Natural Science Forum / Biology / Paleontology / June 2006

On Pliocene Paradox

The following points are made by A.V. Fedorov et al (Science 2006 312:1485):

1) The early Pliocene (5 to 3 million years ago) was similar to and also very different from the world of today. The intensity of sunlight incident on Earth, the global geography, and the atmospheric concentration of carbon dioxide were close to what they are today, but surface temperatures in polar regions were so much higher that continental glaciers were absent from the Northern Hemisphere, and sea level was ~25 m higher than today. This apparent paradox -- that conditions today and those during the early Pliocene are two different climate states in response to practically the same external forcing -- can be addressed with information from such sources as the Pliocene Research Interpretations and Synoptic Mapping Project (PRISM). Although there are some inconsistencies in the information concerning tropical ocean temperatures, most of the discrepancies can be eliminated by considering Pliocene observations in the broader context of earlier and subsequent climate changes, rather than in isolation.

2) Over the past 65 million years, since the beginning of the Cenozoic when temperatures in polar regions were in the neighborhood of 10°C, Earth experienced erratic global cooling. This was a consequence primarily of the drifting of the continents, accompanied by changes in ocean-basin geometry, mountain building, volcanic eruptions, and other phenomena that affect the two factors that determine globally averaged surface temperatures: the albedo of the planet and the atmospheric concentration of greenhouse gases. Superimposed on the global cooling were periodic climate cycles in response to "Milankovitch forcing". This term refers to modest, periodic variations in the distribution of sunlight because of periodic variations in orbital parameters such as the angle of tilt (obliquity) of Earth's axis.

3) The response to Milankovitch forcing involves not only glaciers that wax and wane, but also tropical sea surface temperatures (SSTs) that rise and fall. These phenomena could be separate to some degree, each involving its own physical processes and feedbacks, in the same way that the response to seasonal variations in sunlight involves several different phenomena such as the Indian monsoons, coastal upwelling off southwest Africa, and severe winter storms in central Canada. In other words, an explanation for the monsoons does not explain the other features of the seasonal cycle. It is similarly unlikely that processes involving ice sheets alone can explain all aspects of the response to Milankovitch forcing. The important additional processes are likely to include biogeochemical cycles such as the carbon cycle, as well as tropical ocean-atmosphere interactions.

4) Although Milankovitch forcing has been relatively constant over the past several million years, the amplitude of the climatic response underwent remarkable changes as the long-term global cooling introduced different climate feedbacks. The Pliocene is of special interest because the feedbacks that came into play during that epoch, around 3 million years ago (Ma), started an amplification of the response of climate to orbital forcing. Over the past ~1 million years, this process culminated in drastic oscillations between prolonged glaciations, or ice ages, and brief, warm interglacials. During the warm interglacial periods -- including the current one, which began some 10,000 years ago -- conditions approach those of the early Pliocene. Will the present warm conditions terminate soon, to be followed by the next ice age? Or will the onset of the next ice age be inhibited by the current rise in the atmospheric concentration of greenhouse gases induced by humans? Will that rise restore the warm conditions of the early Pliocene? Answers to these questions require identification of the processes that maintained warm conditions in the early Pliocene and then terminated them.

Source: ScienceWeek
http://scienceweek.com/2006/sw060623-2.htm