Sensitivity of global surface warming to carbon emissions in a suite of climate models

What is it about?

Climate models are used to project how much surface temperature is likely to rise over the next century given the emission of carbon to the atmosphere. The climate models agree on the simple result that the more carbon we emit, the warmer it will become. However, the climate models disagree on the amount of surface temperature rise for a given carbon emission. In this study, we diagnose the response of a suite of 9 Earth system model to increasing atmospheric carbon dioxide over the rest of this century. We employ our theory to understand the different responses of each Earth system model. Our theory consists of a single equation connecting surface temperature rise and the amount of carbon emitted to the atmosphere. The equation is based on a heat balance and a global carbon inventory.

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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 any differences in climate model projections. A combination of our theory and a new framework are used to interpret the climate response over the next century for 9 complex Earth system models, based on an atmosphere–ocean general circulation model, including radiative forcing from a range of greenhouse gases and aerosols, and including a land carbon cycle. We find that inter-model differences in how much the surface warming varies with a carbon emission are mainly due to inter-model differences in the climate sensitivity (determining the long-term warming response to radiative forcing) and how the model takes up heat and carbon. The ocean turns out to provide partially-opposing contributions: the ocean uptake of carbon acts to reduce the radiative forcing for a given carbon emission, while a relative decline in the ocean uptake of heat enhances the fraction of radiative forcing used to warm the surface.

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