Collaborative Research: Using a Hierarchy of Models to Constrain the Temperature Dependence of Climate Sensitivity

气候敏感性通常被定义为从工业化前水平开始，大气二氧化碳(CO2)增加一倍所导致的全球气温上升。气候敏感度是决定人为温室气体排放风险的最重要因素，不能排除高敏感度带来严重后果的可能性。气候敏感度通常被认为在与全球变暖相关的气候状态范围内是恒定的，但如果气候敏感度很高，或者如果温室气体浓度上升到相当于几个二氧化碳倍增的水平(可能在几个世纪内)，这一假设可能不成立。PIS和其他机构最近的研究表明，气候敏感性可能会随着全球气温的升高而增加，这种增加可能会影响大规模变暖的风险。这里开展的工作使用了一个简化的气候模式，社区大气模式版本5(CAM5)耦合到平板混合层海洋，以检查一次、两次和三次二氧化碳倍增对气候敏感性的温度依赖性。该模式被用来产生扰动物理系综(PPE)，其中控制云和对流行为的参数(例如对流的分数质量夹带率)被摄动作为改变气候反馈强度的手段。进一步的工作是在理想域上使用云解析模型(大气建模系统，简称SAM)来确定参数值的合理选择。这项工作还考虑了参数值本身可能会随着气候变暖而变化的可能性，从而产生对温度的敏感性。其他工作着眼于耦合模式比较项目版本5(CMIP5)的温室气体变暖模拟中的气候敏感性，以确定模拟是否表现出与温度有关的气候敏感性。由于高气候敏感性带来的重大社会后果以及限制人为排放可能导致的变暖的可取程度，该工作具有更广泛的影响。依赖温度的敏感性也可能有助于解释过去温暖气候和气候变化的替代记录，因此这项工作具有科学和更广泛的社会影响。此外，该项目还支持一名研究生和一名博士后研究助理，以确保该研究领域的劳动力发展。

Climate sensitivity is typically defined as the increase in global temperature that would result from a doubling of atmospheric carbon dioxide (CO2) starting from the pre-industrial level. Climate sensitivity is the most important factor in determining the risk posed by anthropogenic greenhouse gas emissions, and high values of sensitivity with severe consequences cannot be ruled out.Climate sensitivity is generally assumed to be constant over the range of climate states relevant to global warming, but this assumption may not hold if climate sensitivity is high or if greenhouse gas concentrations rise to a level equivalent to several CO2 doublings (possibly over several centuries). Recent work by the PIs and others suggests that climate sensitivity could increase with global temperature, and this increase could influence the risk of large warming. Work performed here uses a simplified climate model, the Community Atmosphere Model version 5 (CAM5) coupled to a slab mixed layer ocean, to examine the temperature dependence of climate sensitivity over one, two, and three CO2 doublings. The model is used to generate a perturbed physics ensemble (PPE), in which parameters controlling the behavior of cloud and convection (for example the fractional mass entrainment rate for convection) are perturbed as a means of varying the strength of climate feedbacks. Further work uses a cloud resolving model (the System for Atmospheric Modeling, or SAM) on an idealized domain to identify reasonable choices of parameter values. The work also considers the possibility that the parameter values themselves could change as climate warms, thereby yielding a temperature dependence in sensitivity. Additional work looks at climate sensitivity in greenhouse warming simulations from the Coupled Model Intercomparison Project version 5 (CMIP5), to determine if the simulations exhibit temperature dependent climate sensitivity.The work has broader impacts due to the substantial societal consequences by high climate sensitivity and the desirability of constraints on how much warming can result from anthropogenic emissions. Temperature-dependent sensitivity may also help to interpret proxy records of past warm climates and climate change, thus the work has scientific as well as societal broader impacts. In addition, the project supports a graduate student and a postdoctoral research associate, so that workforce development is ensured in this research area.