Continuing to burn fossil fuels at the current rate could bring atmospheric carbon dioxide to its highest concentration in 50 million years, a new study warns. This means jumping from approximately 400 parts per million now to more than 900 parts per million by the end of this century. And if greenhouse gas emissions continue unabated beyond that point, the climate could reach a warming state that hasn’t been seen in the past 420 million years.

Other research suggests that, if humans were to burn through all fossil fuels on Earth, atmospheric carbon dioxide concentrations could hit 5,000 parts per million by the year 2400.

“Not only will the resultant climate change be faster than anything the Earth has seen for millions of years,” says study author Gavin Foster, a professor at the University of Southampton, ” the climate that will exist is likely to have no natural counterpart, as far as we can tell, in at least the last 420 million years.”

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