How the oceans absorb carbon dioxide is critical for predicting climate change.
Air-sea gas exchange is a physio-chemical process, primarily controlled by the air-sea difference in gas concentrations and the exchange coefficient, which determines how quickly a molecule of gas can move across the ocean-atmosphere boundary. It takes about one year to equilibrate CO2 in the surface ocean with atmospheric CO2, so it is not unusual to observe large air-sea differences in CO2 concentrations. Most of the differences are caused by variability in the oceans due to biology and ocean circulation. The oceans contain a large reservoir of carbon that can be exchanged with the atmosphere because the CO2 reacts with water to form carbonic acid and its dissociation products. As atmospheric CO2 increases, the interaction with the surface ocean will change the chemistry of the seawater resulting in ocean acidification.
Evidence suggests that the past and current ocean uptake of human-derived (anthropogenic) CO2 is primarily a physical response to rising atmospheric CO2 concentrations. Whenever the partial pressure of a gas is increased in the atmosphere over a body of water, the gas will diffuse into that water until the partial pressures across the air-water interface are equilibrated. However, because the global carbon cycle is intimately embedded in the physical climate system there exist several feedback loops between the two systems. For example, increasing CO2 modifies the climate which in turn impacts ocean circulation and therefore ocean CO2 uptake. Changes in marine ecosystems resulting from rising CO2 and/or changing climate can also result in changes in air-sea CO2 exchange. These feedbacks can change the role of the oceans in taking up atmospheric CO2 making it difficult to predict how the ocean carbon cycle will operate in the future.
A new study from Columbia University has found that the rate at which the ocean absorbs carbon dioxide can be influenced by human-caused changes to the atmosphere and volcanic eruptions.
The research focuses on resolving the uncertainty around what has caused the ocean to take up varying amounts of carbon throughout the last 30 years. The team’s discoveries will facilitate more accurate measurements and projections of how much the planet might heat in the future, and how much the ocean might offset climate change.
The authors of the study have predicted that, as a result of the lessening global fuel consumption due to the COVID-19 pandemic, the ocean will not continue its recent historic pattern of absorbing more carbon dioxide each year than the year before. They note that it could even take up less in 2020 than in 2019.
“We didn’t realise until we did this work that these external forcings, like changes in the growth of atmospheric carbon dioxide, dominate the variability in the global ocean on year-to-year timescales. That’s a real surprise,” said lead author Galen McKinley, a carbon cycle scientist at Columbia University’s Lamont-Doherty Earth Observatory. “As we reduce our emissions and the growth rate of atmospheric carbon dioxide slows down, it’s important to realise that the ocean carbon sink will respond by slowing down.”
The ocean is the planet’s largest carbon sink – a natural system that absorbs excess carbon dioxide from the atmosphere and stores it. Approximately 40% of the carbon dioxide added to the atmosphere by fossil fuel burning since the dawn of the industrial era has been taken up by the ocean.
There’s variability in the rate at which the ocean takes up carbon dioxide, which isn’t fully understood. In particular, the scientific community has puzzled over why the ocean briefly absorbed more carbon dioxide in the early 1990s and then slowly took up less until 2001, a phenomenon verified by numerous ocean observations and models. By understanding variability in the ocean carbon sink, scientists can improve projections of how the ocean system will slow down.
Pinatubo was the second-largest volcanic eruption of the 20th century. The estimated 20 million tons of ash and gases it spewed high into the atmosphere had a significant impact on climate and the ocean carbon sink. The researchers found that Pinatubo’s emissions caused the ocean to take up more carbon in 1992 and 1993. The carbon sink slowly declined until 2001, when human activity began pumping more carbon dioxide into the atmosphere. The ocean responded by absorbing these excess emissions.
“One of the key findings of this work is that the climate effects of volcanic eruptions such as those of Mount Pinatubo can play important roles in driving the variability of the ocean carbon sink,” said coauthor Yassir Eddebbar, a postdoctoral scholar at Scripps Institution of Oceanography.
Investigating how the Pinatubo eruption impacted the global climate and the ocean carbon sink, and whether the decline in emissions due to COVID-19 is reflected in the ocean are among the research team’s next plans.
McKinley cautions that as global emissions are cut, there will be an interim phase where the ocean carbon sink will slow down and not offset climate change as much as in the past. That extra carbon dioxide will remain in the atmosphere and contribute to additional warming, which may surprise some people.
“We need to discuss this coming feedback,” said McKinley. “We want people to understand that there will be a time when the ocean will limit the effectiveness of mitigation actions, and this should also be accounted for in policymaking.”
To read the full paper, “External Forcing Explains Recent Decadal Variability of the Ocean Carbon Sink”, click here.
Between 1994 and 2007, oceans absorbed 34 gigatonnes of carbon dioxide, or 31 percent of what humans put into the atmosphere during that time, a study published Friday in Science concluded. That means oceans absorbed the weight of 2.6 billion Volkswagen Beetle cars in carbon on average each year during the study period, study author and senior scientist at the National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory in Seattle Richard Feely told The Seattle Times.
"[It's] a huge service the oceans are doing that significantly reduces global temperature," Feely said.
The ocean is able to absorb carbon dioxide in two steps, a press release from ETH Zurich, one of the institutions involved in the study, explained:
Carbon dioxide dissolves in the ocean's surface water
Ocean circulation distributes and sinks it into deeper waters, where it builds up.
Scientists had long thought this was the case, but the study, which looked at more than 100,000 seawater samples from all parts and depths of the ocean, confirmed their models.
"The oceans have been taking up carbon dioxide recently in exactly the way we thought they would," University of Washington associate oceanography professor Curtis Deutsch, who was not involved with the research, told The Seattle Times. "There's nothing really surprising about the results, but they are super important in confirming that we really do understand the system and the way it operates."
So far, the researchers found, oceans have increased the amount of carbon dioxide they absorb as atmospheric levels have increased. However, there will come a point when this will no longer be the case.
"At some point the ability of the ocean to absorb carbon will start to diminish," study author and NOAA climate scientist Jeremy Mathis said. "It means atmospheric CO2 levels could go up faster than they already are."
The ocean sink should continue to work as it does now for the next 50 years.
But it does so at a cost. An increase in carbon dioxide in the ocean leads to ocean acidification, which can dissolve the calcium carbonate that makes up mussel shells and coral skeletons, and interrupt processes like fish breathing.
"Documenting the chemical changes imparted on the ocean as a result of human activity is crucial, not least to understand the impact of these changes on marine life," research team leader and ETH Zurich environmental physics professor Nicolas Gruber said.
This is already harming the ecosystems of the Pacific Northwest. The increasing load of carbon dioxide in the ocean interior is already having an impact on the shellfish industry, particularly along the U.S. West Coast.