上的非均匀冻结形成的，播种可能会导致过度播种，导致变暖而不是变冷。因此，需要评估卷云的特性，特别是在当今气候条件下非均匀冻结对其形成的贡献。

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.