Ocean carbon sink strengthens while land carbon sink stalls new study finds
The ocean is absorbing more carbon dioxide, while the amount taken up by land ecosystems has remained unchanged for more than 30 years, a pioneering new study has shown.
The findings raise questions about the future capacity of land to absorb carbon dioxide, and underline the growing importance of the oceans in regulating the global carbon cycle, through ‘sinks’ – or the net uptake from the atmosphere.
The international study, led by Scripps Institution of Oceanography at the University of California San Diego, and co-authored by experts from the University of Exeter, uses more than 30 years of atmospheric oxygen and carbon dioxide measurements to distinguish between the amounts of carbon absorbed by land and ocean.
It found that the ocean absorbed an average of 2.8 billion tonnes of carbon each year between 1990 and 2019, equivalent to around 35 per cent of fossil fuel emissions over that period. The ocean carbon sink strengthened by approximately 0.63 billion tonnes of carbon per decade.
By contrast, the net land carbon sink showed no notable long-term increase. Land ecosystems absorbed an average of around one billion tonnes of carbon each year over the same period, equivalent to approximately 13 per cent of fossil fuel emissions. The land sink peaked around 2009 and has generally declined since.
The results show that the ocean sink has strengthened substantially more than represented in current model simulations, suggesting that scientists still lack a complete understanding of how the carbon cycle will behave under future climate change.
The lack of growth in the net land sink also challenges many existing land models, which predict increasing carbon uptake as rising atmospheric carbon dioxide and nitrogen stimulate plant growth. The researchers note that the observed trend is particularly striking given that current estimates suggest emissions from deforestation and other land-use changes have been declining.
Professor Andrew Watson, one of the Exeter researchers said “Atmospheric oxygen measurements at the parts per million precision needed for this work are incredibly difficult to do, but the payoff is a unique understanding of the workings of the whole Earth system and how it is responding to human-caused change. These results represent decades of work by a small community of very dedicated scientists who make those observations.”
While the ocean has taken up increasing amounts of carbon dioxide, its future capacity to do so may also be affected by climate change. Warmer seawater holds less carbon dioxide, while warmer surface waters are less efficient at mixing into the deep ocean, where carbon can remain stored for many hundreds of years. The absorption of carbon dioxide also lowers the pH of seawater, contributing to ocean acidification.
The findings highlight the importance of long-term atmospheric and oceanic measurements in improving understanding of the global carbon cycle and its response to climate change, and stable funding for them.
The paper, Atmospheric O2 reveals a strengthening ocean carbon sink and a stagnant net land sink since 1990, is published in Science.
The research was supported by the US National Science Foundation, the National Oceanic and Atmospheric Administration and Schmidt Sciences. Additional support for the University of Exeter researchers was provided by the European Union’s OceanICU project and UK Research and Innovation.
