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

甲烷(CH4)是温室气体，对人为辐射强迫(RF)的贡献仅次于二氧化碳(CO2)。本项目提案的主要主题是首次量化大气CH4作为气候敏感性的辐射反馈组成部分的相关性，该组成部分将RF与全球温度响应联系起来。从初步模型设置得到的近似值表明，CH4的显著负反馈可能会大大加强在二氧化碳驱动的气候变化模拟中已经确立的臭氧负反馈，包括相互作用化学。然而，这样的证据需要更高级的模型来证实。在这个项目中，我们将利用一个经过批准和评估的化学-气候模式(CCM)中新建立的选项，该模式能够使用预定义的CH4排放通量而不是共同固定的下限混合比条件来实现气候敏感性模拟，从而直接计算CH4的气候反馈。我们将主要依靠有效辐射强迫(ERF)的概念，在该概念中，强迫分量被分配快速的辐射调整，包括通过化学物种的辐射调整，只有由海表面温度驱动的辐射变化才被解释为改变气候敏感性的辐射反馈。该项目的具体目标是：首先，确定在有和没有CH4通量边界条件的情况下，CH4驱动的CCM模拟中的快速辐射调整和来自CH4变化的慢辐射反馈。然后，在改变了CH4通量边界条件的情况下，进行了一个等效的CH4驱动的CCM模拟系列，其中CH4既是强迫分量，也是辐射反馈分量。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完整的分析，以便进行模拟。这将提供关于气候敏感性参数对纳入二氧化碳和甲烷驱动的气候变化的相互作用化学的敏感性的进一步信息。总体而言，该项目将推进目前相当稀少的关于由大气化学控制的辐射反馈的知识，将揭示辐射调节与物理和化学来源的反馈之间的潜在相互作用，并将限制甲烷和臭氧在气候敏感性方面的净贡献。这些知识尤其有助于更准确地估计未来CH4排放对气候的净影响，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.