New paper by Rose and Rayborn on effects of ocean heat uptake

The latest paper from our group has been accepted for publication in Current Climate Change Reports. We wrote this piece at the invitation of the section editor Mark Zelinka.

The paper is Rose and Rayborn, “The effects of ocean heat uptake on transient climate sensitivity”. It deals with the phenomenon of time-dependent climate sensitivity, and explores some compelling new ideas about connections between the oceans, atmospheric radiation, and global cloud cover that determine climate sensitivity. Our paper includes substantial review as well as some interesting original results and speculations.

Climate sensitivity here means the amount of warming per unit additional energy flux into the system. When a comprehensive coupled climate model is subjected to a steady radiative forcing (such as an abrupt increase in atmospheric CO2), we typically find that the climate sensitivity increases with time as the model adjusts towards its new, warmer equilibrium state. Why is this? Are new positive feedback processes coming into play as the climate warms? Is it necessary to think about the climate system as fundamentally non-linear?

We argue for a simpler alternative view: the uptake of heat by the oceans tends to be localized to the subpolar regions, and this localized heat sink is something like 2x more effective at altering the global planetary temperature than CO2 – a result that was demonstrated in one of Brian’s earlier papers. We show that an apparent increase in climate sensitivity over time is a natural consequence of the gradual waning of this high-efficacy ocean heat uptake as the climate system warms toward its new equilibrium temperature.

We also argue for a robust physical mechanism linking subpolar ocean heat uptake with changes in subtropical low cloud cover, mediated by changes in the stratification of the atmosphere. See also the recent paper by Rose and Rencurrel for more on this! Understanding the constraints how low cloud changes contribute to global warming (now and in the future) is one of the key goals of modern climate science. Our results suggest that some aspects of low cloud changes may be driven in systematic ways by patterns of heat fluxes in and out of the ocean. If this is true, it may mean that errors and uncertainties in future climate model projections may be more reducible and falsifiable than we thought.