Could Human CO2 Emissions Cause Another PETM?
This conclusion is based on the sharp negative carbon isotope excursion (CIE; >2.5‰ in deep-sea sequences and 5-6‰ in terrestrial and shallow marine settings). The leading hypothesis for the trigger of the greenhouse gas emissions is the rapid dissociation of shallowly buried methane hydrate deposits along continental margins. One of several compelling arguments in favor of this hypothesis is that the magnitude of the CIE requires a highly depleted carbon source in order to affect the entire carbon reservoir very rapidly. An unrealistically large volume of a less depleted CO2 source, including volcanism or combustion of previously buried organic matter, would have been required to generate the CIE. Microbially generated methane (degradation of organic matter by methanogenic bacteria) has a δ13C of <-60‰. This highly depleted δ13C signature led to the hypothesis that microbially generated methane in hydrate deposits was the source of the CIE. The event may have occurred in pulses of carbon release (likely methane) over a period of 10,000 to 37,000 years, with individual pulses occurring very rapidly, likely 1,000 years or less.
When organic material decomposes it release methane, also known as swamp gas. Combine water and swamp gas under low temperatures and high pressures and the result can be a frozen lattice-like substance called methane hydrate. This crystalline combination of a natural gas and water (known technically as a clathrate) looks remarkably like ice but burns if lit with a flame. It is thought that huge amounts of methane hydrate lie under Earth’s oceans and polar permafrost.
Estimates show that oceanic gas hydrates currently hold somewhere between 1,000 and 22,000 gigatons of carbon as methane, with most studies suggesting about 10,000 gigatons. Considering that our atmosphere contains about 700 gigatons of carbon, even the low mass estimates make gas hydrate a major component of the global carbon cycle.
Methane bubbles escaping from the sea floor.
The conclusion that CO2 was not the primary culprit in the PETM temperature spike and extinctions is further corroborated buy research into other similar events. In a paper in the June 19, 2009, issue of Science, investigating the sudden loss of Late Triassic biodiversity in East Greenland, Jennifer C. McElwain, Peter J. Wagner and Stephen P. Hesselbo found a similar, supporting roll for CO2:
The abrupt plant diversity loss between 33 and 37 m is consistent with expected plant responses to a catastrophically rapid rather than gradual environmental change and argues against the currently favored extinction mechanisms invoking gradual CO2-induced global warming due to slow release of CO2 from the mantle associated with extrusion of basalt over an area of >10 million km2 (CAMP; Central Atlantic Magmatic Province)
There is evidence of other massive CO2 releases caused by volcanic activity that predate the PETM. None of these events seem to have triggered a similar spike in global temperatures or resulted in mass extinctions on their own. There was significant volcanic activity around the time of the PETM along the then embryonic plate boundary between Greenland and Europe, so volcanism could have been a significant contributor. Others find possible sources of CO2 in huge forest fires. The following bit of speculation comes from an article on the Geological Society of London web site by Jonathan Cowie:
One possible source of the CO2 could have been the combustion of plants and trees. But as the planet currently supports c. 1840 gigatonnes of dry terrestrial biomass then all would need to be burned to account for the excursion. How about a carbonaceous chondrite impacting the Earth? Alas, if one had, we would expect to see an iridium peak in early Eocene sediments. Three other possibilities remain. A major fossil fuel seam may have ignited. Marine methane hydrates may have destabilised, releasing a massive amount of methane. Or, both possibilities occurred.
To recap the possible cause of the massive carbon spike at the onset of the PETM here are the major proposed causes: massive volcanic eruptions, possibly causing magma interaction with basin-filling carbon-rich sedimentary rocks plate boundary between Greenland and Europe; a release of large volumes of methane from clathrate deposits, much of which would have made it into the atmosphere without being converted into CO2; and the combustion of plants and trees. A sudden shift in ocean thermohaline circulation could also have helped warm the deep ocean enough to release methane from clathrates as well, though the configuration of those currents was significantly different than today because the disposition of land masses was much different then. While there is evidence for all three prospective causes the case for methane hydrate seems strongest because it explains the the rapid onset of anoxic conditions in the oceans and the spike in δ13C.
