The ocean absorbs more heat than expected

November 8, 2018 Our seas are gigantic heat storage systems

The oceans absorb 90 percent of the additional heat energy produced by rising greenhouse concentrations in the atmosphere; the ocean is the main source of thermal inertia in the climate system. Researchers from the University of California, San Diego, Princeton University, the GEOMAR Helmholtz Center for Ocean Research Kiel, and colleagues from France and China have now used a new method to review previous calculations of ocean heat uptake. The result: over the past 25 years, the oceans have absorbed significantly more heat than average values previously calculated.

The ocean acts as a huge buffer in the earth's climate system. It absorbs gases, but also heat energy from the atmosphere, distributes them in its depths and thus slows down changes in the entire Earth system. The amount of energy absorbed by the ocean is therefore also an indication of the total heat absorbed by the planet.

Using a completely new method, the researchers have checked the previous calculations of the heat uptake of the ocean. "It turned out that warming is obviously at the upper end of what older estimates suggest," says Prof. Dr. med. Andreas Oschlies from GEOMAR, one of the co-authors of the study, recently published in the international journal Nature.

To calculate the total heat content of the oceans, research has so far used millions of measurements of water temperature. Many of the data come from autonomous measuring probes, so-called argo-floats. Thousands of them drift with the currents through the oceans, measuring the temperature and salinity of the water. "The Argo program only reached a truly global coverage in 2007. In addition, the floats only cover the upper quarter of the ocean," explains Professor Oschlies.

For the new study, lead-author Professor Laure Resplandy of Princeton University and the others involved did not use ocean measurements. They used - for the first time - high-precision measurements of oxygen and carbon dioxide (CO2) in the atmosphere. Stations around the globe have been carrying them out for several decades.

The process relies on the fact that oxygen and carbon dioxide are less soluble in warmer water. As the ocean warms, the gases are released to the atmosphere. "Taking into account other influences, such as the increase in CO2 due to anthropogenic emissions, but also the increased algae growth from increased inputs of fertilizer into the ocean, the combined amount of oxygen and carbon dioxide in the air is a good indicator of stored heat in the ocean," explains Professor Oschlies.

The method was initially tested using simulations with four different Earth system models. In them, the heat absorption calculated from atmospheric oxygen and CO2 concentrations was in very good agreement with the simulated heating of the ocean by the computer model. The models confirmed the calculations with the new method.

The new calculation of ocean heat uptake is more than 60 percent above the value of the IPCC's latest report, putting it at the top of the list of estimates so far, explains first author Laure Resplandy. Such a warming of the ocean suggests that the Earth is more sensitive to emissions of fossil fuels than previously thought, adds Professor Oschlies.

"Unfortunately, this result also shows that if we limit global warming to 2 or even 1.5 degrees Celsius, we must reduce CO2 emissions even more rapidly and to an even greater extent than the IPCC report," summarizes the Kiel researchers together.

Link to the study: https://doi.org/10.1038/s41586-018-0651-8.