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

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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.