Zachos, who is not involved with the new study, said that between 4,000 and 5,000 gigatons of carbon were released into the atmosphere and oceans over a span of 10,000 to 20,000 years during the ancient warmup. A single gigaton equals about a billion tons.
"There are limited sources for such a large mass of carbon to be released that fast," Zachos said.
Some geologists had previously speculated that a series of vents along the mid-ocean rift between Greenland and Europe injected vast amounts of magma upward into Earth's crust.
The hot magma cooked organic matter in the seafloor sediment. That cooking released the greenhouse gases carbon dioxide and methane into the water, the theory goes.
Those gases turned the oceans fatally acidic and likely resulted in further greenhouse gases rising into the atmosphere—just as cars, coal plants, and other human creations are releasing vast amounts of these gases into the atmosphere now.
The new study, Zachos said, firmly links that bout of North Atlantic volcanism to the start of the PETM warming event.
"[Storey and colleagues] have a compelling case for this serving as a trigger to initiate warming, which could then potentially push the system across some sort of threshold that causes the other additional carbon to be released," he said.
For example, the ocean warmup from these gases may have triggered the release of methane locked up in deposits on ocean shelves surrounding the continents.
Prior to the PETM, Earth's climate was already much warmer that it is today.
The additional greenhouse gases warmed up the planet even more and produced "dramatic changes" on Earth, study co-author Storey said.
For example, sea-surface temperatures in the Arctic Ocean reached as high as 74 degrees Fahrenheit (23 degrees Celsius), he said, and palm trees swayed as far north as Canada.
Studying the PETM climate change event sheds light on "what happens when you pump greenhouse gases on this sort of scale into the atmosphere," Storey said.
"We are pumping carbon dioxide now into the atmosphere at a rate much higher than when this event occurred 55 million years ago," he added.
Today human activity puts an estimated 7 gigatons of carbon into the atmosphere each year.
Over the past century greenhouse gases have helped push Earth's temperature up about 1 degree Fahrenheit (0.6 degree Celsius), many scientists say.
By the end of this century temperatures are likely to rise an additional 3.2 to 7.1 degrees Fahrenheit (1.8 to 4 degrees Celsius), according to a recent report by the UN's Intergovernmental Panel on Climate Change.
Such global warming could trigger additional releases of greenhouse gases, such as methane deposits locked up in the Arctic's permafrost.
"People have certainly worried about feedbacks—amplifying effects that get triggered when climate crosses certain thresholds," the University of California's Zachos said.
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