Coastal vegetation and groundwater flows facilitate the transfer of at least 500 million metric tons of carbon into the ocean each year, according to new scientific modeling that expands their known importance in keeping planet-warming carbon dioxide out of the atmosphere.

Led by South Korean climate scientists, the research published March 15 in Global Biogeochemical Cycles showed that annual land-to-ocean carbon flows totaled 1.4 billion metric tons, more than 50% higher than previous estimates that mostly focused on the carbon carried by rivers.

It was the first to model the global movement of carbon compounds in and out of the ocean by tracking ratios of carbon isotopes, which helped determine the different origins of the organic element.

Carbon, the backbone of many essential life-bearing compounds, flows between the ocean, atmosphere, land and living beings in what is called the carbon cycle. It variously takes the form of organic matter in biological life and its remnants in soil and the oceans, as well as inorganic matter such as carbon dioxide, methane and rocks. 

Tracking the carbon cycle is important for understanding the natural storage of carbon in relation to climate change, said lead author Eun Young Kwon, a project leader at the Center for Climate Physics of South Korea's Institute for Basic Science. The oceans absorb about 31% of carbon dioxide emissions from burning fossil fuels and other human activity, which totals 2.6 billion metric tons per year. Carbon stays in the ocean for several hundred years on average and is often permanently stored when transformed into ocean sediment.

But beyond the land-based carbon carried by rivers to the ocean, how it is released by groundwater and coastal vegetation such as estuaries, salt marshes and mangrove forests is challenging to detect.

"It is difficult to measure the magnitude of non-riverine carbon export because the flow occurs beneath the sea surface at very slow rates, sometimes slower than the instrument detection limits," Kwon said. "Due to these challenges, our understanding of the land-to-ocean carbon transport has been incomplete and highly uncertain."

The new model relied on the ratio between the two stable isotopes of carbon atoms, carbon-12 and carbon-13, which differ by one neutron and are present in nature in different proportions. Plants and the soil they decompose into are relatively low in carbon-13, while the ocean has higher concentrations, so adding land-based organic matter to the ocean dilutes its ratio of the two isotopes.

This change in ratio was used by the paper's authors in their model to indirectly measure the world's carbon flows into and out of the ocean using carbon ratio measurements from previous research.

"Our novel approach led us to a surprising result," Kwon said. "Our estimated terrestrial carbon inputs are twice as much as the previously known riverine carbon inputs to the global ocean."

In addition to the approximately 600 million metric tons of carbon known to be flowing into the ocean from rivers globally, another 800 million metric tons were detected by the model and attributed to groundwater and coastal vegetation outflow. Human-caused effects such as climate change did not meaningfully affect the land-to-ocean carbon flow, the researchers found.

The results included 500 million more metric tons of carbon than 2013 estimates from the Intergovernmental Panel on Climate Change, which estimated 900 million metric tons from both rivers and groundwater. The model showed that 95% of the non-river carbon flows occur in the Pacific and Indian Oceans, where the coasts contain large swathes of vegetation, while the Atlantic Ocean receives most of its land-based carbon from rivers.

The higher-than-expected flow of carbon into the oceans means that plants on the coast play an even more important role in storing carbon, which blunts the rise of carbon dioxide concentrations in the atmosphere, Kwon said.

"If we lose coastal vegetations for coastal developments, we may end up losing this important component of the carbon system, reducing the ocean's carbon storage as well as the carbon storage by coastal vegetations and sediments," she said.

The study also modeled carbon flows between the ocean and the atmosphere and the production of sediments, which largely agreed with previous research that used different methods.

The exact processes behind the movement of carbon from land to water and how changes in the climate and environment affect them are largely unknown, Kwon said. According to her, unanswered questions include where the non-river carbon flows are occurring and what drives that process as well as the transfer of carbon from coastal waters to the open ocean. Kwon's lab and her institute are hoping to tackle these issues with higher-resolution models of the global carbon cycle.

Kwon said there also needs to be more and better direct data on coastal carbon cycles, especially in the Pacific and Indian Oceans, where most of the non-river land-to-ocean outflows are occurring but where observational data is weakest.

"Better observational coverage will help us cross-validate our findings, as well as better address the key research questions," she said.

The study, "Stable Carbon Isotopes Suggest Large Terrestrial Carbon Inputs to the Global Ocean," published March 15 in Global Biogeochemical Cycles, was authored by Eun Young Kwon and Axel Timmermann, Pusan National University and IBS Center for Climate Physics; Timothy DeVries, University of California, Santa Barbara; Eric Galbraith, Autonomous University of Barcelona and Catalan Institution for Research and Advanced Studies; and Jin-Hyeok Hwang and Guebuem Kim, Seoul National University.