by Isobel Leybold-Johnson, swissinfo.ch
Global warming could lead to more of the world’s oceans becoming “dead zones” – where a lack of oxygen leads to marine life dying out.
This was the conclusion of recent analysis of marine oxygen conditions over the past 20,000 years, co-authored by the Federal Institute of Technology Zurich (ETHZ).
Oceans are already oxygen-starved in places: every summer some areas of the northeastern Pacific see huge numbers of dead fish, shrimp or molluscs washed up on beaches.
This is caused by marine animals suffocating because the water contains too little of the vital O2 they need to breathe – or none at all. It is not only an ecological problem, the local fishing industry is affected as well.
Currently around 15 per cent of oceans are considered oxygen-depleted or anoxic “dead zones”.
“There’s been a very longstanding debate about the influence of global warming on the concentration of oxygen in the ocean, basically because the ocean oxygen concentration measurements of the past decades have not been very conclusive,” Samuel Jaccard from the ETHZ’s Geological Institute told swissinfo.ch.
This is why Jaccard and Eric Galbraith from McGill University in Canada decided to go back in time and reconstruct how the oxygen content has changed in oceans in the past 20,000 years, with the focus on the Pacific and Indian Oceans.
Their study, published in Nature Geoscience, showed that the average global temperature rise of around at least two degrees Celsius between the peak and the end of the last Ice Age (between about 10,000-20,000 years ago) had a massive effect on the oxygen content of seawater.
“The warmer the global average temperature, the more extended the oxygen minimum zones are, so the volume of these oxygen-poor water bodies is more extended during warm periods than in cold periods,” Jaccard said.
What is worrying is that, currently, global average temperature is predicted to rise by at least two degrees in the coming century due to climate change. This is of a similar magnitude to the warming the planet has undergone since the last Ice Age 20,000 years ago.
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