Washington: Release of carbon compound methane trapped in marine sediments has been thought to have warmed the planet in the past. Now a new study has found that cycles in Earth`s orbit could be blamed for those ancient heat waves that raised the surface temperature by five degrees Celsius.
Based on chemical clues preserved in rocks, scientists have found that an intense heat wave that hit the Earth some 55 million years ago raised the temperature by 5 degree Celsius. The heat subsided after a relatively short time only to be followed by at least two similar but smaller heat waves.
And the scientists believe this huge amount of greenhouse gas came from the melting of permafrost -- frozen soil packed with organic matter -- after cycles in the Earth`s orbit warmed up the areas near the poles, LiveScience reported.
The melting released a massive amount of carbon into the atmosphere, keeping reflected sunlight from escaping and causing the heat wave, they said.
Previously, scientists have theorised that the release of the carbon compound methane changed the atmosphere. But the new study, published in the journal Nature, argues that not enough methane would have been released to account for the magnitude of the warming.
Other theories include a comet impact, extensive fires or the drying of shallow continental seas, "all these difficult ideas," said study author Mark Pagani, a professor at Yale University.
None of these explain the sequence of progressively smaller heat waves that followed, Pagani argued.
Examining a rock outcrop near Gubbio, Italy that contains evidence of these heat waves, also known as hyperthermals, the team found they lined up with cycles in the Earth`s orbit.
The path of Earth around the Sun and its orientation can vary slightly in cycles that last up to 100,000 years. The researchers found that the timing of three large hyperthermals -- beginning about 55 million years ago-- aligned with periods when the tilt of the Earth`s axis was greatest and when the planet`s orbit was most eccentric (that is, least circular).
This combination meant the high latitudes -- the area closest to the poles -- had warmer or longer summers, "with the potential to thaw vast areas of permafrost once a warming threshold is reached," the researchers wrote.
The cycle became self-reinforcing as more carbon entering the atmosphere encouraged more warming, which encouraged more melting and the release of more carbon.
"Then our climate models show if you have permafrost and you warm the temperatures slowly, there is sort of a sweet spot in the model: When you cross it, the whole thing just goes," Pagani said.
Modern discussions of melting permafrost focus on the Arctic. But 50 million years ago, the world was warmer overall than now, and Antarctica was not yet ice-covered, so the team believes that the southernmost continent probably had its own large stock of carbon tucked away in the permafrost.
After a warming stint lasting some 10,000 years, the carbon from the permafrost could have depleted, resulting in atmospheric carbon dioxide that stuck around for about 200,000 years until natural processes drew it out, cooling the planet down, Pagani added.
Then, a million years later, the process most likely repeated itself, but this time with less permafrost available to melt, leading to smaller warming stints, until the hyperthermals ran themselves out, he said.
These ancient hyperthermals are described by the team as intense bursts of warming, but nowadays the planet is warming more rapidly and scientists anticipate that the melting Arctic permafrost is likely to exacerbate things.