Up to 0.9 gigatonnes more CO2 absorbed by oceans each year than scientists thought

Scientists have been underestimating the amount of carbon our oceans absorb from the atmosphere each year. Previous estimates indicated that about a quarter of the emissions caused by human activity were drawn into the ocean. In other words, more than 2 gigatonnes of CO2 per year.

Researchers supported by the EU-funded 4C and RINGO projects have found that the annual uptake is in fact much higher, exceeding previous estimates by up to 0.9 gigatonnes. These results and the methods used to reach them are explained in a paper published in the journal ‘Nature Communications.’The question is, why are previous figures incorrect? The answer lies in the part of the ocean from which the measurements were taken. Recent efforts by the international scientific community to create a regularly updated global collection of data on marine surface CO2 resulted in the Surface Ocean CO2 Atlas (SOCAT). Containing 28.2 million measurements collected from 1957 to 2020, SOCAT helps scientists determine how much carbon is being absorbed by the oceans. However, previous studies that have used SOCAT data estimated carbon uptake from measurements taken several metres below the ocean surface. The problem with this is that the needed measurements are those taken “right at the ocean surface,” as explained by lead author Andrew Watson of the University of Exeter in an article posted on the ‘SciTechDaily’ website.

While seemingly insignificant, a few metres’ difference leads to changes in temperature and affects the sea’s ability to absorb CO2. “Previous studies have ignored the small temperature differences between the surface of the ocean and the sampling depth, but we know that this has a significant impact on how carbon is held by the oceans in terms of salinity, solubility, stability, and so on,” noted Prof. Watson. “But satellites can measure the temperature more or less exactly at the ocean surface – and when we do this, we find it makes a big difference,” he added.To generate more accurate estimates, the researchers developed a procedure to recalculate the SOCAT data using 1992 to 2018 measurements of the ocean temperature a few millimetres below the surface. The temperature was mainly obtained from satellite infrared observations. Based on the corrected figures, the net annual uptake of carbon into the oceans is between 0.8 and 0.9 gigatonnes more than previously believed – an amount that is sometimes double the uncorrected figures. “These results are consistent with independent estimates of the size of the oceanic carbon sink – those based on global ocean surveys by research ships,” stated co-author Jamie Shutler, also of the University of Exeter, in the same article. “Now that these two separate estimates of the size of the carbon dioxide ocean sink agree pretty well, we can view and use their results with greater confidence, and trust that they are most likely giving us an accurate picture of what is going on.”

This study forms part of the 4C (Climate-Carbon Interactions in the Current Century) project’s efforts to quantify the main processes regulating the carbon-climate system. It also furthers the RINGO (Readiness of ICOS for Necessities of integrated Global Observations) project’s goal to generate accurate data on carbon fluxes between the atmosphere, the Earth and our oceans. RINGO concludes at the end of 2020 and 4C in 2023.

For more information, please see:

4C project website

RINGO project website