Oceans Absorb a Growing Share of Fossil Carbon as Land Sinks Stall, Scripps Study Finds

Since 1990, the ocean has taken up a growing amount of the carbon dioxide released by burning fossil fuels, while the net uptake by forests, soils and other land ecosystems has not grown at all. That is the central finding of a study led by scientists at UC San Diego's Scripps Institution of Oceanography, published Nov. 12 in Science.

The work rests on more than 30 years of air measurements from the Scripps O2 Program. Researchers have collected regular air samples at remote sites around the world, shipped them back to Scripps, and measured the oxygen and carbon dioxide in each. The two gases together carry the signal. Burning coal, gas and oil produces CO2 and consumes oxygen. On land, photosynthesis draws down CO2 and releases oxygen. In the ocean, CO2 is absorbed mainly through reactions in seawater that do not affect oxygen. By following long-term trends in both gases, the team could separate how much carbon went into the sea from how much went into land ecosystems.

From 1990 to 2019, the ocean absorbed an average of 2.8 billion metric tons of CO2 per year, about 35% of fossil fuel emissions over that period. The ocean sink strengthened by 0.63 billion metric tons of carbon per year with each passing decade. That uptake slows the rise of CO2 in the atmosphere, but it carries a cost for the ocean itself, including lower pH through ocean acidification.

The net land sink told a different story. Land ecosystems took up an average of 1 billion metric tons of carbon per year over the same three decades, about 13% of fossil emissions. There was no notable long-term growth, though the sink swung widely in strength from one decade to the next. It peaked in the decade centered on 2009 and has generally declined since.

That result conflicts with many computer models of land absorption, which show the natural land sink growing as rising CO2 and nitrogen fertilize plant growth. For those models to hold, emissions from deforestation and other land-use change would have had to rise enough to cancel the growth. Current estimates show those emissions falling. "It was very surprising to see how the net land sink has not kept pace with rising CO2 levels," said Eric Morgan, a Scripps project scientist and the study's lead author. "Our data underscore that the land sink may be pretty fragile."

The ocean's growing role may not last. Warmer water holds less CO2, and warmer surface water mixes less efficiently into the deep ocean, where carbon can be locked away from the atmosphere for many hundreds of years. Together, the land and ocean sinks have offset roughly half of fossil fuel emissions, slowing warming. "Without the land and ocean absorbing half the CO2 from fossil fuel emissions, we would be living on a very different planet," Morgan said, noting that the sinks cannot be observed directly.

The study stresses that representing these sinks accurately matters, because climate models define the emissions-reduction pathways needed to meet a temperature target. Ralph Keeling, a Scripps geosciences professor and co-author, said keeping such measurements going year after year is not easy, but the payoff for understanding the carbon cycle is huge.

Keeling founded the O2 Program in 1989. It now maintains 10 sampling sites along a Pacific transect from the high Arctic to the South Pole, and it is a companion to the Keeling Curve, the CO2 record begun at Mauna Loa by Charles David Keeling in 1958. Funding came from the U.S. National Science Foundation, NOAA and Schmidt Sciences, with added support for the University of Exeter authors from the EU's OceanICU and UK Research and Innovation. Co-authors include researchers from NCAR and UC Santa Barbara, Exeter, the Rochester Institute of Technology, the Max Planck Institute for Biogeochemistry, Japan's National Institute for Environmental Studies, and NOAA and CIRES at the University of Colorado Boulder.