From an initially warm climate, with no ice sheets in either Antarctica or Greenland, by the end of the era the planet had entered a cold phase, with glaciers cyclically covering vast continental areas with thick layers of ice during the last 2 million years of the Cenozoic. This cooling was accompanied (both as a cause and an effect) by an almost continuous loss in atmospheric CO2, which dropped from a concentration of around 2,000 ppm at the beginning of the era to just under 300 ppm during the last million years (Pagani, 2005).

This drop in mean temperature levels did not occur steadily, in a uniform manner. Rather, there were periods in which the temperatures actually rose and the ice sheets retreated. Furthermore, a number of very short events (or climatic aberrations, if you like) also occurred in which temperatures reached all-time highs and lows. Three such events can be distinguished: an intense warming period occurring 55 million years ago; a cooling period occurring 34 million years ago; and another cooling phase, this time occurring 23 million years ago (Zachos, 2001).

Figure 18. The general cooling of the Cenozoic (Tertiary and Quaternary).

...

The first epoch of the Tertiary, the Palaeocene (65 Ma-54 Ma), enjoyed a climate very similar in nature to that of some of the warmer Cretaceous periods.

Crocodiles and turtles inhabited the arctic latitudes and palm trees grew on the Kamchatka peninsula. The Arctic Ocean was smaller than it is now and had a more precarious link with the Atlantic. Its waters were much shallower, less salty and much warmer. Seawater was several degrees warmer than today, both on the surface and in the depths.

The habitat of the subtropical plankton of the Atlantic stretched 15º of latitude further north than it does today and corals occupied a much wider tropical band than at present. Ocean currents and the thermohaline circulation were also different (Diekmann, 2004; Thomas, 2004).

A number of different factors may have contributed to this warm climate at the beginning of the Tertiary:

1. a) A more zonal atmospheric circulation (stronger westerly winds around the Arctic).
   

Models recreating the early Tertiary climate attribute the warmth of higher latitudes to a reinforcement of westerly winds around a closed Arctic Ocean, where, unlike today, there was a permanent band of low pressure.  The strong westerly winds would have increased the movement of warm, humid air masses from the Pacific and Atlantic towards North America and Eurasia (respectively) and would have heated the inland areas of the continent (Sewell, 2001).

Furthermore, the low pressure of the Arctic region would have triggered a circulation of surface currents in the Arctic ocean which would have weakened what is now known as the Beaufort Gyre and hindered the formation of ice. Thus, during this first part of the Tertiary period, the AO (Arctic Oscillation) index would have been extremely positive (due to the intense low pressure of the Arctic), which would in turn have fostered strong westerly winds. The AO index depends on pressure differences between Arctic lows and the subtropical highs of the northern hemisphere (the AO index is sometimes called the NAM - the Northern Hemisphere Annular Mode) (Wallace, 2002; Thompson, 2001).

The Arctic Ocean was too closed off to benefit from a direct influx of oceanic heat from the tropics. There was a link between the large Tethys Sea and the Arctic, through a shallow sea located in Western Siberia, as well as through the strait dividing Greenland and Scandinavia, but it was only a very small one. This made it difficult for ocean currents to transport much tropical heat towards the mid-level and higher latitudes.

b) High concentrations of carbon dioxide and methane.

Another possible cause of the warming trend is the high concentration of CO2. The existence of a high concentration of atmospheric CO2 at the beginning of the Eocene, which according to some authors was over 2,000 ppm, may have been the result of an intense period of magmatic degasification, in turn caused by a higher-than-usual level of tectonic plate movement during that era, particularly in the Atlantic opening between Greenland, Iceland and Norway, as well as in Alaska and the Asian region, where the northern edge of the Indian plate sunk below the southern edge of the Asian one (Pearson, 2000; Storey, 2007).

A more recent theory suggests that the high concentration of CO2 was the result of a series of giant fires which, in the warm, dry climate of the period, may have ravaged the peat bogs which then covered vast areas of land (Kurtz, 2003). The event would have been similar, although on a much larger scale, to that which occurred in Indonesia in 1997, which significantly increased CO2 levels during that year.

The rise in carbon dioxide may also have been caused by the oxidation of large quantities of methane escaping from the marine subsoil, following an initial warming of the waters located near the seabed. The low carbon-13 levels found in the sediments dating from this time seem to corroborate this theory. On the coast of Norway, researchers have recently found a series of geological structures dating from this period which seem to correspond to underwater cracks through which methane trapped in the form of frozen hydrates would have escaped when the clathrates defrosted (Svensen, 2004).

Once in the atmosphere, the methane would have oxidised and been converted into CO2 and water vapour, thus doubling or even tripling the concentration of atmospheric carbon dioxide (Zachos, 2003). When dissolved in water, this carbon dioxide would have altered the oceans’ carbon chemistry. This dissolution acidified the  seawater and lowered the concentrations of carbonate ion. To compensate, the lysocline rose and a new dissolution of marine calcite occurred, as evident in ocean sediments dating from that period (Zachos, 2005).

