The ocean absorbs about 30 percent of the carbon humanity is emitting into the atmosphere, and it takes up over 90 percent of the excess heat that increasingly abundant greenhouse gases trap on earth. The rate at which this carbon and heat uptake occurs is thus crucial in regulating how quickly the planet warms. The ocean’s circulation plays a key role in setting the uptake rates, because the currents connect the surface ocean, where carbon and heat are taken up, with the vast reservoir of the interior ocean, where the bulk of the carbon and heat is stored.
As a vast body of turbulent fluid, the ocean has a circulation that displays a complex hierarchy of current systems, ranging in size from planetary-scale overturning to centimeter-scale turbulence. This broad range of scales — and interactions between the scales — is the fundamental challenge to understanding what drives ocean currents, how the circulation might change in the future, and how the ocean may have changed in the past. Oceanographers employ a range of methods to meet this challenge: much of our understanding is encapsulated in simplified theory, but all theory is strongly informed by observations, ranging from satellite-based remote sensing to ship-based in situ sampling, and by numerical models of isolated processes or the ocean as a whole.