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The riddle of the ancient heat waves Periods of high world temperatures more than 50 million years ago raise questions about the carbon cycle and global warming Alan Bailey Petroleum News
When, a couple of years ago, the Intergovernmental Panel on Climate Change published “Climate Change 2007 — The Physical Science Basis,” this massive and erudite report became in some ways the authoritative text describing the scientific rationale for theories of global warming and predictions of future climate change.
But the 1,100 pages of the report only include 1.4 pages of text and two figures that speak to the Earth’s climate history prior to the Quaternary Period, the most recent period of the Earth’s history, Gerald Dickens, professor in the Department of Earth Science at Rice University, told a meeting of the Alaska Geological Society Feb. 19.
And that’s a pity, because geologists have been making some astonishing discoveries about massive global warming that occurred tens of millions of years before the onset of the Quaternary, the period that began around 1.8 million years ago and that has been characterized by a series of ice ages (and more recently by the greenhouse gas effect of coal burning and other human activity).
Hypothermal events Starting in 1991, geologists uncovered irrefutable evidence for what they term “hypothermal events,” in which the Earth warmed by around 6 degrees Celsius (11 degrees Fahrenheit). Warming typically occurred over a period of 10,000 to 30,000 years, before the Earth cooled down again over the subsequent 200,000 years or so, Dickens said.
Geologists use the presence and chemical attributes of certain tiny organisms to infer these ancient temperature changes.
The first of the hypothermal events to be discovered, referred to as the Paleocene-Eocene Thermal Maximum or PETM, occurred about 55.5 million years ago, while another similar event, the ETM-2, happened about 2 million years later. Evidence from some rock sequences suggests that several other similar events may have happened around this same general period in the Earth’s history.
Clear evidence for the PETM and the ETM-2 can be found at multiple locations around the world, thus demonstrating that the massive heating was a global phenomenon.
“We are virtually certain that in the geologic record we have these short-term … (events) of extreme global warming. … And that appears to occur at all latitudes,” Dickens said.
And these events occurred during a period when the Earth was already relatively warm and becoming warmer, as evidenced by the fossil record.
“That’s not news to anyone who looks at the paleontology,” Dickens said. “You can go find reptile fossils up on Ellesmere Island, palm trees in Wyoming etc.”
Massive carbon release Examination of ratios of carbon isotopes from organic material deposited during the hypothermal events indicates massive increases in atmospheric carbon as temperatures rose, with subsequent declines in carbon levels as temperatures dropped back to more normal levels. Geologists have also found evidence that submarine carbonate deposits were destroyed during the events, as a consequence of ocean acidity resulting from dissolved carbon dioxide.
“We are now virtually certain that these were massive inputs of carbon,” Dickens said.
In fact, geologists think that the total input of carbon during the perhaps 20,000-year onset of an event such as the PETM could have been broadly similar to the approximately 2,400 gigatons of carbon that the human race may inject into the Earth’s natural carbon cycle over, say, 300 years of fossil fuel use.
So, what does the climate record of a 55 million-year-old hypothermal event tell us about our current climate change predicament?
At first blush it’s tempting to conclude that the short but massive outbursts of carbon emissions between 50 million and 60 million years ago simply caused massive carbon dioxide greenhouse gas effects that subsided as the carbon dioxide became absorbed back into the Earth’s natural carbon cycle. That’s an explanation that seems to tally with current theories on climate change and matches nicely with predictions of the impact of global warming on climate systems.
For example, climate change theory predicts that, as the Earth warms, precipitation in the Arctic will increase in volume but occur over shorter time periods. Sediments found in some PETM-related deposits seem consistent with this prediction, Dickens said.
Climate model mismatch Unfortunately, however, to account for the 6 degrees Celsius temperature rise in events like the PETM, the computer climate models used for contemporary climate predictions require much more carbon dioxide to be present in the atmosphere than is indicated by the ancient carbon isotope data. Or, alternatively, the sensitivity of the ancient climate to increases in atmospheric carbon dioxide would have needed to be much higher than climate models indicate, Dickens said.
And evidence from a PETM-related rock sequence in New Jersey suggests another explanation for the linkage between global warming and massive increases in atmospheric carbon levels — the fossil record in these rocks shows clearly that the Earth started warming before the carbon levels started to escalate.
“Clearly about halfway along the warming trend is when the carbon input actually happens,” Dickens said.
That suggests that, rather than carbon dioxide driving the global warming, a rise in global temperatures from some other cause actually drove a carbon buildup, he said. Presumably, the rising temperatures would have caused carbon to be released into the atmosphere from various sources. However, as carbon levels increased it is likely that the continued warming was at least in part attributable to a greenhouse gas effect from atmospheric carbon, Dickens said.
No explanation But, to date, geologists have not been able to determine a satisfactory explanation for either the warming or the release of massive quantities of carbon into the atmosphere.
Wildlife, including various mammals, flourished during the era of the hypothermal events and there is no evidence for a mass extinction, Dickens said. That would seem to preclude the possibility of worldwide catastrophes such as the mass burning of peat or worldwide forest fires, he said. And explanations involving the release of carbon tied up in living things — carbon release by forest fires, for example — fail to account for the total volume of carbon believed to have been injected into the atmosphere.
Maybe, as the Earth slowly warmed at around the time of events like the PETM, the temperature passed certain trigger points where carbon-bearing materials decomposed — gas hydrates might have disassociated on a global scale, for example. But that type of explanation does not gel with scientists’ current understanding of the Earth’s carbon cycle, Dickens said.
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