Illustration: Sarah Grillo/Axios
Editor's note: This piece has been corrected due to a correction posted by the authors for Nature. Please see full explanation at bottom.
The world's oceans have absorbed about 60% more heat during the past 25 years than previously estimated, according to a new study published in the journal Nature. The study takes advantage of a new method that can serve as a whole ocean thermometer.
Why it matters: If the ocean is absorbing even more heat than observed, it would suggest future global warming will track on the upper end of projections — possibly as high as 5°C, or 9°F, by 2100 if emissions are not significantly curtailed.
The oceans are absorbing about 93% of the extra energy from increasing amounts of greenhouse gases in the atmosphere.
What they did: For the study, scientists, led by Princeton University geochemist Laure Resplandy and Ralph Keeling from the Scripps Institution of Oceanography, devised a new way of taking the global ocean temperature. It relies on precise atmospheric measurements of oxygen and carbon dioxide, which date back to 1991.
- The scientists examined the combined amount of oxygen and carbon dioxide in the air, which they term "atmospheric potential oxygen," or APO.
- Both of these gases are less soluble in warmer water, and as the ocean warms, these gases are released into the air, which increases the APO.
- This contrasts with other methods that use millions of observations from ocean sensors, including buoys and ship-based instruments. These measurements have considerable uncertainty, particularly the further back you go in time.
What they found: The amount of carbon dioxide that can be emitted by human activities before putting the goals of limiting warming to under 2℃, or 3.6℉, out of reach is about 25% less than what was previously calculated.
“Imagine if the ocean was only 30 feet deep,” said Resplandy in a press release. “Our data shows that it would have warmed by 6.5℃, or 11.7℉, every decade since 1991. In comparison, the estimate of the last IPCC assessment report would correspond to a warming of only 4℃, or 7.2℉, every decade.”
What they're saying: Keeling told Axios the results, “Imply that there’s likely to be more warming in the future.”
- This is because the warming oceans eventually transfer much of that extra heat into the atmosphere, resulting in accelerated global warming and stronger storms that can deliver heavier precipitation.
"This study shows that estimates of this ocean heat uptake are probably on the high side of what we previously thought," said NASA climate scientist Kate Marvel. "Why is this scary? Because of something called 'climate sensitivity:' a measure of how hot the planet will eventually get."
Pieter Tans, who closely tracks the amount of carbon dioxide in the Earth's atmosphere for NOAA and was not involved in the new study, told Axios the paper is valuable for providing a new, independent estimate of global average ocean warming.
"In case the larger estimate of ocean heat uptake turns out to be true, adaptation to, and mitigation of, our changing climate would become more urgent."— Pieter Tans, NOAA
However, Tans cautioned that the new estimate is still uncertain, and needs to be replicated by subsequent studies. So too did Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research who was not involved in the new study.
Ultimately, the calculations, Trenberth says, "have implications, because the planet is clearly warming and at faster rates that previously appreciated, and the oceans are the main memory of the climate system (along with ice loss)."
After this study and the Axios story based on it was published, the authors submitted a correction to Nature, showing that the margins of error on their calculations of ocean heat uptake were larger than previously stated. According to lead author Ralph Keeling, the errors "Do not invalidate the study’s methodology or the new insights into ocean biogeochemistry on which it is based."
Instead of showing a higher amount of ocean heat increase than other studies, the study amounts to an independent confirmation of what other studies have found. We have updated the headline to reflect this.