Another concordant hypothesis is that tectonic subduction movements in the Arctic may have caused sediments rich in organic matter to sink to the depths of the ocean, thus creating methane which escaped into the air through vents in the strata (Clift, 2002).

c) More stratospheric clouds

Finally, some experts speculate about the existence of a vast, dense blanket of stratospheric clouds which acted as a barrier trapping tropospheric heat underneath. Researchers from the University of Santa Cruz have alluded to this factor as a possible reason for the warm climate of high latitudes during the early Eocene. The greenhouse effect of the clouds would have triggered a relative warming of the polar regions, slowing up the formation of continental and sea ice, decreasing the albedo and, finally, contributing to the creation of a warmer, more humid climate across the whole globe (Sloan, 1998).

These clouds would have formed in the stratosphere, at high latitudes and at a height of around 15 kilometres, where temperatures are very low. They would have produced a greenhouse effect since, while being translucent to solar radiation, they are nevertheless fairly opaque to infrared terrestrial radiation, absorbing it and then sending it down once again to the surface. Two conditions must be met in order for polar stratospheric clouds to form: the air temperature must be very low (below -75º C) and the humidity level sufficiently high. This latter condition is by no means easy to achieve, since the stratosphere is generally very dry. This is because tropospheric water vapour hardly ever penetrates that far, since before reaching the tropopause (the frontier between the troposphere and the stratosphere) it condenses into water droplets or ice crystals and falls as rain or snow. However, tropospheric methane does reach the stratosphere. As a result, the majority of stratospheric water in fact comes from the oxidation of methane. Therefore, the high levels of methane production during the Eocene epoch, part of which rose up to the stratosphere and provided moisture through oxidation, would explain the abundance of stratospheric clouds. It is also possible that changes in the general circulation, prompted by an increase in CO2, further boosted the accumulation of water vapour in the stratosphere (Kirk-Davidoff, 2002).

Palaeocene-Eocene Thermal Maximum (PETM)

Right on the boundary between the Palaeocene and the Eocene, around 55 million years ago, the temperature rose even higher, prompting a short temperature peak known as the Palaeocene-Eocene Thermal Maximum.

It was a sudden global warming event which only lasted around 80,000 years, but nevertheless it had an enormous influence on the evolution of animal life. The episode coincided with a major wave of extinctions among the existing fauna, both on the continents and in the oceans, and is coincident with the emergence of many new mammalian orders which have dominated the animal kingdom ever since.  Flora adapted by changing the physiognomy of their leaves and by migrating to higher latitudes (Wing, 2005).

Continental temperatures, already high, rose again by between 5º C and 7º C. In the seas, the temperature of coastal surface waters in the Antarctic rose from 13º C to 20º C, and in the Arctic, they reached as high as 24º C. Although the waters of subtropical regions also became warmer, the effect was much more noticeable in the higher latitudes.

Figure 19. Map of the Palaeocene and beginning of the Eocene, around 55 million years ago (Brinkhuis, 2006).

Deep water temperatures also rose (as during the warm mid-period of the Cretaceous) to around 12º C higher than the current day mean (Lear, 2000). This was probably due to a change in the principal location at which deep waters were formed, which moved from the cold seas of the southern hemisphere to the warmer ones of the northern hemisphere. Carbon-13 analyses of sediments provide evidence pointing to this abrupt circulatory change (Nunes, 2006).

It is believed that the PETM peak may have been caused by a sudden increase in methane or carbon dioxide. The most reliable evidence of this sudden increase in methane seems to lie in an abrupt high-low oscillation of sedimentary carbon-13, since methane, due to its biological origin, is very poor in this isotope.

The sudden release of methane into the atmosphere would have come from the methane enclosed in ice crystals located in the sediments of the ocean floor. The eruption of the gas may have occurred after the temperature of oceans' deep waters passed a specific heat threshold, thus enabling the defrosting of methane hydrates. It is possible that a change in ocean circulation triggered this process (Tripati, 2005).

Nevertheless, the abundance of methane may also have been the result of intense bacterial production in either the wetlands that covered vast areas of tropical and mid-level regions during that period or the peat bogs which formed in higher latitudes. However, the suddenness of the episode seems to support the theory of the fusion of hydrates frozen in the marine subsoil (Bains, 1999; Katz, 2000).

Initial Eocene Thermal Maximum (IETM)

Following the temperature peak at the end of the Palaeocene, the temperature dropped, although it remained high throughout the whole first part of the Eocene, until around 50 million years ago. Particularly striking is the situation of the Arctic, which remained free of ice and enjoyed much milder winters than today. Recent studies carried out by the ACEX (Arctic Coring Expedition) project indicate the existence of sedimentary microfossils near the North Pole which are typical of waters with a temperature of 20º C (Moran, 2006).

  (prev)                              (back to contents)                              (next)