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follow that have no precedent in 420 million years of evolution.

Planetary temperatures

And the agency at work is the ratio of carbon dioxide in the atmosphere, which has hovered at around 280 parts per million (ppm) for almost all human history.

Carbon dioxide is a greenhouse gas that was once present in the atmosphere at far greater levels.

Once humans started to burn coal and oil – based on plant material sequestered during the Carboniferous era – they also started to return ancient CO2 to the atmosphere, to create a heat trap. Carbon dioxide ratios have risen to more than 400ppm, and planetary average temperatures have risen by almost 1°C.

The latest research, published in Nature Communications journal, contains a grim warning for humankind – but it was driven at least in part by curiosity about the coupling of atmosphere and evolution during the emergence of complex life.

“We cannot directly measure CO2 concentrations from millions of years ago,” says Gavin Foster, professor of isotope geochemistry at the University of Southampton in the UK, who led the study. “Instead we rely on indirect ‘proxies’ in the rock record.

“In this study, we compiled all the available published data from several different types of proxy to produce a continuous record of ancient CO2levels.”

During the half billion years, planetary temperatures alternated between extended cold snaps with low CO2 levels, and intense “greenhouse” temperatures at which CO2 levels rose to 3,000 ppm.

“The resultant climate change will be faster than anything the Earth has seen for millions of years”

But these changes were immensely slow, and the study emphasises the speed of human impact in what geologists would like to call the Anthropocene period.

Research like this is fundamental: it tells climate scientists something about the dynamics of atmosphere and sunlight over the millennia. And one of the puzzles of evolution is that, in the early days of life, the Sun must have been fainter than it is now.

“Due to nuclear reactions in stars, like our Sun, over time they become brighter,” explains Dan Lunt, professor of climate science at the University of Bristol, UK, and a co-author of the report.

“This means that, although carbon dioxide concentrations were high hundreds of millions of years ago, the net warming effect of CO2 and sunlight was less. Our new CO2 compilation appears on average to have gradually declined over time by about 3-4 ppm per million years.

“This may not sound like much, but it is actually just about enough to cancel out the warming effect caused by the Sun brightening through time, so in the long term it appears the net effect of both was pretty much constant on average.”

So the coincidence of a greenhouse atmosphere and a cooler Sun created conditions in which life emerged, evolved and adapted to its environment. Plants consumed and sequestered carbon dioxide, and animals benefited from the oxygen released in the process.