Thawing permafrost 50 million years ago led to warm global events, says new study

A new study led by the University of Massachusetts Amherst and involving the University of Colorado Boulder proposes a simple new mechanism to explain the source of carbon that fed a series of extreme warming events on Earth about 50 million years ago called the Paleocene-Eocene Thermal Maximum, or PETM, as well as a sequence of similar, smaller warming events afterward.

“The standard hypothesis has been that the source of carbon was in the ocean in the form of frozen methane gas in ocean-floor sediments,” said lead study author Rob DeConto of the University of Massachusetts Amherst. “We are instead ascribing the carbon source to the continents in polar latitudes where permafrost can store massive amounts of carbon that can be released as CO2 when the permafrost thaws.”

The new view is supported by calculations estimating interactions of variables such as greenhouse gas levels, changes in the Earth’s tilt and orbit, ancient distributions of vegetation, and carbon stored in rocks and in frozen soil.  A paper on the subject appears in the April 5 issue of Nature.

While the amounts of carbon involved in the ancient soil-thaw scenarios were likely much greater than today, implications of the study appear dire for the long-term future as polar permafrost carbon deposits have begun to thaw due to burning fossil fuels, DeConto said. “Similar dynamics are at play today. Global warming is degrading permafrost in the north polar regions, thawing frozen organic matter, which will decay to release CO2 and methane into the atmosphere. This will only exacerbate future warming in a positive feedback loop.”  

He and colleagues at CU-Boulder, Yale, Penn State, the University of Urbino, Italy, and the University of Sheffield in England designed a model to account for the source, magnitude and timing of carbon release at the PETM and subsequent warm periods, which now appear to have been triggered by changes in the Earth’s orbit.

CU-Boulder researcher Kevin Schaefer, a Nature paper co-author, led a 2011 study showing up to two-thirds of Earth’s permafrost could disappear by 2200 as a result of warming temperatures, unleashing vast quantities of carbon into the atmosphere.

“We found in this new Nature study that changes in Earth’s orbit triggered massive releases of carbon dioxide and methane from thawing permafrost in Antarctica,” said Schaefer, a research associate at CU-Boulder’s National Snow and Ice Data Center, an arm of the Cooperative Institute for Research in Environmental Sciences. “If the Arctic permafrost thaws out, it will release carbon dioxide and methane into the atmosphere and amplify warming due to the burning of fossil fuels.”

Earth’s atmospheric temperature is a result of energy input from the sun minus what escapes back to space. CO2 in the atmosphere absorbs and traps heat that would otherwise return to space. The PETM was accompanied by a massive carbon input to the atmosphere, with ocean acidification, and was characterized by a global temperature rise of about 9 degrees Fahrenheit in a few thousand years, according to the study.

The research team used a new, high-precision geologic record from rocks in central Italy to show that the PETM and other warming events occurred during periods when Earth’s orbit around the sun was both highly eccentric and tilted. Orbit affects the amount, location and seasonality of solar radiation received on Earth, which in turn affects the seasons, particularly in polar latitudes, where permafrost and stored carbon can accumulate.

They then simulated climate-ecosystem-soil interactions, accounting for gradually rising greenhouse gases and polar temperatures plus the combined effects of changes in Earth’s orbit. Their results show that the magnitude and timing of the PETM and subsequent busts of warming can be explained by the orbitally triggered decomposition of organic carbon soil material in Antarctica and the area surrounding the Arctic.

The massive carbon reservoir at the poles had the potential to repeatedly release billions of tons carbon to the atmosphere-ocean system once a long-term warming threshold was reached just prior to the PETM,” DeConto and colleagues say. Until now, Antarctica, which today is covered by miles of ice, has not been recognized as an important player in such global carbon dynamics, according to the team. 

In the past, Antarctica and high elevations adjacent to the Arctic region were suitable locations for massive carbon storage, according to the study. “During long-term warming, these environments eventually reached a climatic threshold,” with permafrost thaw and the sudden release of stored soil carbon triggered during the Earth’s highly eccentric orbits coupled with high tilt, according to the study. 

Janet Lathrop, UMass media relations, 413-545-0444
Robert DeConto, 413-545-3426
Kevin Schaefer, 303-492-8869
Jim Scott, CU media relations, 303-492-3114

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