The Paleocene-Eocene Temperature Maximum (PETM)

Using a variety of temperature proxies, including the ratio of the different stable isotopes of carbon, ¹³C and ¹²C, where higher temperatures favor the lighter isotope, ¹²C and one would expect to see a negative δ¹³C. Another indicator is the negative δ¹⁸O > 1% present in foraminifera shells that grew both in the surface and deep oceanic waters that occurred during this same time period. Likewise, analyses of the foraminifera Mg/Ca ratio and ratios of particular organic compounds used for TEX₈₆^1,2, a paleothermometer based on the composition of marine picoplankton, Crenarchaeota³. There is clear evidence of a massive injection of ¹³C-depleted carbon at the Paleocene-Eocene transition. First, the previously mentioned negative δ¹³C change in carbon compounds at this time, about 55 million years ago. Second, carbonate dissolution which indicates the acidification of the ocean that occurs when atmospheric CO₂ increases. The source of this carbon dioxide is unknown. Massive volcanic activity might have been to blame, but there is no evidence of sufficiently large volcanic activity such as the ash and lava deposits one would expect. There are volcanic traps in Siberia that were formed about this time, but there that activity alone would not account for the volume necessary to account for the likely CO₂ content in the atmosphere of over 1700 ppm (parts per million) at its peak. This is based on a base value of about 1000 ppm near the end of the Paleocene and an addition of about 3,000 GtC (gigatonnes of carbon). This amount of carbon was determined by taking into account the carbonate compensation depth (CCD), a value determined by the carbonate depletion of shallow ocean bottom cores.⁴ A comet or asteroid impact that caused continent-wide fires was considered and rejected because there is no layer of soot that has been found in ocean bottom cores as would be expected. Nor has a crater been found of the right age to account for this. A third possibility is that the large deposits of methane hydrate that exist on the deep continental shelves were destabilized and released. Methane hydrate is a single molecule of methane, CH₄, enclosed in a "cage" of water molecules, H₂O. This compound is only stable in a narrow range of temperatures and pressures. It would only have taken a small change in temperature or a physical disturbance of the water surrounding the methane hydrate to cause it to become unstable and release the CH₄. Methane is 20X as effective as CO₂ at retaining heat, so with a large release of methane, there would have been an immediate rapid surge in temperature. Methane decays over a period of about ten years to carbon dioxide which would then linger for over one hundred years, heating the atmosphere more slowly for a longer period of time. The total temperature of the ocean water, both shallow and deep was about 9^oF,reaching 77^oF in the surface waters near the poles.  There was virtually no ice anywhere on Earth at this time since the temperature had already been warm prior to the PETM.  The poles warmed more than the rest of the planet because with the lack of snow, the albedo increased, causing a larger temperature excursion.¹

The transition from the Paleocene epoch to the Eocene epoch Is marked by sharp decrease in the percentage of ¹³C, δ¹³C, dissolution of carbonate in all the ocean basins, the extinction of many terrestrial mammals, and foraminifera in the mud on the ocean floor (i. e., benthic).  It was also marked by the origin of several new mammalian orders, including primates, artiodactyls (even-toed ungulates), and perissodactyls (odd-toed ungulates). The δ¹³C and dissolution of carbonate indicate that the ocean experienced a geologically rapid acidification and temperature rise. The acidification strongly implies a rapid rise in the amount of carbon in the atmosphere. This increase occurred over a period of about 20,000 years, a rate that is about one-tenth of the rate of increase of CO₂ that we have experienced during the past 150 years.
Humans are currently adding about 30 GtC annually. At this rate, anthropogenic carbon additions will equal what was released at the P-E boundary in 100 - 200 years, some 100 times faster than occurred 55 million years ago.^1,5 because we have geologic evidence of what the climate was like during this P-E boundary time period, we have an idea of what the climate will be like in our near future if we continue on our current path. Periods of 100^o temperatures in the North American Southwest will last for months around the clock. Drought will be longer and more frequent than now. Flooding will be worse in those areas that commonly flood. All the glaciers will melt, raising the sea level by about 200 feet - it was 220 feet higher at the P-E Boundary than it is now. Since there are vast stores of methane under the permafrost, if the permafrost melts, and that now seems inevitable, the released methane will just exacerbate the problem and hasten the warming. There are also large deposits of methane hydrate on the continental shelves that could be released if the ocean temperature exceeds the threshold.
Only broad features of the climate during the 150,000 year transition from the Paleocene to the Eocene until CO₂ levels returtned to the levels of the late Paleocene after this large carbon dioxide excursion. One can hope that by further study, a more detailed understanding will be gained along with indications of how this scenario might be mitigated

1. National Geographic, October 2011, pp. 90 – 109
2. http://en.wikipedia.org/wiki/Paleocene%E2%80%93Eocene_Thermal_Maximum
3. http://en.wikipedia.org/wiki/TEX86
4. http://www.realclimate.org/index.php/archives/2009/08/petm-weirdness/
5. http://www.realclimate.org/index.php/archives/2004/12/how-do-we-know-that-recent-cosub2sub-increases-are-due-to-human-activities-updated/