PETM

• Earth's orbital variations--the most common form of natural global warming
• Pumping huge amounts of carbon into the atmosphere coincided with extreme climate warming during the PETM
• Computer climate models fail to reproduce the PETM
• Comparison with today
• For more information
• References

One frequently hears comments like, "Earth has had many periods of warmth far exceeding the warmth of today's climate, so we should not be surprised if the current warming of the globe is a natural phenomenon". This view is especially prevalent among geologists, who take a very long view of history and are among the most skeptical scientists regarding the reality of human-caused climate change. It is true that Earth's past has had many episodes of natural global warming that we can learn from. But the greatest natural global warming episode of the past 65 million years, the Paleocene-Eocene Thermal Maximum (PETM) event of 55 million years ago, presents us with a cautionary tale of how massive releases of greenhouse gases--similar in scale to what humans are now producing--may cause extreme warming of the climate.

Earth's orbital variations- the most common form of natural global warming

The most common cause of natural global warming over time has been changes in Earth's orbit. Three oscillations in Earth's orbit with periods of 26,000, 41,000, and 100,000 years (called Milankovitch cycles) cause ice ages to be triggered when summer sunshine at 65°N reaches a minimum. The reduced sunlight over Canada, Siberia, and Scandinavia allows winter snowfall to persist through the summer, and thus accumulate and build Northern Hemisphere ice sheets. There isn't enough land in the Southern Hemisphere to allow large, land-based ice sheets to build there, so it is the growth and decay of Northern Hemisphere ice sheets that has controlled the timing of ice ages and warm inter-glacial periods over the past three million years. Earth's orbit is currently in a phase where the amount of sunlight falling at 65°N is changing very little. Thus, the primary mechanism for past natural global warming events is not to blame for the current warming. According to the "official" word on climate, the 2007 report of the U.N.'s Intergovernmental Panel on Climate Change (IPCC), the amount of sunlight falling at 65°N is expected to change little over the next 30,000 years, and "it is very unlikely that the Earth would naturally enter into another ice age for at least 30,000 years".

Pumping huge amounts of carbon into the atmosphere coincided with extreme climate warming during the PETM

Natural global warming has also occurred in the past due to changes in solar brightness, and natural emissions of natural carbon dioxide (CO2) and methane (methane is the primary component of the natural gas we use to heat our homes, is a potent greenhouse gas 20 - 25 more effective at heating the Earth than than CO2, with a lifetime of about 9 years in the atmosphere before reacting with the OH radical to form CO2). I discussed one example of natural global warming in my previous post--volcanoes have emitted enough CO2 over time to account for a large portion of Earth's natural greenhouse effect. However, volcanoes only put about 1 - 3% as much CO2 per year into the atmosphere as human activities do. What, then, does Earth's past tell us about what might happen if we dump 100 times more carbon than volcanoes do into the atmosphere, over a period of a few centuries?

The end of Earth's Paleocene era, 55 million years ago, was a time of great warmth on planet Earth. Subtropical vegetation grew in Greenland and Patagonia, and crocodiles swam off the coast of Greenland. Sea surface temperatures at the North Pole were a toasty 64°F (18°C). Tropical palm forests in northern Wyoming played host to early primates. Despite the fact that the sun put out 0.5% less energy than today (equivalent to a global temperature that would be 0.5°C cooler), there was no polar ice cap or Greenland Ice Sheet. The higher temperatures of that era were probably due to high carbon dioxide levels of 560 - 600 ppm. This is far higher than the 280 ppm seen in the 1800s, and the 383 ppm as of 2009. The continents had a different configuration due to continental drift, and this may have kept the world warmer as well.

Figure 1. Global temperature change (right scale) as inferred from oxygen-18 isotope measurements (left scale) from fossil ocean microorganisms (Zachos et al., 2001). Oxygen-18 levels in these fossils are proportional to the temperatures of the era when the fossils were formed. The Paleocene-Eocene Thermal Maximum (labeled PETM) shows a sharp upward spike in temperatures occurred. This spike is likely to be understated by a factor of 2 - 4 due to coarse sampling and averaging in this data set. For more detail, see the Wikipedia entry for the PETM.

