It's no secret that our planet is getting hotter due to heat-trapping carbon dioxide (CO2) emissions, but a new study suggests that current global warming trends could produce a climate not seen in almost half a billion years of Earth's history.
Researchers say that if humanity continues to exploit all available fossil fuels on the planet, by the year 2250 we could be facing levels of atmospheric CO2 not seen since the Triassic period some 200 million years ago – and by 2400, Co2 levels could exceed anything on the geological record.
To gauge how the amount of carbon in the atmosphere has changed over the past 420 million years, researchers led by the University of Southampton in the UK compiled approximately 1,500 estimates of atmospheric CO2 levels from 112 published studies.
"We cannot directly measure CO2 concentrations from millions of years ago," says geochemist Gavin Foster from the University of Southampton in the UK.
"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 CO2 levels."
Among other things, the analysis shows that while CO2 levels are much lower now than they have been at other, hotter points in Earth's history, they're rising incredibly quickly.
Concentration of CO2 stood at 280 parts per million (ppm) in the atmosphere during the pre-industrialisation era, but this rose to above 400 ppm in 2016.
Despite the unprecedentedly sharp rise, that's still significantly lower than the concentration got during Earth's past 'greenhouse' periods – where it has risen as high as 3,000 ppm.
But aside from how quickly human-caused CO2 emissions have increased in the past two centuries – ending a slow, natural decline in CO2 levels that lasted hundreds of millions of years beforehand – there's another somewhat scary factor to be aware of.
"Due to nuclear reactions in stars, like our Sun, over time they become brighter," says climate scientist Dan Lunt from the University of Bristol in the UK.
"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."
In other words, the level of solar output that affects Earth's atmosphere – called total solar irradiance – has been growing the past hundreds of millions of years as the Sun has grown brighter, but Earth's climate has kept stable, due to atmospheric CO2 gradually dropping over the same period.
That is, until now.
With CO2 levels and total solar irradiance both rising – collectively called climate forcing – the team thinks we're headed into uncharted and potentially very dangerous climactic territory in the future.
"We found no past time period when the drivers of climate, or climate forcing, was as high as it will be in the future if we burn all the readily available fossil fuel," Foster explains in The Conversation.
"Nothing like it has been recorded in the rock record for at least 420m years."
If fossil fuel use continues unabated, the researchers estimate we'll reach about 2,000 ppm of CO2 by about 2250 – levels not seen since around 200 million years ago during the Triassic period: a hot and dry era in Earth's history in which the planet's polar regions were free of ice, and the first dinosaurs emerged.
"However, because the Sun was dimmer back then, the net climate forcing 200 million years ago was lower than we would experience in such a high CO2 future," Foster explains in a press release.
"So not only will the resultant climate change be faster than anything the Earth has seen for millions of years, 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."
As for what such an environment would look like on the ground, it's difficult to say for sure.
But a study published in 2015 found that if humanity turned its back on renewable energy and proceeded to burn through all the remaining fossil fuel resources on Earth, Antarctica would effectively melt, causing sea levels to rise by up to 60 metres.
That's something we don't want to have happen, people. We're making the future right now – let's not make one we can't live in.
The findings are reported in Nature Communications.