How do you take the ocean’s temperature?
The question might sound like the prelude to a children’s joke. But for climate scientists, the answer has serious consequences.
Climate change is rapidly warming the world’s oceans, killing off aquatic organisms — like coral reefs and kelp forests — that anchor entire ecosystems. The warmer waters also cause sea levels to rise and make extreme weather events like hurricanes more destructive.
If scientists can more accurately measure the speed at which oceans are warming, they can better predict the future effects of climate change. And a study published Wednesday in the journal Nature suggests that oceans are warming far faster than the estimates laid out by the Intergovernmental Panel on Climate Change, the global organization for climate data.
The study, led by Laure Resplandy, a biogeochemical oceanographer at Princeton University, found that between 1991 and 2016 the oceans warmed an average of 60 percent more per year than the panel’s official estimates.
In October, the panel released a major report predicting that some of the worst effects of climate change, including coastal flooding, food shortages and a mass die-off of coral reefs, could come to pass as soon as 2040 if human greenhouse-gas emissions continue at current levels. The IPCC report showed that scientists may have been underestimating the severity of the world’s present climate trajectory.
The new ocean temperature estimates, if proven accurate, could be another indication that the global warming of the past few decades has exceeded conservative estimates and has been more closely in line with scientists’ worst-case scenarios.
The researchers used a new approach that derived ocean temperatures by measuring the levels of carbon dioxide and oxygen in the atmosphere.
Those gases dissolve in ocean waters, but the amount the ocean can hold depends on its temperature. “As the ocean has been warming, it’s basically pushing out oxygen and carbon dioxide,” said David Nicholson, an associate scientist at the Woods Hole Oceanographic Institution who was not involved in the study.
As Resplandy put it: “If you leave a Coke outside in the sun, it’s going to warm and it’s going to lose the gas. It’s a little bit the same idea.”
Scientists normally measure ocean temperatures using thermometers, but stitching together a global temperature record requires thermometers around the globe. Global temperature records were spotty before 2007, when an international consortium began a program, known as Argo, creating an international network of ocean-temperature-measuring instruments.
But a group from Scripps Institution of Oceanography had been taking careful measurements of atmospheric carbon dioxide since 1991, for unrelated reasons. Resplandy and her team used that data set for this study.
Nicholson said the study was an example of how collecting data now can have unexpected benefits later. “It kind of supports the importance of collecting these long-term time series even if it isn’t apparent at the start what the outcome will be,” hesaid.
Scientists already know that the world’s oceans absorb 90 percent of the excess heat trapped on Earth by human greenhouse gas emissions. In its recent report, the IPCC used one of the lower available estimates of how much the oceans have warmed. Resplandy and her team found that the upper estimate is more likely what is happening.
“Their estimates overlap with previous estimates, but it’s aligned with some of the higher estimates,” Nicholson said.“It’s not like completely changing our understanding of what the ocean might be taking up — it’s a new type of measurement that’s weighing in toward the higher end of that.”
There are some caveats. This is a novel approach, and it is unclear if it will hold up to further scrutiny. Kevin E. Trenberth, a senior scientist in the Climate Analysis Section at the National Center for Atmospheric Research, noted that the methodology works best over long periods of time but does not detail what happens year to year.
Still, Trenberth’s own research found that the IPCC’s measurements for observed ocean heat were too low. “This is a new complementary method, and the results are quite compatible with our estimates for the most part,” he wrote in an email.
Resplandy said her work did not upend the IPCC report’s warnings that humanity has only a couple of decades to ward off some of climate change’s most catastrophic effects.
“It doesn’t change the results,” she said. “What it does is that it makes it harder to get there.”
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