Emergent Constraints on Climate Sensitivities
Scientists at the University of California Los Angeles, University of Exeter, and the UK Centre for Ecology and Hydrology have synthesized the current state of knowledge surrounding the climate model evaluation technique known as emergent constraints. The team finds over fifty different proposed constraints spanning a wide variety of geophysical quantities and highlights key gaps in existing research. They also examine the underlying theory and statistical underpinnings of the method, and how emergent constraints link to other fields.
An important aspect of this manuscript is the attempt to convey the emergent constraint methodology to an audience outside of the geosciences by publishing in Reviews of Modern Physics. The team hopes that this review will encourage physicists to contribute to the growing field, bringing important physical insights. Given the rapid uptick in emergent constraint development in recent years, this paper will also serve as an important reference to climate scientists for best practices to follow.
Despite major advances in climate science over the last 30 years, persistent uncertainties in projections of future climate change remain. Climate projections are produced with increasingly complex models that attempt to represent key processes in the Earth system, including atmospheric and oceanic circulations, convection, clouds, snow, sea ice, vegetation, and interactions with the carbon cycle. Uncertainties in the representation of these processes feed through into a range of projections from the many state-of-the-art climate models now being developed and used worldwide. Here a promising way to make use of the ensemble of climate models to reduce the uncertainties in the sensitivities of the real climate system is reviewed. The emergent constraint approach uses the model ensemble to identify a relationship between an uncertain aspect of the future climate and an observable variation or trend in the contemporary climate. This review summarizes previously published work on emergent constraints and discusses the promise and potential dangers of the approach. Most importantly, it argues that emergent constraints should be based on well-founded physical principles such as the fluctuation-dissipation theorem.