CAREER: Understanding the Time- and State-Dependence of Climate Sensitivity

CAREER奖的教育部分旨在将气候科学引入高中、大学和研究生院的课堂。该项目与高中科学教师合作，通过在华盛顿大学校园举办的研讨会开发教育材料。目标是以一种易于理解的形式介绍气候系统，在保留基本要素及其相互作用的同时尽可能地简化它。我们开发了一些简单的模式，使用一两个方程来表示全球温度响应辐射强迫的演变，其中包括反馈效应和海洋蓄热造成的时间延迟等因素。尽管这些模型很简单，但它们与前沿研究密切相关，因为这些模型通常用于理解在完全复杂的气候模型和观测记录中发现的行为。模型和随附材料还向学生介绍了简单的微分方程、科学计算、假设检验和科学方法。在里奇兰西北太平洋国家实验室的科学家和教育专家的帮助下，为华盛顿东部的高中科学教师组织了一个类似的项目。该奖项的研究部分致力于了解全球温度对温室气体浓度变化的敏感性。这种敏感性，通常被称为气候敏感性，被量化为二氧化碳（CO2）比工业化前水平增加一倍所产生的变暖。气候敏感性是温室气体增加带来的风险的一个重要指标，对其价值的估算已经投入了大量的努力。但是，温室气体增加对全球温度的影响在多大程度上可以用一个固定的敏感性值来表示，目前还不清楚。几项研究表明，随着气候变暖，全球温度对二氧化碳的辐射效应变得更加敏感。在这种情况下，二氧化碳加倍最终造成的变暖大于在温度完全上升之前任何时间计算的敏感性估计所期望的变暖。当气候系统完全适应二氧化碳加倍的辐射效应时所达到的温度升高被称为平衡气候敏感性（ECS）。该术语将ECS和瞬时气候敏感性（ICS）区分开来，瞬时气候敏感性指的是在二氧化碳增加一倍的过程中，由增量辐射增加引起的温度增量增加。ECS是一个固定值，而ICS随着时间的推移而变化，随着最终温度翻倍的实现，接近ECS。虽然有证据表明ICS总是低于ECS，但ECS超过ICS的原因尚未得到令人满意的解释。CAREER奖的研究旨在了解ICS随时间变化的原因，以及为什么它一直低于ECS。研究中的一个关键问题是气候敏感性对特定区域的反馈机制的依赖。一个简单的例子是海冰反照率反馈，其中变暖导致海冰退缩，用较暗的海洋表面取代明亮的反射冰盖，导致更多的太阳能被保留在气候系统中，并导致进一步变暖。由于反照率反馈只发生在冰雪覆盖区域，反馈强度取决于极帽的变暖，而不是全球平均温度的升高。PI假设，ECS-ICS差异的大部分是由于温室气体增加造成的变暖速率的区域差异造成的：如果不同地区有不同的反馈过程，并且一些地区变暖的速度比其他地区快，那么在向更暖气候过渡的不同阶段，不同的区域反馈机制将是突出的。随着时间的推移，反馈过程的混合变化可以解释ICS的演变。因此，本研究主要通过分析气候模式输出和一系列重点气候模式实验来探讨气温上升的空间格局。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The educational component of the CAREER award seeks to introduce climate science to the classroom at the high school, college and graduate school levels. The project works with high school science teachers to develop educational materials through workshops held on the University of Washington campus. The goal is to introduce the climate system in an accessible form, simplifying it as much as possible while retaining the essential elements and their interactions. Simple models are developed which use one or two equations to represent the evolution of global temperature in response to radiative forcing, incorporating factors such as feedback effects and time delay due to ocean heat storage. Despite their simplicity the models are closely related to cutting-edge research, as such models are commonly used to understand behaviors found in full complexity climate models and the observational record. The models and accompanying materials also introduce students to simple differential equations, scientific computation, hypothesis testing and the scientific method. A parallel effort is organized for high school science teachers in eastern Washington with the help of scientists and education specialists at the Pacific Northwest National Laboratory in Richland.The research component of the award is an effort to understand the sensitivity of global temperature to changes in greenhouse gas concentration. This sensitivity, commonly referred to as climate sensitivity, is quantified as the warming produced by a doubling of carbon dioxide (CO2) over its pre-industrial level. Climate sensitivity is an important indicator of the risk posed by greenhouse gas increases, and much effort has been devoted to estimating its value.But the extent to which the impact of greenhouse gas increases on global temperature can be represented by a single, fixed sensitivity value is unclear. Several lines of research suggest that global temperature becomes more sensitive to the radiative effect of CO2 increases as climate warms. In that case that the warming ultimately produced by CO2 doubling is greater than what would be expected from sensitivity estimates calculated at any time prior to the full temperature increase.The temperature increase achieved when the climate system has fully adjusted to the radiative effect of CO2 doubling is termed the equilibrium climate sensitivity, or ECS. The terminology distinguishes between ECS and instantaneous climate sensitivity (ICS), meaning the incremental temperature increase from an incremental radiative increase during the ramp-up to CO2 doubling. ECS is a fixed value whereas ICS evolves over time, approaching ECS as the ultimate doubling temperature is realized. While there is evidence that ICS is always less than ECS, the excess of ECS over ICS has not yet been satisfactorily explained.Research under this CAREER award seeks to understand why ICS changes over time and why it is consistently lower than ECS. A key issue in the research is the dependence of climate sensitivity on feedback mechanisms that are specific to particular regions. A simple example is the sea ice albedo feedback, in which warming causes sea ice retreat, replacing bright reflective ice cover with the darker ocean surface, causing more solar energy to be retained in the climate system and causing further warming. Since the albedo feedback only occurs within ice covered regions, the feedback strength depends on the warming of the polar cap rather than the increase in globally averaged temperature. The PI hypothesizes that much of the ECS-ICS difference is due to regional differences in the rate of warming produced by greenhouse gas increases: if different regions are home to different feedback processes and some regions warm faster than others, different regional feedback mechanisms will be prominent during different stages of the transition to a warmer climate. Changes over time in the mix of feedback processes can then account for the evolution of ICS. The research thus focuses on the spatial pattern of temperature increase, which is explored primarily through analysis of climate model output and a set of focused climate model experiments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.