Then, 55 million years ago, the fossil record shows that an extraordinary drop in the ratio of carbon-13 to carbon-12 occurred, indicating that a massive amount of "light" carbon with low levels of the carbon-13 isotope was emitted into the atmosphere in a very short amount of time--just 500 - 20,000 years. The most likely source of carbon-13 depleted carbon would have been methane from ocean sediments or land vegetation. If it was methane, about 1,000 - 2,000 gigatons of carbon would have had to be injected into the atmosphere, in order to account for the observed fossil deposits. For comparison, the total amount of carbon in today's atmosphere, primarily as CO2, is a factor of two or three less--about 810 gigatons. The fossil record shows that extreme climatic warming occurred nearly simultaneously with this massive release of carbon into the atmosphere. Global average temperatures rose 9°F (5°C) in a geological instant--1,000 - 10,000 years (Sluijs et al., 2007). Average sea surface temperatures at the North Pole reached 74°F (23°C). The warmth lasted 120,000 - 220,000 years before weathering of silicate rocks was able to remove the CO2 from the atmosphere and return the climate to its former state. This was the largest global warming event since the time of the dinosaurs, 65 million year ago (Moran et al., 2006). The resulting impact on Earth's climate was so severe that a new geological era was born--the Eocene. The warming event has been dubbed the Paleocene-Eocene Thermal Maximum (PETM), since it occurred at the boundary of these two eras. Ocean circulation patterns changed radically during the first 5,000 years of the PETM (Nunes and Norris, 2006), and the deep oceans became 11°F (6°C) warmer, severely depleted in oxygen, and more acidic. A mass extinction of deep ocean microorganisms resulted, though the exact reasons remain unclear. The PETM did not cause mass extinctions on land of plants and animals, but a major turnover in mammalian life occurred at that time. Many of today's major mammalian orders emerged in the wake of the PETM. The new geological era it ushered in, the Eocene, is named for the Greek goddess of the dawn (Eos), since this was the dawn of the era of large mammals.

It is extremely difficult to explain the warmth of the PETM without assuming that the huge amount of "light" carbon pumped into the atmosphere created intense warming due to the greenhouse effect. The controversial question is, how did this carbon get into the atmosphere? Did PETM happen because of the greenhouse effect from all the carbon added to the atmosphere, or did the carbon get released into the atmosphere in response to climatic warming from another cause (and boosting the warming that was already occurring?) The mystery is a difficult one to unravel, since our vision of what happened so long ago is very fuzzy. A recent high-resolution study of ocean sediments laid down in New Jersey during the PETM (Sluijs et al., 2007) argued that about half of the PETM warming occurred 1,000 - 1,500 years before the 1,000 - 2,000 gigatons of "light" carbon got injected into the atmosphere. The authors theorize that global warming due to some other cause heated up the deep oceans enough to release methane stored in the form of methane hydrate, a form of methane 'ice' that forms in cold bottom water under great pressures and is widely distributed and plentiful in sediments on the outer edges of continental margins. The methane released was the huge pulse of "light" carbon seen in the fossil record, and this methane warmed the planet even further via the greenhouse effect. The authors argued that the warming that triggered the PETM could have been due to a variation in Earth's orbit, or due to a pulse of greenhouse gases that didn't happen to be enriched in light carbon. A wide variety of other theories abound. Dickens (2004) theorizes that a volcano in the North Atlantic erupted through a huge fossil fuel deposit in overlying ocean sediments, releasing massive amounts of the stored carbon into the atmosphere. Pancost et al. 2006 found evidence that the warming of the PETM significantly increased as carbon stored on land in wetlands was released in the form of methane. Huber (2008) argued that temperatures got so hot during the PETM that a huge die-off of tropical vegetation resulted, creating vast deserts and putting thousands of gigatons of carbon dioxide into the atmosphere, further increasing temperatures.

Figure 2. Photographs of methane hydrate as nodules, veins, and laminae in sediment. Intense warming of the deep oceans during the PETM may have released huge quantities of methane gas from ocean sediments with methane hydrates in them. Image credit: United States Geological Survey.

Computer climate models fail to reproduce the PETM

A big concern about the climate models that we are using to forecast climate for the coming century is that they do a poor job of reproducing the climate of the Eocene, and, in particular, the PETM. These models fail to reproduce the high temperatures observed in the polar regions relative to the tropics during the PETM. However, in the words of climate scientists Daniel Schrag and Richard Alley in a 2004 article in Nature, "It would be a grave mistake to take these lessons from ancient climates as a reason to disregard the projections from climate models." If the observations of the climate in this far-ago era are correct, the reason that the climate models fail to correctly simulate this past climate is because the climate is more sensitive to CO2 than believed. There is a missing "feedback" causing increased warming near the pole that the models are missing. Sluijs et al. (2006) theorize that the models may be missing how hurricanes transport heat to the poles, or how polar stratospheric clouds may act to trap heat over the poles. In short, the failure of the models to correctly simulate the PETM may mean that our current estimates of the amount of global warming likely over the coming century (1.1°C - 6.4°C) are far too low. The other possibility, mentioned by Huber (2008) is that the models are correct, but the temperatures inferred for the tropics from the fossil record are in error. This wouldn't be the first time that measurements were found to be in error and the models vindicated.

