What caused the Paleocene-Eocene Thermal Maximum (PETM)?
About 56 million years ago, Earth’s temperature was a lot warmer than it is today–as much as 21°F higher than today (see the graph). Earth’s temperature is rising today, likely because of human emissions of greenhouse gases. But 56 million years ago, there were no human emissions; there were no humans. What caused the big increase in Earth’s temperature? And could it happen again today?
Researchers at Rice University suggest that the temperature increase could well be due to releases of stored methane from the oceans.
Methane is a powerful greenhouse gas and a natural product of bacterial decomposition. In the oceans, methane sinks into the sediments and freezes into a slushy gas hydrate, stabilized in a narrow band under the seafloor.
According to calculations done by the Rice University scientists, the warmer oceans resulted in more methane hydrate being stored. At warmer temperatures, bacteria decompose organic materials faster, resulting in more methane in a shorter period of time. They estimate that, just before the PETM, there was as much methane hydrate stored as there is today, in a smaller band than exists today.
If this band is disturbed, as by a meteor impact or earthquake, the methane can be rapidly released into the atmosphere. More greenhouse gases in the atmosphere result in increased warming. But there’s no evidence of there having been an impact. So what happened to release the methane 56 million years ago?
Nobody really knows, but the significance is clear.
“I’ve always thought of (the hydrate layer) as being like a capacitor in a circuit. It charges slowly and can release fast — and warming is the trigger. It’s possible that’s happening right now,” said Gerald Dickens, a Rice professor of Earth science and an author of the study.
That makes it important to understand what occurred in the PETM, he said. “The amount of carbon released then is on the magnitude of what humans will add to the cycle by the end of, say, 2500. Compared to the geological timescale, that’s almost instant.”
“We run the risk of reproducing that big carbon-discharge event, but faster, by burning fossil fuel, and it may be severe if hydrate dissociation is triggered again,” Guangsheng Gu, lead author of the study, said, adding that methane hydrate also offers the potential to become a valuable source of clean energy, as burning methane emits much less carbon dioxide than other fossil fuels.
Learn more about the feedback loops involved in climate change in the High-Adventure Science climate investigation.