Oceans soaked up 35% of fossil fuel carbon dioxide as land absorption declined

A new study published in the journal Science has provided some of the strongest evidence to date as to how Earth’s carbon sinks have changed over time. Researchers at UC San Diego’s Scripps Institution of Oceanography analyzed more than 30 years of atmospheric oxygen and carbon dioxide (CO₂) measurements. According to the data, oceans absorbed 35 percent of fossil-fuel CO₂ emissions between 1990 and 2019, compared with 13 percent absorbed by land ecosystems. Together, the two sinks absorbed roughly half of those emissions. If lands and oceans weren’t absorbing half the CO₂ from fossil fuel emissions, “we would be living on a very different planet,” according to the study’s lead author Eric Morgan. “But these sinks can’t be observed directly. Figuring out exactly how much they have absorbed, and how they have changed over time, is a central goal of carbon cycle research,” Morgan told Phys.org. Ocean uptake increased According to the study, oceans absorbed an average of 2.8 billion metric tons (3.1 billion US tons) of carbon dioxide per year between 1990 and 2019, about 35 percent of total fossil fuel emissions. Each decade, researchers observed that annual absorption increased by about 630 million metric tons (694 million US tons). Land ecosystems absorbed around 1.0 billion metric tons (1.1 billion US tons) of carbon dioxide a year, with an uncertainty of ±0.5 billion metric tons (0.55 billion US tons), far less than the oceans. The land sink peaked in the decade around 2009, and has declined ever since. “It was very surprising to see how the net land sink has not kept pace with rising CO₂ levels. Our data underscore that the land sink may be pretty fragile,” added Morgan. How researchers measured the sinks As fossil fuels burn, they consume atmospheric oxygen and release carbon dioxide. On land, plants absorb carbon dioxide through photosynthesis and release oxygen in the process. Ocean uptake, however, occurs primarily through reactions in seawater that do not affect oxygen levels. The researchers started their work by studying observations collected through the Scripps O₂ Program. By tracking changes in these gases, they estimated how much carbon entered the ocean and land ecosystems. The Scripps O₂ program was launched in 1989, it measures changes in atmospheric oxygen levels from air samples collected at stations along the Pacific from the high Arctic to the South Pole. “It is not easy to keep these kinds of measurements going year after year, but the payoff for improving our understanding of the carbon cycle is huge,” said co-author Ralph Keeling, who is also a professor at Scripps. Implications for climate models Their findings challenge many climate models that simulate carbon uptake by land and oceans. Some of them have predicted that land uptake would increase. However, the data show that land sinks have not shifted that significantly. The amount of carbon they have absorbed has, in fact, declined. One theory points to deforestation and other land-use changes that may have released more carbon dioxide into the atmosphere. Now, thanks to this research, climate scientists might have to adjust climate models to accurately represent how land ecosystems absorb carbon. As for the major consequences of CO₂ emissions, Phys.org notes that ocean carbon sinks contribute to acidification. As the ocean absorbs more CO₂, the chemical reactions involved lower the pH of seawater, making it more acidic. Warmer waters hold less carbon dioxide than colder waters. As temperatures rise, the oceans might become less capable of absorbing carbon. Warmer surface water doesn’t mix with deeper, colder ocean water, which could be a long-term carbon storage facility that may no longer work. As the lead study author previously said, without these carbon sinks, “we would be living on a very different planet.” The latest research will hopefully equip scientists with the most up-to-date information about how the planet is dealing with the rising carbon dioxide levels. The study concludes that its observations question the ability of some process-based models to accurately predict future climate conditions. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Originally from LA, Maria Mocerino has been published in Business Insider, The Irish Examiner, The Rogue Mag, Chacruna Institute for Psychedelic Plant Medicines, and now Interesting Engineering.