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  • 08-Aug-09 What Caused the PETM (Paleocene-Eocene Temperature Maximum)
  • SEPP Science Editorial #24-2009
    (in TWTW Aug 8, 2009)

    S. Fred Singer, Chairman and President , Science and Environmental Policy Project (SEPP)

    What Caused the PETM (Paleocene-Eocene Temperature Maximum)

    Aug 8, 2009

    One of the striking features of the thermal history of the earth is the unusually rapid and strong warming about 55 million years ago, termed the PETM. It was recently again discussed in a paper by Zebee et al in Nature Geoscience online: 13 July 2009 | doi:10.1038/ngeo578

    The paper brought great joy and jubilation to both climate skeptics and climate alarmists. Skeptics latched on to the authors' statement that GH models could not explain the rapid temperature rise in relation to the observed rise of CO2. Alarmists, on the other hand, warned that such rapid and strong temperature excursions might even be possible today unless we restrain CO2 emissions.

    Of course, it is difficult to be certain about the direction of cause-effect from a correlation of temperature and CO2, since the data lack adequate time resolution. It might therefore be appropriate to develop a different hypothesis, which happens to make use of two papers I already published (in 1971 and 1988). Many authors seem to accept that the cause of the temperature rise was the rapid release of methane trapped in clathrates in ocean sediments, which then was oxidized to CO2. The problem with this simple idea is there may not be sufficient oxygen, particularly in the deep ocean, to accomplish this chemical transformation. This will be particularly true if large bursts of methane are released in bubbles that travel rapidly to the sea surface.

    Once in the atmosphere, methane released in these large quantities could survive for a long time, simply by depleting the available hydroxyl (OH) radicals, which exist only in minute concentrations in the steady state. As a consequence, not only would this methane exert a strong GH effect, but large amounts of methane could percolate into the stratosphere, and there be photolyzed by solar ultraviolet radiation to eventually form both water vapor and CO2, and contribute to destruction of ozone ("Stratospheric Water Vapour Increase Due to Human Activities." Nature 233:543-547. 1971).

    These large amounts of water vapor released into the normally dry stratosphere can lead to important consequences, including the formation of cirrus clouds (consisting of ice particles) in the vicinity of the cold tropopause. Tabulated physical measurements give us the "complex refractive index" of water and ice. Therefore, a direct calculation based on Mie theory can provide the optical properties of the cirrus cloud cover (Re-Analysis of the Nuclear Winter Phenomenon. Meteorology and Atmospheric Physics 38:228- 239. 1988).

    If the cloud cover is very thick, it could exhibit an appreciable optical albedo. But my analysis shows that as the cloud thins, it retains a large infrared opacity, sufficient to cut off any thermal radiation from the earth's surface in the IR window of the atmosphere (8-12 microns). Such a GH effect is quite powerful for warming the global climate; it depends, of course, also on the areal coverage of the cirrus cloud. It might be strong enough to enhance the warming of the earth and therefore accelerate a further release of methane from the ocean, a kind of positive feedback that could explain the observed large temperature increase. But so far all of this is simply hypothesis and speculation. Some obvious questions remain:

  • How to test this hypothesis? One would expect to find some evidence concerning anoxic effects in the ocean, including a die-off of marine organisms. The CO2 increase observed could partly be caused by a degassing of a warming ocean.
  • And could such an effect happen now? Not likely. We have to remember that temperatures near the P-E boundary had been unusually high for long periods of time. In fact, the earth was completely ice-free, including also the polar regions. This is quite different from the present situation. Further, nothing of the sort has happened during the much warmer (compared to today) Holocene Temperature Optimum, 8000-5000 years ago.

    View The Week That Was in which this editorial appeared.

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