A framework to understand the transient climate response to emissions

  • Richard G Williams, Philip Goodwin, Vassil M Roussenov, Laurent Bopp
  • Environmental Research Letters, January 2016, Institute of Physics Publishing
  • DOI: 10.1088/1748-9326/11/1/015003

A framework to understand how global surface warming connects to carbon emissions

What is it about?

Climate models reveal that global surface warming is nearly proportional to cumulative carbon emissions on multi-decadal to centennial timescales. What is unclear though is the cause of this relationship and the relative importance of different climate processes. To gain insight, we focus on the dependence of surface warming on carbon emissions, which is simply the slope of surface warming when plotted against how much carbon is emitted. This dependence of surface warming on carbon emissions is defined by the product of three terms: the dependence of surface warming on radiative forcing, the radiative forcing divided by the radiative forcing from atmospheric carbon dioxide, and the dependence of radiative forcing from atmospheric carbon dioxide on cumulative carbon emissions.

Why is it important?

Complex Earth system models are used to provide projections of how the climate will change with carbon emissions. However, it is difficult to understand the underlying cause of a climate model projection. This framework is used to interpret the climate response over the next century for two Earth System Models of differing complexity, both containing a representation of the carbon cycle: an Earth System Model of Intermediate Complexity, configured as an idealised coupled atmosphere and ocean, and an Earth System Model, based on an atmosphere–ocean general circulation model and including non-CO2 radiative forcing and a land carbon cycle. There is limited change in the dependence of surface warming on carbon emissions due to the ocean and terrestrial system acting to sequester both heat and carbon: carbon uptake acts to decrease the dependence of radiative forcing from carbon dioxide on carbon emissions, which is partly compensated by changes in ocean heat uptake acting to increase the dependence of surface warming on radiative forcing.

Perspectives

Professor Richard G Williams
University of Liverpool Department of Earth Ocean and Ecological Sciences

We hope that this article provides a theoretical underpinning to understand the response of climate models. While the detailed projections of climate models are sensitive to different model choices and forcing, the overall response of the climate models is easy to understand: the more carbon we emit, the warmer it will become. This study brought together authors with different skills and resources. Our theory and framework is tested using climate models of differing complexity. The underlying controls of the climate response though turned out to be broadly similar, determined mainly by the climate feedback parameter and how the ocean takes up heat and carbon. This article was invited after a special session at the American Geophysical Union meeting at San Francisco in December 2014.

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http://dx.doi.org/10.1088/1748-9326/11/1/015003

The following have contributed to this page: Professor Richard G Williams