Estimating the Impact of the Radiative Feedback from Atmospheric Methane on Climate Sensitivity

甲烷（CH4）是仅次于二氧化碳（CO2）的对人为辐射强迫（RF）贡献第二大的温室气体。本项目提案的主要议题是首次量化大气CH4作为气候敏感性辐射反馈成分的相关性，这将RF与全球温度反应联系起来。从初步的模型设置中得到的一个近似值表明，甲烷的负反馈很大，这可能会大大增强在二氧化碳驱动的气候变化模拟（包括相互作用化学）中已经确立的臭氧负反馈。然而，这种证据需要更先进的模型来证实。在这个项目中，我们将利用一个新建立的选项，在一个批准和评估的化学气候模式（CCM），使气候敏感性模拟与预定义的甲烷排放通量，而不是常见的固定的下边界混合比条件，从而直接计算气候反馈的甲烷。我们将主要依靠有效辐射强迫（ERF）概念，其中快速辐射调整，包括通过化学物质的调整，被分配给强迫分量，只有由海表温度驱动的辐射变化被解释为改变气候敏感性的辐射反馈。在有和没有CH4通量边界条件的CO2驱动的CCM模拟中，确定CH4变化的快速辐射调节和缓慢辐射反馈。然后，一个等效的CH4驱动的CCM模拟系列将与改变CH4通量边界条件，其中CH4作为强迫以及辐射反馈组件。在任何一种情况下，都将对所有物理和化学驱动的辐射调整和反馈进行全面分析，以进行模拟。这将产生关于气候敏感性参数对纳入CO2和CH4驱动的气候变化的相互作用化学的敏感性的进一步信息，总的来说，该项目将增进目前关于受大气化学控制的辐射反馈的相当稀少的知识，将揭示辐射调整与物理和化学来源的反馈之间的潜在相互作用，并将限制CCM中甲烷和臭氧反馈对气候敏感性的净贡献。这种知识尤其有助于更精确地估计未来甲烷排放的净气候影响，这是潜在缓解概念的先决条件。

Methane (CH4) is the greenhouse gas with the second most important contribution to anthropogenic radiative forcing (RF), next to carbon dioxide (CO2). The main topic of the present project proposal is to quantify, for the first time, the relevance of atmospheric CH4 in its role as a radiative feedback component for the climate sensitivity, which links the RF to the global temperature response. An approximation available from a preliminary model setup suggests a significant negative CH4 feedback that may considerably enhance the already well established negative ozone feedback in CO2-driven climate change simulations including interactive chemistry. However, such evidence requires confirmation from more advanced models. In this project we will make use of a newly established option in an approved and evaluated chemistry-climate model (CCM) that enables climate sensitivity simulations with predefined CH4 emission fluxes instead of the common fixed lower boundary mixing ratio conditions and thereby a direct calculation of the climate feedback of CH4. We will mainly rely on the effective radiative forcing (ERF) concept, where rapid radiative adjustments, including those via chemical species, are assigned to the forcing component and only radiative changes driven by the sea surface temperature are interpreted as radiative feedbacks modifying the climate sensitivity.The particular objectives of the project are, first, to determine the rapid radiative adjustment and the slow radiative feedback from CH4 changes in CO2-driven CCM simulations with and without CH4 flux boundary conditions. Then, an equivalent CH4-driven CCM simulation series will be performed with changed CH4 flux boundary conditions, where CH4 works as the forcing as well as a radiative feedback component. In either case a complete analysis of all physically and chemically driven radiative adjustments and feedbacks will be carried out for the simulations. This will yield further information about the sensitivity of the climate sensitivity parameter to the inclusion of interactive chemistry for CO2- as well as for CH4-driven climate change.In total, the project will advance the present, rather sparse, knowledge on radiative feedbacks controlled by atmospheric chemistry, will reveal the potential interactions between radiative adjustments and feedbacks of both physical and chemical origin, and will constrain the net contribution of methane and ozone feedbacks in CCMs on the climate sensitivity. Such knowledge will, in particular, allow more precise estimates of the net climate impact of future CH4 emissions, which is a prerequisite for potential mitigation concepts.