So what was the cause of the PETM? I like Douglas Erwin’s explanation for the cause of the Permian-Triassic Extinction, the worst extinction event in Earth’s history. He proposed the “Murder on the Orient Express” model. In other words, it was all these bad things-each by itself incapable of explaining all the evidence-happening at once. Volcanic activity could have helped warm the oceans to release methane and also caused the forest fires. You might ask, “how improbable is that?” But remember, complex life has been plentiful on Earth for more than half a billion years, which is plenty of time for any improbable coincidence to happen several times. What ever the actual chain of events that triggered the PETM it looks like CO2, even 2,000 gigatons of it, wouldn’t have been released fast enough and couldn’t have cause the sudden temperature spike on its own.
One final point comes from a paper titled “The Paleocene-Eocene Thermal Maximum in the Southern Ocean: Middle Bathyal Constraints from ODP Sites 689 and 738,” written by S. Schellenburg, j. Zachos and D. C. Kelly for the American Geophysical Union Fall Meeting in 2003. In it they state:
We hypothesize that these patterns reflect carbonate saturation profile changes in response to pulsed pCO2 increases from distinct methane releases. Specifically, given that wt% carbonate values remain suppressed at Site 690, we interpret the “double-dip” pattern at Sites 689 and 738 as two distinct lysocline shoalings to at least middle-bathyal depths. If true, current PETM models may underestimate the magnitude of carbonate undersaturation, and thereby the methane volume released, during the PETM.
This hints that much larger volumes of methane were released than the normal estimated amounts. Estimates all tend to be in the range of 1,200 – 5,000 gigatons of carbon with 2,000 gigatons often cited and at least one estimate as high as 10,000 gigatons. So could human GHG emissions cause an event similar to the PETM? Let’s do some “back of the envelope” calculations and see.
Humans are currently emitting around 4 Gt of carbon a year that doesn’t get reabsorbed by the carbon cycle. To reach the nominal figure of 2,000 Gt of carbon will take 500 years at this rate, which in geologic terms is very rapid. Ignoring an improbable future in which we will continue to emit as much or more carbon as we do today for five more centuries, it should be obvious that we have some time to deal with the problem before things get out of hand. Aside from that, the 500 year period seems to be a good fit for the carbon “pulses” that occurred at the start of the PETM. Call this estimate the naive lower bound. But remember, to trigger the PETM scientists have concluded that much of the carbon was released as methane, not CO2.
Let us suppose that half of the carbon was released as CH4 making the mix 1,000 Gt of carbon dioxide and 1,000 Gt of methane. Using the IPCC’s Global Warming Potential (GWP) for methane in a 500 year time frame, which is 7 times the potency of CO2, we find the effective carbon dioxide level is 8,000 Gt CO2e. This level of human emissions would take mankind 2,000 years at our current rate. Moreover, the estimated fossil fuel carbon sink is only 6,000 Gt, so reaching the required total would mean finding some other source of carbon based fuels-perhaps ocean floor methane hydrates (nice bit of irony there, eh). Let’s call this result the probable lower bound given what we know from the scientific evidence.
Finally, lets do an upper bound estimate on our ability to cause a PETM type event. Using the high end estimate of 10,000 Gt of carbon and keeping half of the emissions in methane we get an new total of 40,000 Gt CO2e. At the current emissions rate this would take us 10,000 years! I see no way that this is remotely plausible given the limited amount of fossil fuel reserves and the march of technological progress, but that’s what upper bounds are for.
To restate the question, could Human CO2 emissions cause another PETM? The best answer I can give is not bloody likely. There are those who will yammer on about tipping points, but that moves from informed speculation to flat out guess work. No one knows what tipping points exists (if any) or what conditions are necessary to make them trip (see Tiptoeing Through The Tipping Points for a discussion of tipping points). Based on the best scientific knowledge available there is little danger of mankind’s puny CO2 emissions triggering another PETM, no matter what the alarmists say.
Be safe, enjoy the interglacial and stay skeptical.
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