Something strange is going on with the planet’s oxygen levels, which has left researchers scratching their heads as to the cause.

Scientists testing the concentration of oxygen in the atmosphere have found that levels have dropped by almost 0.7 per cent over the past 800,000 years, compared to modern levels.

What’s more, the rate of this decline has sped up over the last century, dropping by a further 0.1 per cent.

The concentration of oxygen in earth's atmosphere (illustrated) has dropped by almost 0.7 per cent over the past 800,000 years, compared to modern levels. Researchers also found the rate has increased rapidly over the last century

Oxygen levels currently stand at around 21 per cent, but have fluctuated greatly over the planet’s 4.3 billion-year history, with two major spikes linked with the explosion of life.

To sample the ancient atmosphere, a team led by researchers at Princeton University in New Jersey, studied bubbles of gas frozen in the ice of Greenland and Antarctica thousands of years ago.

By measuring changes in the atmospheric concentrations of oxygen and nitrogen, they showed a subtle declining trend over thousands of years.

The researchers believe that burning fossil fuels has led to the rapid increase seen over the last century – by consuming oxygen and releasing large quantities of carbon dioxide into the atmosphere – but the cause of the longer term decline has been trickier to pin down.

To sample the ancient atmosphere, the team studied bubbles of gas frozen in the ice cores (stock image pictured) of Greenland and Antarctica thousands of years ago

GEOLOGICAL TIMESCALE One potential explanation put forward by the scientists for the declining oxygen in the recent geological past is an increase in erosion, which would lead to freshly exposed sediment being oxidised by the atmosphere, reducing atmospheric oxygen levels. Another long term process which is interaction with the oceans. With lower average global temperatures in the past, the world’s oceans would have been able to absorb more oxygen, with cooler waters able to soak up more gas. The researchers believe a series of slow chemical reactions between the atmosphere and rocks, known as silicate weathering, could explain the apparent lack of carbon dioxide in the atmosphere. Advertisement

Unlike the recent decline, there is no evidence to suggest carbon dioxide levels increased substantially during the period.

But the researchers believe a series of slow chemical reactions between the atmosphere and rocks, known as silicate weathering, could explain this lack of carbon dioxide.

‘The planet has various processes that can keep carbon dioxide levels in check,’ said Dr Daniel Stolper, a geoscientist at Princeton.

Over thousands of years, carbon dioxide in the atmosphere reacts with exposed rock to form calcium carbonate minerals, trapping the carbon in a solid form. In geological timescales, this process soaks up atmospheric carbon, locking it away in rock.

Scientists believe that as more carbon dioxide has been released into atmosphere from burning fossil fuels, the increasing temperature has led to the weathering process occurring more rapidly.

But human activity is releasing carbon dioxide into the atmosphere so quickly that we may cause this slow, long-term geological process to ‘short-circuit’, and so they cannot keep up.

With lower average global temperatures in the past, the world’s oceans would have been able to absorb more oxygen, as cooler waters able to soak up more gas. Stock image pictured

One potential explanation put forward for the declining oxygen in the recent geological past is an increase in erosion, which would lead to freshly exposed sediment being oxidised by the atmosphere, reducing atmospheric oxygen levels.

Another long term process to factor in is interaction with the oceans.

With lower average global temperatures in the past, the world’s oceans would have been able to absorb more oxygen, as cooler waters able to soak up more gas.

‘This record represents an important benchmark for the study of the history of atmospheric oxygen,’ said Dr John Higgins, co-author of the study.

‘Understanding the history of oxygen in Earth’s atmosphere is intimately connected to understanding the evolution of complex life. It’s one of these big, fundamental ongoing questions in Earth science’