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.

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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.

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