Continuing to burn fossil fuels at the current rate could bring atmospheric carbon dioxide to its highest concentration in 50 million years, jumping from about 400 parts per million now to more than 900 parts per million by the end of this century, a new study warns.

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.

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

The new study speaks to the power of human influence over the climate. It suggests that after millions of years of relative stability in the absence of human activity, just a few hundred years of anthropogenic greenhouse gas emissions are on track to cause unprecedented warming.

To come to these grim conclusions, which were published Tuesday in the journal Nature Communications, the researchers constructed a continuous record of atmospheric carbon dioxide concentrations spanning the last 420 million years. They created the record by compiling more than 1,500 estimates of atmospheric carbon dioxide concentrations drawn from 112 published studies.

According to lead author Gavin Foster, a geochemistry professor at the University of Southampton, those estimates were constructed mainly using the carbon isotope composition of ancient soil samples or examining the abundance of pores on the leaves of fossilized plants, an indicator of how much carbon dioxide was available for them to draw from the air while they were alive.

The findings suggest that, until humans started rapidly burning fossil fuels with the advent of the Industrial Revolution, Earth’s climate had been relatively stable for millions of years, and carbon dioxide concentrations were declining. Thanks to the human emission of greenhouse gases, though, that’s all changing at record-breaking pace.

Current concentrations of CO2 emissions are at their highest in human history,  currently hovering around 400 parts per million and continuing to rise. Before the Industrial Revolution, carbon dioxide concentrations had settled into an average of about 280 parts per million.

On a business-as-usual pathway — a trajectory in which greenhouse gas emissions would continue at their current rate — carbon dioxide concentrations will hit a level that hasn’t been seen in 50 million years, according to the research. 

The warming that will be brought on by the continued emission of greenhouse gases will only be compounded by an increase in solar radiation as the sun continues to grow brighter in the future, the researchers said.

A business-as-usual trajectory suggests that carbon dioxide levels could exceed 2,000 parts per million by the year 2250, concentrations that were last seen about 200 million years ago. But thanks to the combined influence of a hotter future sun, the planet’s resulting warming will probably be greater than at almost any point in the past 420 million years.

Additionally, at least one study has suggested that concentrations could be as high as 5,000 parts per million by the year 2400 if humans were to burn through all the fossil fuels on Earth, and that would result in both the highest carbon dioxide levels and the highest temperatures seen throughout the study period.

The Intergovernmental Panel on Climate Change has presented estimates of how much the Earth might warm under a business-as-usual trajectory over certain time periods. It  suggests that by the year 2300, the Earth could warm by nearly 9 degrees Celsius (or about 16 degrees Fahrenheit). But there are many factors that could affectl temperature trends in the long-term that remain uncertain, Foster suggested, such as changes in terrestrial vegetation or the amount of carbon dioxide the ocean has room to absorb in the coming centuries. 

As a result, he said, long-term warming could end up being even more intense than we estimate now.

The new study helps address a kind of paradox in the Earth’s climate history. Based on our knowledge of the way stars generate energy, scientists know that our solar system’s young sun would have been much dimmer millions of years ago. Over time, its intensity has increased, and will likely continue doing so for millions or even billions of years.  

If the sun has been getting hotter for millions of years, though, then one would expect the planet’s climate to have steadily warmed during that time as well, Foster noted. But there is ample evidence from the fossil record to suggest that the planet’s climate actually remained mostly stable for millions of years before humans began burning fossil fuels. Scientists have hypothesized that this stability comes from a long-term reduction in atmospheric carbon dioxide levels, which offset the warming caused by a brightening sun.

And the new study supports this idea. The researchers’ record suggests that, while there have been fluctuations throughout history, the long-term average carbon dioxide concentration generally declined right up until the Industrial Revolution as a result of natural processes related to the formation of the terrestrial Earth.

Thanks to human activity, however, carbon dioxide levels are now on the rise again, and they’re on track to break millennial-scale records if serious mitigation efforts aren’t undertaken, the study suggests. 

Foster emphasizes that the new historical record is not necessarily perfect. There are still gaps and uncertainties, that could be filled in with more discoveries over time.

But, he added, “just as far as we know, that the [warming] in the future is going to be unprecedented.”

Correction: This article previously contained an incorrect conversion from Celsius to Fahrenheit. We regret the error and it has been corrected.