By Isobel Leybold-Johnson
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.
“So we would assume that if, indeed, temperatures are increasing in the next 100 years, these oxygen minimum zones would also increase in volume and that the general oxygen concentration of the ocean will decrease,” Jaccard said.
And what is more: “our analysis has shown that not only was absolute temperature important, but also the rate of change, so the faster the warming, the more expanded these zones are”.
Oxygen in seawater mainly comes from gas exchange between the water’s surface and the atmosphere. As temperatures at the surface increase, the dissolved oxygen supply below the surface gets used up more quickly. It’s a little like turning down the oxygen pump in a fish tank, says Jaccard.
Dead zones are a topic well known to green campaigners and are not just limited to the biodiversity-rich deep oceans, explains Jochen Lamp, a marine expert at WWF in Germany.
They also affect the shallow seas like the Baltic Sea, which are subject to eutrophication: when nutrients from the land and agriculture cause over-enrichment of the water and the growth of algal blooms. These blooms then deplete the water’s oxygen.
But whereas it is easier to tackle shallow seas dead zones by controlling nutrient input, such as by having low nutrient agriculture, climate change adaptation is “a much more long lasting and complicated process”, said Lamp.
Even if countries such as Switzerland can agree on measures – the Rio+20 conference on sustainable development is scheduled for June – the change in the trend may not be seen for 50-70 years, he added.
Overall the world’s oceans are suffering: there is also overfishing and the other effects of climate change like the acidification of the waters.
Oceans are a delicately balanced ecosystem. “We hope that the balance will re-establish, but there is a lot of human impact in this imbalance and we do not yet know what will happen in reality during the next decades,” warned Lamp.