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New Understanding of Past Global Warming Events

Posted on 9 April 2012 by John Hartz

This is a reprint of a press release posted by the University of New Hampshire  on April 2, 2012.

A series of global warming events called hyperthermals that occurred more than 50 million years ago had a similar origin to a much larger hyperthermal of the period, the Pelaeocene-Eocene Thermal Maximum (PETM), new research has found. The findings, published in Nature Geoscience online on April 1, 2012, represent a breakthrough in understanding the major “burp” of carbon, equivalent to burning the entire reservoir of fossil fuels on Earth, that occurred during the PETM.

“As geologists, it unnerves us that we don’t know where this huge amount of carbon released in the PETM comes from,” says Will Clyde, associate professor of Earth sciences at the University of New Hampshire and a co-author on the paper. “This is the first breakthrough we’ve had in a long time. It gives us a new understanding of the PETM.” The work confirms that the PETM was not a unique event – the result, perhaps, of a meteorite strike – but a natural part of the Earth’s carbon cycle.

Working in the Bighorn Basin region of Wyoming, a 100-mile-wide area with a semi-arid climate and stratified rocks that make it ideal for studying the PETM, Clyde and lead author Hemmo Abels of Utrecht University in the Netherlands found the first evidence of the smaller hyperthermal events on land. Previously, the only evidence of such events were from marine records.

 Photo of Bighorn Basin in Wyoming

These geological deposits make the Bighorn Basin area of Wyoming ideal for studying the PETM. Credit: Aaron Diefendorf

“By finding these smaller hyperthermal events in continental records, it secures their status as global events, not just an ocean process. It means they are atmospheric events,” Clyde says.

Their findings confirm that, like the smaller hyperthermals of the era that released carbon into the atmosphere, the release of carbon in the PETM had a similar origin. In addition, the warming-to-carbon release of the PETM and the other hyperthermals are similarly scaled, which the authors interpret as an indication of a similar mechanism of carbon release during all hyperthermals, including the PETM.

“It points toward the fact that we’re dealing with the same source of carbon,” Clyde says.

Working in two areas of the Bighorn Basin just east of Yellowstone National Park – Gilmore Hill and Upper Deer Creek – Clyde and Abels sampled rock and soil to measure carbon isotope records. They then compared these continental recordings of carbon release to equivalent marine records already in existence.

During the PETM, temperatures rose between five and seven degrees Celsius in approximately 10,000 years -- “a geological instant,” Clyde calls it. This rise in temperature coincided exactly with a massive global change in mammals, as land bridges opened up connecting the continents. Prior to the PETM, North America had no primates, ancient horses, or split-hoofed mammals like deer or cows.

Scientists look to the PETM for clues about the current warming of the Earth, although Clyde cautions that “the Earth 50 million years ago was very different than it is today, so it’s not a perfect analog.” While scientists still don’t fully understand the causes of these hyperthermal events, “they seem to be triggered by warming,” Clyde says. It’s possible, he says, that less dramatic warming events destabilized these large amounts of carbon, releasing them into the atmosphere where they, in turn, warmed the Earth even more.

“This work indicates that there is some part of the carbon cycle that we don’t understand, and it could accentuate global warming,” Clyde says.

The article, “Terrestrial carbon isotope excursions and biotic change during Palaeogene hyperthermals,” was published online in Nature Geoscience. In addition to Clyde and Abels, co-authors were Philip Gingerich from the University of Michigan, Frederik Hilgen and Lucas Lourens from Utrecht University, Henry Fricke from Colorado College, and Gabriel Bowen from Purdue University. Clyde received funding for this work from the National Science Foundation.

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Comments 1 to 5:

  1. Feedbacks, feedbacks everywhere you look. I'm still struggling with some of the rates, however: the temperature/CO2 feedback works pretty fast, and the cloud/H2O vapor feedback works in concert and at similar rates. One question about this I have is this: Where is the silicate weathering (carbon sequestration) signal in all this?

    Is it: warming reduces ice and increases available rock for weathering, but that is a much slower process?

    Or is it: warming reduces ice, increases available rock, but only until sea level rise covers more rock than is exposed by ice loss?

    Or is it: absent mountain building on a globally significant scale, warming/CO2 release/water vapor increase effects will swamp the silicate weathering signal over long periods of geological time?
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  2. Curious why a land bridge appeared during warming - wouldn't that indicate higher sea level, and therefor fewer land bridges?
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  3. I think those land bridges were tectonic in origin, rather than sea-level related...

    1) continental collision, subduction and plate motion brought new territories into reasonable proximity, and in some cases my have opened sub-aerial connections..

    2) Loss of ice in the Arctic, coupled with still connected Norway/Spitzbergen/Greenland/Canada (in short, North Atlantic rifting was nowhere near complete, and Labrador Sea was no nearly open...) gave rise to passable pathways from Asia, through N. Europe, to N. America.... that's the basic idea here (http://www.pnas.org/content/103/30/11223.full ).

    One aspect of the PETM that is not discussed here is the impact of North Atlantic Volcanic Province activity - massive shallow submarine flood basalt eruption.... there are those who point to that as the trigger for a lot of the early Eocene craziness.
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  4. danielc @1, currently the rock weathering process is approximately in balance with global CO2 emissions from volcanoes. We know this because of the very nearly stable CO2 concentrations over the holocene. That means that the rock weathering process removes approximately 0.224 to 1.1% of annual anthropogenic CO2 emissions every year. Put differently, for each year of emissions at current rates it will take between 90 and 450 years to remove the emitted CO2 from the atmosphere. At least it would if there were no ongoing CO2 emissions from volcanoes.

    Put another way, if the rate of CO2 removal by rock weathering where doubled while the rate of CO2 emissions from volcanoes remained the same and humans stopped all CO2 emissions, it would take on average 5500 year to remove the emissions of the last 20 years from the surface reservoirs of CO2 (ocean, soil, biosphere and atmosphere).

    Fortunately increased warmth does increase the rate of rock weathering by increasing chemical reaction rates directly, and by increasing the strength of the water cycle, thus increasing the rate of erosion. But it may still be from tens to hundreds of thousands of years before CO2 levels are restored to pre-industrial levels.
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  5. I'm a tad suspicious of the sudden appearance of land bridges without some sort of proof. However, in a fairly recent copy of New Scientist it was reported that under the sediment in an area between Iceland and Great Britain, sonar studies have discovered a reticulation of river beds. Apparently the hot spot that keeps Iceland above the surface of the earth sent a blob in this direction and there was for a time, land above the sea. Rivers developed and then as the magma cooled, this land sunk. They also point out other areas in the world where they think this is likely. Still not land bridges, though.
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