The rate of carbon emissions is higher than at any time in fossil records stretching to the age of the dinosaurs, according to a study.
Scientists say the pace of emissions even eclipses the onset of the biggest-known natural surge in fossil records, 56 million years ago, that was perhaps driven by a release of frozen stores of greenhouse gases beneath the seabed.
That ancient release, which drove temperatures up by an estimated 5C and damaged marine life by making the oceans acidic, is often seen as a parallel to the risks from the build-up of carbon in the atmosphere from burning fossil fuels.
"Given currently available records, the present anthropogenic carbon release rate is unprecedented during the past 66 million years," the scientists say in the journal Nature Geoscience.
The dinosaurs went extinct about 66 million years ago, perhaps after a giant asteroid struck the Earth.
Lead author Richard Zeebe, of the University of Hawaii, said geological records were vague and "it's not well known if/how much carbon was released" in that cataclysm.
Present carbon emissions, mainly from burning fossil fuels, are about 10 billion tonnes a year, against 1.1 billion a year spread over 4000 years at the onset of the fast warming of 56 million years ago, the study found.
The scientists examined the chemical make-up of fossils of tiny marine organisms in the seabed off New Jersey in the United States to gauge that ancient warming, known as the Paleoeocene-Eocene thermal maximum (PETM).
UN studies project temperatures could rise by up to 4.8C this century, causing floods, droughts and more powerful storms, if emissions rise unchecked.
Carbon dioxide forms a weak acid in seawater, which threatens the ability of creatures such as lobsters or oysters to build protective shells.
"Our results suggest that future ocean acidification and possible effects on marine calcifying organisms will be more severe than during the PETM," Zeebe said.
"Future ecosystem disruptions are likely to exceed the relatively limited extinctions observed at the PETM," he said.
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