A new study has found that carbon emissions are now so high, the last time they hit similar levels was 66 million years ago, when many species of dinosaur were wiped off the face of the planet.
And carbon dioxide emissions are still rising, albeit at a slower rate than in recent years. Researchers have been looking at the geological evidence to compare current levels of CO2 with those of the past - they used sediment cores collected from the New Jersey coast to re-evaluate the carbon-13 and oxygen-18 isotope records, which indicate atmospheric carbon concentrations and global temperature respectively.
The team, led by Richard Zeebe from the University of Hawaii at Manoa, found that rises in carbon emissions and global warming happened more or less simultaneously during the Palaeocene-Eocene Thermal Maximum (PETM) period around 56 million years ago, as the supercontinent Pangea was breaking apart.
That's an important marker, because scientists think the climate changes we're currently experiencing are similar to changes that occurred during the PETM.
The fact that carbon emissions and global warming were closely linked suggests a slow and steady increase in CO2 during that time - if a spike in CO2 levels did occur, the overall climate would take longer to catch up, which discounts the possibility of massive amounts of methane gushing from the seafloor as Pangea split up.
Zeebe and his colleagues then used their findings to calculate that the annual carbon emissions rate during the PETM was between 0.6 and 1.1 billion tonnes per year. Compare that to the 10 billion tonnes (and rising) that are annually pumped into the atmosphere today, and you can easily see the severity of the problem.
As for the event that killed off the dinosaurs, it's hard to say exactly what the levels of CO2 were in the atmosphere at the time (around 10 million years before the PETM), says Zeebe, because geological records get progressively worse as far as older events are concerned. It's possible that the 9.6-km-wide (6-mile-wide) asteroid that slammed into Earth's surface saw similar CO2 emissions to those we're seeing today - but we can't be sure.
So in reality, we're actually looking at carbon emission levels that are higher than they've been since the time of the dinosaurs, and possibly even earlier than that.
"We conclude that, given currently available records, the present anthropogenic carbon release rate is unprecedented during the past 66 million years," conclude the researchers. "We suggest that such a 'no-analogue' state represents a fundamental challenge in constraining future climate projections. Also, future ecosystem disruptions are likely to exceed the relatively limited extinctions observed at the PETM."