Efficiency of ocean as a heat sink, atmospheric gases sponge
The ocean is tempering the effects of greenhouse gas warming of the atmosphere by absorbing heat and gases. Over the last ten years, the ocean has absorbed one-fourth of human emissions of carbon dioxide as well as 90 percent of additional warming due to the greenhouse effect. Acting like a massive sponge, the ocean pulls from the atmosphere heat, carbon dioxide, and other gases (e.g., chlorofluorocarbons, oxygen, and nitrogen) and stores them in their depths for decades to centuries and millennia.
Using the NASA-Goddard Institute for Space Studies (GISS) and the MIT ocean general circulation models with prescribed surface forcing, NASA and MIT scientists found that gases are more easily absorbed over time than heat. As the Atlantic Meridional Overturning Circulation (AMOC) slows down, the ocean absorbs less of both atmospheric gases and heat, though its ability to absorb heat is more greatly reduced.
Specifically, the two models were forced with chlorofluorocarbons (CFC-11) boundary conditions at the surface of the ocean with realistic and idealized time evolution in order to tease apart the effects of the changing thermohaline circulation strength to uptake passive tracers and heat. The researchers found that although both heat and passive tracers scale with the strength of the AMOC, their temporal evolution differs.
As a result, estimating how much heat the ocean absorbs by only using a tracer may not be accurate. Scientists need to think differently about how the ocean responds to taking up heat and passive tracers or greenhouse gases. By refining the understanding of how efficiently gases and heat are taken up, scientists will be able to improve global climate model projections for future climate scenarios.
As most of the excess heat and greenhouse gases from climate change or even chemical pollutants will go into the ocean, ocean large-scale currents will recirculate that extra load and, at some point, will release some of it back to the atmosphere, where it will keep raising temperatures, regardless of future carbon dioxide emissions scenarios. This eventual release of buried gases and heat from the ocean is sometimes called the “warming in the pipeline” or “warming commitment.”
Role of the ocean's AMOC in setting the uptake efficiency of transient tracers (Geophysical Research Letters)
2NASA Goddard Institute for Space Studies
3Massachusetts Institute of Technology