Oceanic Constraints on Global Surface Warming Across Timescales

The magnitude of future atmospheric surface warming driven by greenhouse gas forcing depends on (i) the rate of emissions, (ii) the strength of atmospheric radiative feedbacks, and (iii) the rate of global ocean heat uptake. A central aim of climate research has been to reduce uncertainty in radiative feedback processes, given their dominant role in setting Earth’s equilibrated warming response to greenhouse gas emissions — warming reached many centuries from now.

In contrast, far less focus has been placed on understanding the ocean heat uptake processes that govern the rate of warming – which is arguably of greater relevance to society and climate policy. The ocean-heat-uptake driven influence on surface warming can be quantified by the Ocean Heat Uptake Efficiency (OHUE), defined as the global mean rate of ocean heat uptake divided by the global mean surface temperature anomaly.

OHUE has widely been interpreted as representing how efficiently ocean dynamics move heat from the surface ocean to depth, with its time dependence being the primary control on the pace of climate warming. However, there is a wide spread in the magnitude and time dependence of OHUE across global circulation models (GCMs), resulting in divergent predictions of the rate at which long-term warming is reached.

This spread reflects the fact that the dynamical ocean processes underlying OHUE and its time dependence are not well understood. This project will employ a hierarchy of numerical tools to (i) characterize the time dependence and spread in OHUE across GCMs and over a range of timescales (ii) identify the underlying mechanisms governing OHUE across different timescales and dynamical regimes, and (iii) identify new observational constraints, which can be used to reduce uncertainty in OHUE across various time-horizons.

The scientific outcomes of the project will have far-reaching community and societal relevance. In the near term, the project will generate practical metrics that, when leveraged with oceanic observations, will reduce uncertainty in the future global surface warming rate across different timescales. Further, the collaborators involved are chosen to solidify lasting connections between synergistic research efforts at Caltech, University of Washington, and New York University.

Thus, in a longer-term sense, this project will ensure ongoing collaborative efforts and scientific output. The project would also support an early career scientist as a Senior Research Associate. In doing so, it would facilitate her ongoing graduate student mentorship and enable her to advise an undergraduate student under the Caltech Summer Undergraduate Research Fellowships (SURF) Program.

The project’s SURF student component is structured to impart valuable insight into ocean dynamical theory and climate sensitivity, as well as scientific research skills. More broadly, the idealized modeling components of this project would provide effective teaching tools to be incorporated into coursework at Caltech, amplifying the impact of the project’s scientific goals.

This project aims to rectify a major gap in climate research by targeting OHUE, the primary source of uncertainty in future warming rates. It will lead to an improved understanding of oceanic processes governing the rates of ocean heat uptake and the atmospheric surface warming across a range of timescales. Through the targeted use of a novel numerical model hierarchy, this study will advance the process-level understanding of OHUE and yield a deeper theoretical grasp on the role of oceans in climate.

A central hypothesis is that the time dependence of OHUE is set not only by the efficiency of processes moving heat downward away from the surface ocean, but also by the processes that determine how long that heat is sequestered before re-emerging at the surface. This work will shed new light on future climate evolution, namely by identifying the drivers and the timescale of a key multi-centennial transition in OHUE dynamics— from a “downward” heat transport regime” to an “upward” regime.

The work could thus provide a physical theory for why the rate of climate equilibration and surface warming under greenhouse-gas forcing is spread so widely across GCMs, and in doing so, improve our understanding of one of the major uncertainties in climate projection.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.