Comparison with today

Since the beginning of the Industrial Revolution, humans have pumped about 500 gigatons of carbon into the atmosphere. There are about 5,000 gigatons in the planet's coal reserves, while oil and traditional natural gas deposits are hundreds of gigatons each (Rogner, 1997). Given that humans are now adding about 10 gigatons of carbon to the atmosphere each year (Global Carbon Project, 2007), we will surpass the 1,000 gigaton mark 50 years from now at current emission rates. This is at the lower end of the 1,000 - 2,000 gigatons of carbon that are estimated to have been added to the atmosphere during the PETM--the most extreme natural global warming event of the past 65 million years. Though our view of events so long ago is very fuzzy, the PETM should serve as a cautionary tale. We cannot rule out the possibility that continuation of our current rates of fossil fuel burning will lead to an extreme climatic warming event like the PETM. In particular, we need to keep a careful eye on the huge reservoirs of methane hydrate stored in marine sediments (500 - 10,000 gigatons of carbon) and stored in permafrost (7.5 - 400 gigatons). Continued warming of the planet could trigger substantial releases of these massive reservoirs of greenhouse gases, leading to a repeat of the PETM event. However, a 2008 study by the U.S. Climate Change Science Program (CCSP, 2008) concludes that there is currently no evidence that a sudden catastrophic release of methane stored in ocean sediments or in permafrost will happen over the next century. It should take at least a century for global warming to penetrate the deep oceans and permafrost regions containing these significant reservoirs of methane hydrates. The study concludes, "Catastrophic release of methane to the atmosphere appears very unlikely in the near term (e.g., this century)...Although the prospect of a catastrophic release of methane to the atmosphere as a result of anthropogenic climate change over the next century appears very unlikely based on current knowledge, many of the processes involved are still poorly understood, and developing a better predictive capability requires further work. On a longer time scale, methane release from hydrate reservoir is likely to be a major influence in global warming over the next 1,000 to 100,000 years". So, the bottom line is: don't expect global warming to be able to cause huge releases of methane hydrates in the coming century, such as may have occurred during PETM. But it is wise to ponder that a release of greenhouse gases similar in magnitude to what we are doing now coincided with the most extreme global warming event of the last 65 million years. We should not be surprised if our human greenhouse gas emissions cause a similar massive climate perturbation over the next 1,000 years, leading the dawn of a new geological era--the Anthropocene.

For more information

One of the best sources for researching this is a December 2008 report from the U.S. Climate Change Science Program (CCSP), titled, Abrupt Climate Change. Chapter 5, "Potential for Abrupt Changes in Atmospheric Methane" [1.3 Mb] was the relevant chapter.

References

Archer, D., 2007, "Methane hydrate stability and anthropogenic climate change", Biogeosciences, 4, 521544,.

Dickens, G.R., 2004, "Hydrocarbon-driven warming", Nature 42, 429, pp513-515, 3 June 2004.

Huber, M., 2008, "A Hotter Greenhouse?", Science 321, no. 5887, pp. 353-354, DOI: 10.1126/science.1161170

Moran, et al., 2006, "The Cenozoic palaeoenvironment of the Arctic Ocean", Nature 441, 601-605 (1 June 2006) | doi:10.1038/nature04800.

Nunes, F. and R.D. Norris, 2006, "Abrupt reversal in ocean overturning during the Paleocene/Eocene warm period", Nature 439 (7072): 603. doi:10.1038/nature04386

Pancost, R.D., et al., 2006, "Increased terrestrial methane cycling at the Palaeocene-Eocene thermal maximum", Nature 449, 332-335 (20 September 2007) | doi:10.1038/nature06012

Rogner, H.-H, 1997, "An assessment of world hydrocarbon resources", Annu. Rev. Energy Environ., 22, 217-262.

Sluijs, A., et al., 2006, "Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum", Nature 441, 610-613 (1 June 2006) | doi:10.1038/nature04668

U.S. Climate Change Science Program (CCSP), 2008: Abrupt Climate Change. A report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research [Clark, P.U., A.J. Weaver (coordinating lead authors), E. Brook, E.R. Cook, T.L. Delworth, and K. Steffen (chapter lead authors)]. U.S. Geological Survey, Reston, VA, 459 pp.

Zachos, J. C., U. Rohl, S.A. Schellenberg, A. Sluijs, D.A. Hodell, D.C. Kelly, E. Thomas, M. Nicolo, I. Raffi, L.J. Lourens, H. McCarren, and D. Kroon, 2005, "Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum", Science, 308, 1611-1615