Two months ago, NASA’s Timothy Hall and colleagues published a study that described how they had estimated the amount of manmade carbon dioxide absorbed by the ocean since the start of the industrial era.
Oceans absorb about a third of the carbon dioxide that humans release into the atmosphere, so sorting out a long-term record of carbon uptake is of great interest to climate scientists.
To create their record of the ocean’s uptake of carbon, Hall and Samar Khatiwala, the lead author of the study, devised a clever mathematical technique that proved to be a considerable advance. When Hall’s study appeared in the journal Nature, he assumed the creation of this new long-term, continuous record would headline the news.
But journalists gravitated toward something else entirely: a brief mention that the amount of carbon dioxide absorbed by the ocean seemed to be experiencing, as the researchers put it, “a small decline in the rate of increase in the last few decades.”
“Seas Grow Less Effective at Absorbing Emissions”, one headline trumpeted. Another article compared the world’s oceans to a fish “stuffed to the gills” with carbon dioxide and another reported a “sudden and dramatic drop in the amount of carbon dioxide being absorbed by the sea.“
Given the caveats included in the original study, all of this caught Hall slightly off guard. I’ll let Hall, who summarized his reactions to the coverage for What On Earth, pick the story up from here:
My coauthors and I had viewed the ability to estimate the history of ocean uptake of anthropogenic carbon as the highlight of the paper. Previously, observationally-based estimates had only provided a few snapshots in time, and we were proud of the cleverness of our techniques.
It seems clever mathematical techniques, however, don’t make good press releases. Interestingly, coverage of the paper has not focused on the fact that we can estimate the uptake history. Instead it has focused on apparent reductions in the rate of uptake over the last 2 decades.
The figure below shows our estimate of ocean uptake since 1775. The first impression is the rapid increase since 1950, coinciding with the rapid rise in carbon emissions to the atmosphere. The oceans have prevented about 1/3 of anthropogenic carbon emissions from accumulating in the atmosphere. A closer reading of the curve reveals a reduction in the uptake’s rate of increase after about 1980, even while emissions continue to increase.
Scientists have long suspected that ocean carbon uptake would eventually be unable to keep pace with rising emissions. Basic aqueous chemistry tells us that, as dissolved carbon in seawater increases, seawater becomes less able to absorb new carbon. Eventually, the absorption saturates. The slowing down of the increase rate may be an early signal of this saturation.
However, recent changes in uptake were not our focus when we performed the study, and more importantly we did not analyze the statistical significance of the slowdown. We plan further analysis on these trend variations. What we can say is that there are physical reasons to suspect a reduction in the ocean’s capacity to keep pace with increasing carbon emissions, and that there are now strong observational hints for recent reductions.
Hall advises reading this story, which also appeared in Nature. It’s less dramatic and more technical than most of media accounts, but it is a more accurate representation of the paper.