Connecting surface warming to carbon emissions - atmospheric and oceanic viewpoints

What is it about?

Climate model projections reveal a simple answer - the increase in surface temperature is nearly proportional to the increase in the total amount of carbon emitted to the atmosphere and ocean. While this empirical relationship has been identified over the last 10 years, the reasons for this relationship are not well understood. In this study, we reconcile two different views of how the surface warming increase with carbon emissions may be related either to changes in atmospheric properties or ocean properties. Our work sets out the following view: The increase in surface temperature with the amount of carbon emitted to the atmosphere depends on the uptake and storage of heat and carbon. Ocean heat uptake acts to strengthen surface warming, as the ocean becomes more stratified in time. Carbon uptake by the ocean and terrestrial system acts to weaken surface warming by removing carbon from the atmosphere. The proportionality of surface warming to carbon emissions may be written in terms of a thermal contribution multiplied by a carbon contribution. The carbon contribution depends on the increase in the atmospheric carbon inventory plus the maximum amount of carbon that the ocean may hold. To understand the role of ocean chemistry, we diagnose the response of climate models of differing complexity over centennial and millennial timescales. In all the models, there is a similar carbon response: During emissions, the ocean surface acidifies and the maximum amount of carbon that the ocean can hold decreases, which weakens the carbon contribution to the proportionality of surface warming to carbon emissions. Hence, ocean carbonate chemistry is important in controlling the proportionality of surface warming to carbon emissions and its evolution in time.

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Why is it important?

We need to gain more insight into the response of climate models with increasing carbon emitted to the atmosphere. Here we provide theory to understand the climate model response either in terms of atmospheric or oceanic properties. This work reconciles two different ongoing ways of understanding the climate response. Both atmospheric or oceanic views are formally equivalent to each other, each offers different insight into the underlying processes controlling the surface warming response to carbon emissions. In particular, this study emphases how carbon dioxide reacts in seawater, forming bicarbonate and carbonate ions, and how this partitioning alters with increasing atmospheric carbon dioxide driving increased ocean acidity. We explore this response in a hierarchy of climate models and reassuringly find a similar carbonate chemistry response in all of them. Consequently, inter-model differences in the surface warming response to carbon emissions are unlikely to be due to this ocean carbonate chemistry part of the climate system.

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