Southern Ocean Carbon Sink Stronger than Previously Thought

Two new studies, one published in Geophysical Research Letters (GRL), the other in Science, find that the Southern Ocean carbon sink has become stronger rather than weaker, contrary to what some scientists previously thought.

In climate parlance, a “carbon sink” is anything that absorbs more carbon dioxide (CO2) from the atmosphere than it releases, while a “source” is anything that emits more CO2 than it absorbs. Oceans, forests, and soils comprise the world’s major sinks. The balance between sinks and sources determines the “airborne fraction” – the annual percentage of CO2 emissions that accumulates in the atmosphere.

The GRL paper, which focuses on the Drake Passage, the roughest and windiest part of the Southern Ocean, is based on more than one million observations made by ocean-going vessels during 2002-2015. The Science paper, which examines the entire Southern Ocean, incorporates the GRL study data plus other data going back almost three decades.

As summarized by the American Geophysical Union, which publishes GRL, the studies “conclude that the Southern Ocean has increasingly taken up more carbon dioxide during the last 13 years.” That’s a big deal. Although covering only 26% of total ocean area, the Southern Ocean accounts for nearly 40% of all CO2 absorbed by the world’s oceans.

The increasing efficiency of the Southern Ocean carbon sink conflicts with one of the Obama administration’s rationales for upping its “social cost of carbon” (SCC) estimates by roughly 60% between 2010 and 2013.

The administration bases its SCC estimates on three so-called integrated assessment models – computer programs combining speculative climatology with speculative economics. For its 2013 estimates, the administration used a version of the Dynamic Integrated Climate-Economy (DICE) model that assumes a weakening of ocean sinks. 

As noted in our comment letter on the 2013 estimates, empirical studies find that the CO2 airborne fraction has held constant in recent decades, indicating no decline in the efficiency of ocean sinks. The new Southern Ocean studies provide additional evidence that ocean sinks are keeping up with anthropogenic emissions.