(From The Resilient Earth / By Doug L. Hoffman) — As the Cenozoic progressed a cooling trend set in leading up to the formation of permanent ice caps and the Pleistocene Ice Age we are still experiencing. But before the world started to ice up our planet underwent one of the most dramatic bouts of global warming known to science-the Paleocene Eocene Thermal Maximum or PETM. Recently, global warming activists have tried to liken human CO2 emissions to the cause of the PETM, 55 million years ago. Is it true, that our actions may trigger a sudden sharp rise in global temperature?
The mid-Cretaceous (~125-85 million years ago) and the early Paleogene are among the best known ancient “greenhouse” climate intervals-times when Earth’s average temperature was significantly higher than they are today. During the Cenozoic (the last 65 million years) the global climate has cooled substantially, up to a main cooling step at the Eocene-Oligocene transition (~34 mya), which included the development of the first glaciation at a continental scale in Antarctica. Another main cooling step occurred in middle Miocene (14 mya) and was a significant step in the development of the Antarctic continental glaciation.
In addition, these intervals contain some of the most abrupt and transient climatic changes in the geologic record, including the Paleocene-Eocene Thermal Maximum (PETM), the mid-Maastrichtian deepwater event (MME), and the early Aptian Oceanic Anoxic Event (OAE1a). For more information on these and other similar events see the detailed write up of Ocean Drilling Program (ODP) Leg 198 on Shatsky Rise. These sudden shifts in Earth’s ecology and climate involved dramatically modified oceanic circulation patterns, profound changes in geochemical cycling, abrupt changes in marine life and the emergence of new animal species on land. The event from this time which is most well known is the PETM, also known as the Late Paleocene Thermal Maximum (LPTM), and it is the main subject of this posting.
According to the Deep Earth Academy’s The PETM in Review, the PETM was likely triggered by the rapid emission of greenhouse gases, probably methane (CH4), which was rapidly oxidized to carbon dioxide (CO2). The result of this sudden increase in atmospheric greenhouse gases was global warming. The consequences of the PETM were significant in magnitude and truly global in scope:
Global warming; atmospheric temperatures warmed by 5°-9°C globally (6°-9°C warming of southern high latitude sea surface temperatures, 4°-5°C warming of the deep-sea, tropical sea surface temperatures, and Arctic Ocean, and ~5°C warming mid-latitude continental interiors).
Perhaps the most staggering result was that at times during the early Eocene warm episode the Arctic sea surface temperature soared to 24°C. The evidence suggests that the PETM marked possibly the warmest time at the North Pole for over 100 million years-certainly it has not been as warm since. Today’s circum-polar ecosystems could not exist in such a climate regimen.
Ocean acidification (the carbonate compensation depth [CCD] rapidly shoaled by more than 2 km [<10,000 years] and recovered gradually (>100,000 years)).
Sudden onset of anoxic conditions in deep ocean waters.
Increased intensity of the hydrologic cycle and erosion rates (based in part on changes in clay mineral assemblages).
Major extinctions of benthic foraminifera in the deep-sea (30-50% of species). Turnover and evolution of calcareous plankton (calcareous nannofossils and planktic foraminifers).
Migration of terrestrial organisms to the high latitudes.
Turnover and evolution of terrestrial animals and plants. New mammal lineages first appear in the earliest Eocene, including the earliest horse in North America.
A number of different possible causes for the PETM have been proposed by scientists. These include massive continuous volcanic eruptions, world wide outbreak of forest fires, sudden reversals of circulating ocean currents and the release of deep sea deposits of methane held in methane-ice compounds known as clathrates.
Several investigators have suggested that early Cenozoic global warming would have altered deep-ocean circulation patterns by reducing the density of surface waters at high latitudes (Kennett and Shackleton, 1976; Wright and Miller, 1993; Zachos et al., 1993). This, in turn, would permit increased downwelling of highly saline but warmer waters in subtropical oceans. Such reversals or switches in circulation probably occurred suddenly rather than gradually. In fact, it has been suggested that a sudden change in intermediate-water circulation patterns may have occurred just prior to the PETM, possibly triggering the dissociation of clathrates (Karen L. Bice and Jochem Marotzke, 2002).