Climate change: Death zones in the ocean are getting bigger

New model study predicts drastic oxygen loss

Oxygen in the World Ocean: Regions with very low oxygen are marked violet and are found mainly in the tropics. © IFM-GEOMAR
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A new study on the effects of future climate changes on the oceans is causing alarming results: in addition to the increasing acidification of the oceans, oxygen-depleted zones, in which life is no longer possible, will expand significantly in the future.

So far, marine scientists have assumed that the significant impact of increasing CO2 concentrations on the ocean is in increasing acidification of seawater.

The new study by the international research team headed by Professor Andreas Oschlies from the Leibniz Institute for Marine Sciences (IFM-GEOMAR) in Kiel shows that the human-caused increase in CO2 could also have drastic effects on the oxygen-poor zones of tropical oceans.

"Our research shows that the zones with low-oxygen water will expand by up to 50 percent by the end of this century, " reports Oschlies in the international journal "Global Biogeochemical Cycles". "We were a bit surprised by the clarity of the signal, " the researcher continued. "Although we have assumed in our calculations that people continue to do business as usual (so-called 'business-as-usual scenario'), we still would not have expected this effect to this extent."

Fertilizing effect on the sea surface

The strong decrease in oxygen concentrations at a few hundred meters depth is due to the bacterial degradation of sinking organic matter. This becomes increasingly carbon-rich as a result of the fertilizer effect of the CO2 at the sea surface, thereby consuming more oxygen during decomposition than the conventional protein-rich biomass. display

If Oschlies and his colleagues are right and people do not change their behavior significantly, then the "dead zones" in the oceans, in which due to a low oxygen content no higher life is possible, will increase significantly.

For the study, the researchers used a global climate model, with ocean, atmosphere and integrated carbon and nutrient cycles. The model was fed with data obtained from field experiments using so-called mesocosms. These mesocosms, which look like oversized test tubes, are experimental systems that allow us to use time-lapse experiments to study the influence of the increasing uptake of carbon dioxide in the ocean under real conditions.

Measuring campaigns off Peru and West Africa

"The next thing we need is more observation data, " explains Kai Schulz, co-author of the study. This allows us to calibrate and improve our models better.

As part of the new Collaborative Research Center 754 "Climate Biogeochemical Interactions in the Tropical Ocean", intensive measurement campaigns are being conducted on this topic. "Our colleagues are currently traveling with the research ship METEOR off the coast of Peru in order to collect important data in the low-oxygen upwelling area, " says Oschlies.

At the same time, another research team with MARIA S. MERIAN is examining the conditions in the Atlantic Ocean off the coast of West Africa in the Cape Verde region. These expeditions come at exactly the right time. Because Oschlies and his colleagues are already waiting for the data ...

(idw - Leibniz Institute for Marine Sciences, 13.11.2008 - DLO)