Measurements in the oceans and model calculations show equally that the oxygen content of the oceans is decreasing. However, the models underestimate this decrease significantly. That makes predictions for the future difficult. In a study published today in the international journal Nature Geoscience, four GEOMAR researchers reveal the gaps in the models and identify further, previously underestimated causes of oxygen loss.
The oceans lose oxygen. Numerous studies at local, regional and global level confirm this trend. For example, a comprehensive data analysis published by Kiel oceanographers at the beginning of 2017 has shown that the oceans have lost two percent of their oxygen content worldwide in the past 50 years. Even computer models of the oceans and the earth system show this trend and predict an acceleration in the future. But there is a problem. “The models available to us do not succeed in exactly following the previous development. They show significantly less oxygen loss than the actual measured values, “says Prof. Dr. med. Andreas Oschlies from the GEOMAR Helmholtz Center for Ocean Research Kiel.
This discrepancy makes forecasts for the future uncertain. In the international journal Nature Gesoscience, Professor Oschlies publishes today together with his colleagues Prof. Dr. med. Peter Brandt, dr. Lothar Stramma and dr. Sunke Schmidtko of the GEOMAR a study, which shows the gaps of the models and thus at the same time further, so far underestimated drivers of the oxygen loss. “The comparison with our observation data reveals various inadequacies of the models and gives us indications in which direction we must concentrate our research efforts,” says co-author Peter Brandt.
It is certain that global warming is the main cause of oxygen depletion. But it affects the oceans in several ways. Among other things, it influences the solubility of oxygen in the water. The warmer the water, the less gasses it can absorb. “This process mainly affects the uppermost layers of water, which are in direct contact with the atmosphere,” explains Dr. med. Schmidtko. This effect can explain up to 20 percent of the previous oxygen decrease and is in the models already well understood, he continues.
But warming is also changing patterns of global ocean circulation. Since the complex system of surface and deep currents supplies oxygen to the deeper areas of the ocean, these changes could affect the oxygen content throughout the ocean. “Many models have problems here because transport processes are often not resolved well enough or reproduced incorrectly,” says co-author Dr. Lothar Stramma.
Even the extremely complex interactions between biological, chemical and physical processes in the ocean have so far been insufficiently represented in the models. “We often lack the data or the knowledge about the many factors that interact in the ocean’s response to global warming,” emphasizes Andreas Oschlies, who specializes in the modeling of biogeochemical processes. “Our study shows that previous models significantly underestimate the effects of this interaction, at least on the oxygen distribution.”
To close these gaps, the four authors argue for more intensive and internationally coordinated ocean observation. “We need multidisciplinary process studies to better understand the delicate balance of oxygenation and oxygen consumption in the ocean,” emphasizes Andreas Oschlies, “international initiatives such as the Global Ocean Oxygen Network are helpful.”
Improving the models’ oxygen budget By the way, oceans also have another advantage: “Oxygen is ideal for calibrating models that calculate the uptake of carbon dioxide in the ocean. At the same time, we would improve our knowledge of the carbon cycle, “says Oschlies.