An Alarming New Study Suggests the Oceans Are Warming 60 Percent Faster. A Statistician Disagrees.
Is the study's worst-case climate scenario wrong?
"Startling new research finds large buildup of heat in the oceans, suggesting a faster rate of global warming," said the Washington Post headline last week. The Los Angeles Times headline declared: "Oceans warming faster than anticipated, giving even less time to stave off worse impacts of climate change, study finds." Both newspapers were covering a new study in Nature by the Princeton geoscientist Laure Resplandy and her team.
Resplandy and company had found a novel way to measure the amount of heat being absorbed by the world's oceans. Heretofore, climate researchers have sought to deduce warming trends in the oceans using data collected from thermometers, recently including a network of 3,200 Argo floats deployed throughout the world's waters. Resplandy's method takes advantage of the fact that as the oceans warm they release both oxygen and carbon dioxide into the atmosphere, a quantity they call "atmospheric potential oxygen," or APO.
"As the ocean warms, these gases tend to be released into the air, which increases APO levels," explains the press release accompanying the study. "APO also is influenced by burning fossil fuels and by an ocean process involving the uptake of excess fossil-fuel CO2. By comparing the changes in APO they observed with the changes expected due to fossil-fuel use and carbon dioxide uptake, the researchers were able to calculate how much APO emanated from the ocean becoming warmer. That amount coincides the heat-energy content of the ocean."
What's great about this new technique is that it is an independent measure of the oceans' heat content.
The team's research "suggests that ocean warming is at the high end of previous estimates, with implications for policy-relevant measurements of the Earth response to climate change, such as climate sensitivity to greenhouse gases and the thermal component of sea-level rise." How much higher? Resplandy and company calculate that the amount of heat being absorbed by the oceans is more than 60 percent higher per year than the estimates offered by the United Nations' Intergovernmental Panel on Climate Change in 2014.
That would be worrisome, because it would mean that equilibrium climate sensitivity (ECS) is higher than many other researchers had thought. ECS represents how much the average global temperature would ultimately increase if the amount of carbon dioxide in the atmosphere doubles above the preindustrial level. The figure has huge implications for policy. If future warming is at the low end, humanity has more time to adapt and to shift energy production away from the fossil fuels that are loading up the atmosphere with extra carbon dioxide. If it's at the high end, we'd need to speed up our efforts to adapt and shift energy production to low-carbon sources.
"The new ocean temperature estimates," reports The New York Times, "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."
If proven accurate is the relevant phrase. The study was peer-reviewed and published in one of the world's most prestigious scientific journals, but the developers of any new scientific technique recognize that it must undergo skeptical scrutiny before it can be accepted as valid by the wider scientific community.
As it happens, independent climate researcher and statistician Nic Lewis thinks that he has identified a major flaw in the Nature ocean heat uptake study. Lewis is no stranger to the scientific debate over climate sensitivity. He and former Georgia Tech climatologist Judith Curry published a study earlier this year in The Journal of Climate concluding that "for any future emissions scenario, future warming is likely to be substantially lower than the central computer model-simulated level projected by the IPCC, and highly unlikely to exceed that level." Specifically, they think the models are running almost two times hotter than the analysis of historical data suggests that future temperatures will be.
So what error does Lewis think that he has identified in the new Nature study? Using the APO values and the data in the article itself, Lewis derives an ocean heat uptake trend that is basically the same as other researchers have found using ocean temperature data, not 60 percent higher. He can't identify exactly why the values he derives differ from those reported, but he speculates that the computer code used in the study might not have taken into account the fertilization effect of anthropogenic aerosol deposition—which promotes marine photosynthesis—and the changes in solubility, biology, and ocean circulation due to warming.
I have reached out to both Resplandy and Lewis seeking comment. Via email, Lewis responded: "I've had no substantive response from Professor Resplandy, just a non-committal reply saying that they were looking into the questions I had raised and if they found anything that needed correction they would address it. Unfortunately, they have every incentive to conclude that they don't need to take any action! So do Nature; journals don't like being made to look foolish."
I have not heard back from Resplandy yet. I expect that she and her colleagues need time for a careful evaluation of Lewis' arguments.
This is an ongoing scientific debate, and I will keep readers informed as it proceeds.
