The world's oceans close to being starved of oxygen - and even that could lead to mass sea life extinction which could last a million years.

University of Exeter scientists fear the modern ocean is 'on the edge of anoxia' - when the oceans are depleted of oxygen.

And while this dramatic drop in oceanic oxygen comes to a natural end, it takes about a million years.

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The world's ocean are at the tipping point of being starved of oxygen leading to mass sea life extinction which could last a million years. University of Exeter scientists fear the modern ocean is 'on the edge of anoxia' - when the oceans are depleted of oxygen

THE TIPPING POINT Scientists believe the modern ocean is 'on the edge of anoxia' - and the Exeter researchers say it is 'critical' to limit carbon emissions to prevent this. A study last year by researchers from the GEOMAR Helmholtz Centre for Ocean Research, showed that while the levels of carbon dioxide are increasing, the concentration of oxygen in the oceans is decreasing. Researchers analysed over 50 years’ worth of data looking at a range of parameters from ocean salinity to temperature. From this, they calculated that over this period, the world’s oceans have lost an average two per cent of their oxygen. The main process through which the oceans are losing their oxygen is the heating of the water. As oceans warm, they lose their ability to trap dissolved oxygen. And because the warming is normally contained to the upper levels of the water, it decreases the density of the surface water, preventing it from dropping to the depths and taking oxygen with it. Advertisement

Studying what happened during the Jurassic period, they found the drop in oxygen causes more organic carbon to be buried in sediment on the ocean floor.

This eventually leads to rising oxygen in the atmosphere which ultimately re-oxygenates the ocean. But it took a million years to get the balance right again.

Lead researcher PhD student Sarah Baker said it was now 'critical' for modern humans to limit carbon emissions to prevent this.

She said: 'Once you get into a major event like anoxia, it takes a long time for the Earth's system to rebalance.

'This shows the vital importance of limiting disruption to the carbon cycle to regulate the Earth system and keep it within habitable bounds.'

The researchers studied the Toarcian Oceanic Anoxic Event, which took place 183 million years ago.

This was characterised by a major disturbance to the global carbon cycle, depleted oxygen in Earth's oceans and mass extinction of marine life.

Numerical models predicted that increased burial of organic carbon - due to less decomposition and more plant and marine productivity in the warmer, carbon-rich environment - should drive a rise in atmospheric oxygen, causing the end of an anoxic event after one million years.

Testing the theory they examined fossil charcoal samples to see evidence of wildfires - as such fires would be more common in oxygen-rich times.

These were taken at Mochras in Wales and Peniche, Portugal.

They found a period of increased wildfire activity started one million years after the onset of the anoxic event, and lasted for about 800,000 years.

Ms Baker added: 'We argue that this major increase in fire activity was primarily driven by increased atmospheric oxygen.

The researchers examined fossil charcoal samples to see evidence of wildfires - as such fires would be more common in oxygen-rich times. They found a period of increased wildfire activity started one million years after the onset of the anoxic event, and lasted for about 800,000 years

'Our study provides the first fossil-based evidence that such a change in atmospheric oxygen levels could occur in a period of one million years.'

The increase in fire activity may have also helped end ocean anoxia by burning and reducing the amount of plants on land.

This is because plants can help to erode rocks on the land that contain nutrients needed for marine life - therefore with fewer plants, fewer nutrients are available to be carried to the sea and used to support marine life in the oceans.

Less marine life - that would use oxygen to breathe - would mean less oxygen being used in the oceans, and could therefore help the oceans to build up a higher oxygen content, ending anoxia.

It may therefore be essential to maintain the natural functioning of wildfire activity to help regulate the Earth system in the long-term.