Climate experts had been reading and rereading a report that was published in Nature before the end of 2018. Laure Resplandy, a researcher at Princeton University’s Department of Geosciences, was the primary author. The findings were so striking that they deviated from the typical incremental contributions to the literature on ocean heat. Her team’s findings revealed that the world’s oceans were absorbing almost 60% more heat each year than earlier UN climate predictions had indicated. Not a small change. a significant discrepancy between the findings of an independent measurement approach and what the models and reporting frameworks had been telling policymakers.
Understanding the methodological decision that made the study unique is important since it contributed to the significance of the findings. The majority of estimates of ocean heat content are based on direct observations of water temperature, mostly from the Argo float network, a global network of around 4,000 free-drifting devices that profile salinity and temperature across the water column. Resplandy’s team took a different tack, measuring air CO2 and oxygen concentrations from 1991 to 2016 and using those signals as a stand-in for ocean heat content. The ratio of oxygen to carbon dioxide in the air above varies as the ocean warms, providing information on the conditions in the water below. The ocean float data has no bearing on the atmospheric signal. The main finding was that it offered a different story than the float-based estimations.
The study’s results were substantial enough to necessitate context. For the 25-year study period, Resplandy’s team calculated 13 zettajoules of heat intake annually, which is roughly 150 times the yearly electrical generation of all human civilizations put together. While surface temperatures increased at the relatively slow rate that has been the focus of climate change discussions, the ocean has been silently absorbing energy at that scale. The ocean has been absorbing the heat instead, which is largely why surface air temperatures have not increased as quickly as the total greenhouse gas forcing would suggest. The Earth system’s sensitivity to ongoing emissions is directly impacted by how much heat it has actually been absorbing; if the Resplandy discovery was verified, it suggested the system was more sensitive than previous models had predicted.
A further investigation headed by Lijing Cheng and published in Science in January 2019 came to a similar but different conclusion: ocean warming had been escalating at a rate that was around 40% faster than what the 2014 IPCC Fifth Assessment Report had predicted. These two conclusions—the Cheng analysis and the Princeton atmospheric proxy study—came from different approaches, addressed various facets of the ocean heat issue, and yielded comparable but distinct estimates. Both indicated that earlier calculations of the rate at which the ocean was absorbing extra heat had been overly conservative.
Finding that the ocean absorbs heat more efficiently than the models predicted has rather paradoxical climate implications. The atmosphere warms more slowly, buying time, and a warmer ocean acting as a greater heat buffer seems like good news. However, climate sensitivity operates in the opposite manner. The system’s final surface warming response to a given amount of CO2 accumulation is bigger than the models predicted if the globe has been accumulating heat more quickly than the models predicted and the surface temperature signal remains unchanged. According to the Resplandy analysis, reducing emissions by about 25% would be necessary to reach a warming target of 1.5 or 2 degrees Celsius. The safety buffer was not as large as anticipated.
Following publication, the study did undergo a statistical correction, which involved recalculating the uncertainty ranges that the initial analysis had underestimated. Although the precise numbers changed and the confidence ranges expanded, the fundamental directional finding—that ocean heat uptake was greater than earlier estimates—remained valid following the revision. The episode attracted attention as a science story as well as an illustration of how the publication and correction process in climate research actually works: a finding of great interest, quick scrutiny by the scientific community, open correction, and a revised but still significant result. It is not a retraction. An improvement.
