Climate change

Rate of CO2 absorption by ocean impacted by human-caused atmosphere changes

A new study from the Earth Institute at 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.

ocean carbon sink rate

“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.

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