New study finds parallels between past and present climate change

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The banded sedimentary rock layers of the Willwood formation in Wyoming provide clues as to future climate change (Photo: Scott Wing, Smithsonian Institution)

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There's an element of déjà vu in the most recent political news on climate change: UN-led talks, like the recent Lima summit, that end with disgruntled environmentalists and plans for yet another summit. At this point, our best hope is to mitigate the effects of global warming (which is occurring faster than previously thought) and, if possible, keep temperature rises to a maximum of 2° C (3.6° F). While the future of the planet looks uncertain with unpredictable climate patterns, U.S researchers looking to the past to gain a better understanding of modern climate change have found the rate of modern, human-caused global warming resembles that which occurred almost 56 million years ago much more closely than previously thought.

A new study out of the University of Utah says the mechanisms behind the so-called Paleocene-Eocene thermal maximum (PETM), a global warming event that took place around 55 million years ago when temperatures rose by 5° C (9° F) to 8° C (14.4° F), are similar to current warming trends. The PETM was first discovered in 1991 and the new study sheds light on how it happened, providing clues to the climate change the planet is now undergoing.

Digging for clues

The researchers analyzed carbonate and limestone nodules in sediment cores found in Polecat Bench in northern Wyoming’s Bighorn Basin, which is a site that has been excavated by paleontologists studying mammals for more than a century. The site features a stratigraphic sequence of more than 2 km (1.2 miles) of rocks, dating from 65 to 52 million years ago. The Paleocene-Eocene warming is recorded in the rock and soil layers of Willwood formation, a sedimentary sequence deposited during the late Paleocene to early Eocene, more specifically in the round, gray to brown-gray carbonate nodules in those rocks, varying from 0.1 to 2 in (0.25 to 5.1 cm) in diameter.

History repeats

The new study suggests the warming was caused by two pulses of carbon release that took place over 1,500 years. Previous analysis of seafloor sediments in other parts of the world also indicated two Paleocene-Eocene carbon pulses. When they measured carbon isotope ratios in the nodules, the Utah researchers found that during each 1,500-year carbon release, the ratio of carbon-13 to carbon-12 in the atmosphere declined, an indication that two large releases of carbon dioxide or methane came from the biosphere, just like fossil fuel carbon does. The decline was three parts per thousand for the first pulse, and 5.7 parts per thousand for the second.

The rate of carbon release is what led the researchers to make parallels with the contemporary Anthropocene period. It involved the average annual release of a minimum of 0.9 petagrams (1.98 trillion pounds, or 0.9 trillion kilos) of carbon to the atmosphere, and probably much more over shorter periods.

The rate for anthropogenic carbon emissions is measured at 9.5 petagrams (Pg), or 20.9 trillion pounds (9.5 trillion kg), per year. Between 1900 and 2010, the burning of fossil fuels emitted an average of 3 Pg/year. Although at face value the old and modern figures seem too far apart, a deeper analysis makes the comparison between past and present more relevant.

“Our estimate for the PETM is a conservative one," Gabe Bowen, lead author and geochemist at the University of Utah, tells Gizmag. "The values we get depend on the assumptions we make about where the new carbon is coming from and what its carbon isotope signature is.

"We made assumptions that gave us a minimum estimate (we assumed all the carbon came from methane), but if some or all of the carbon was coming from other sources (soils, plants), as some researchers have argued, the total amount added and the rate at which it was added during the PETM could have been three to four times greater (3 to 4 Pg/year)."

He adds that, as a natural process, the rates of release fluctuated up and down over time. There were periods of time with much higher and some with lower release rates within that 1,500-year window, although the study could not pin down the variation precisely. However, if in the future someone were to calculate the average of man-made warming during the 530 to 2030 interval, when there would be several near zero-emission phases, the overall rate would probably drop to about 0.5 Pg/year or less.

Back at the PETM, the atmospheric carbon levels took a few thousand years to normalize again after the first pulse, probably as carbon dissolved in the ocean. But it took much longer after the second pulse, somewhere around 200,000 years. In that warm period, some areas were stormier while others were more arid. It was period of massive migration of animals and plants. There was not much extinction caused by the warming, except for one cell organisms and deep-sea foraminifera that couldn't move easily. It was also the period when some modern mammals started showing up, including ancestral primates and hoofed animals, and the oceanic acidity increased to the level it is now.

The researchers say the second pulse of carbon may have been caused by feedbacks. "The fact we have two releases may suggest that second one was driven by the first, perhaps, for example, if the first warming raised sea temperatures enough to melt massive amounts of frozen methane,” Bowen says. "Understanding the connection between the two pulses in more detail will be key to better understanding the true implications of this pattern, Hopefully, the fact that we see some evidence for positive feedbacks and amplification of global change by the natural system can help reinforce the many calls already made for attention and action in response to ongoing carbon emissions.”

Show cause

According to previous theories, carbon release could have lasted less than a year or tens of thousands of years as some of the hypothesized causes included an asteroid impact, slow melting of permafrost, burning of organic-rich soil or drying out of a major seaway. But these would have been either too fast or two slow.

Bowen says the two relatively rapid carbon releases are more consistent with warming oceans or an undersea landslide triggering the melting of frozen methane on the seafloor and large emissions to the atmosphere, where it became carbon dioxide within decades. Another possibility is a massive intrusion of molten rock that heated overlying organic-rich rocks and released a lot of methane.

There are a couple of differences between then and now that need to be taken into account, though. When the Paleocene-Eocene warming began, the global climate was already much warmer than today’s, Besides, there were no icecaps, so warming happened on a setting that was quite different from today's.

Lessons to be learned

All this knowledge can be valuable to face present challenges. “The body of research on the PETM and its climate and biotic impacts is rich, and there is a lot of information in those scientific studies that could begin to help us understand what might be required to adapt to climate change,” Bowen says. He believes the greatest lesson to be learned is to be flexible.

"During the PETM, land plants and mammals seemed to cope with the massive environmental change by moving. They had to shift huge distances, but it seems they were largely able to do so, and they survived the event. In contrast, some groups that were more rooted in space and less flexible (coral reefs, deep-ocean foraminifera) suffered major extinctions. The first step in adaptation is being willing to change," he notes. "But change is sometimes hard."

Details of the study were published in the latest issue in the journal Nature Geoscience.

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