Pliocene


The warmer climate facilitates hurricane activity. This amounts to a positive feedback, which can potentially lead to multiple climate states – one with permanent El Nino-like conditions and strong hurricane activity and the other corresponding to modern climate with a cold equatorial Pacific.” Fedorov, et al., 2010.
The Pliocene epoch spans 3 million years of Earth’s history between 5.4 and 2.4 million years ago. In the early Pliocene our earliest pre-human ancestors discovered so far Ardipithecus ramidus appeared. And the first humans Homo habilis and possibly even Homo erectus were around at the end of the Pliocene for the transition into the ice ages of the Pleistocene. While Homo habilis died out long before the appearance of Homo sapiens sapiens (us) Homo erectus was still around by the time we evolved.
At the beginning of this epoch, the Mediterranean Sea evaporated and remained dry for about 170,000 years. The African continental plate had been colliding with the European continental plate since about 85 million year ago when the ancient Mediterranean was still the Tethys Ocean. Five million years ago the African continental plate slid under Spain uplifting it and causing the Mediterranean to be cut off from the Atlantic Ocean [Govers, 2009]. Since the Mediterranean Sea loses more water to evaporation than is supplied by all the rivers which feed into it, in a few tens of years it had virtually dried up forming a deep hole some 3 miles below sea level at its deepest point. The average depth of the Mediterranean is about 1 mile. Imagine some pre-human following the edge of the Mediterranean Sea and all of its bounty a mile or so below sea level, perhaps a successful strategy for tens of thousands of years and thousands of generations. When the Atlantic finally breached the Gibraltar dam the flooding must have been dramatic catching millions of animals unaware.
But the most interesting thing about the Pliocene is its climate. It turns out that the Pliocene epoch is the best analog for the current Earth climate of all the 4.55 billion year history of our planet. The sun’s luminosity was nearly the same as it is today, the atmospheric carbon dioxide level was between 300 and 400 parts per million by Volume (ppmV), and the continents were in approximately the same location. We have discussed the faint young sun in a previous article [climate factors]. And if you recall, our sun has been steadily increasing in luminosity as the original hydrogen in its core has been fusing into helium. Because of this, during the Pliocene the solar forcing may have been about 0.2 W/m2 less than today or about the same as during the little ice age [Wang, 2005, Krivova, 2007]. Despite the slightly cooler sun, the early Pliocene was 4oC warmer than today and the mid Pliocene was about 2oC warmer. The carbon dioxide forcing had to account for the warm climate and the slightly cooler sun. This is an enigma since the current estimate for the equilibrium climate sensitivity, or the amount that the temperature would increase with a doubling of atmospheric carbon dioxide, i.e, to about 560 ppmV is about 3oC. Atmospheric carbon dioxide levels today are about 390 ppmV, or at the upper end of the Pliocene values, yet the Pliocene climate was hotter than the accepted equilibrium climate sensitivity would predict.
While the continents were nearly in the positions they are in today, there were some differences. The difference which probably affected the climate the most is that the Isthmus of Panama between North and South America did not close the connection between the Pacific and the Atlantic Oceans, the Central American Seaway, until about 3 million years ago [Murdock, 1997]. This closure impacted ocean circulation of heat.
Recently a new paper suggests another feedback mechanism associated with increased warmth in the early Pliocene. Fedorov et al propose that increased hurricane activity contributed to the warm climate as part of a positive feedback mechanism that maintained the warmth with permanent El Nino-like conditions [Fedorov, 2010]. In a review article, Ryan Sriver writes “These results may provide clues to understanding not only the climate of the early Pliocene, but also the nature of future climate change in a greenhouse world.” [Sriver, 2010]
Perhaps the most important difference, not discussed in the Fedorov paper, is that the Earth’s climate had been gradually cooling since the hot house Eocene 50 million years ago whereas our climate is recovering from an ice age. The last glacial maximum was only 20,000 years ago. I asked Kerry Emanuel of MIT and a co-author of the Fedorov paper about this and he replied to me: “Although CO2 levels were similar then to today’s, that climate had plenty of time to equilibrate to the forcing whereas ours clearly has not. It is also plausible that the climate exhibits hysteresis and multiple equilibria, so that approaching 370 ppm of CO2 from a warmer state may yield a different climate than approaching it from a colder state.”
James Hansen of NASA GISS, has suggested that the accepted value for the sensitivity of the Earth’s climate only accounts for fast feedbacks such as increasing water vapor and not very slow feedbacks. Hansen suggests that equilibrium climate sensitivity might be closer to 6oC [Hansen, 2008] when slow feedbacks are accounted for. A related aspect is that most of the trapped energy is currently warming the oceans as shown in figure 1 and it takes a very long time for these bodies of water to warm up or cool down [Murphy, 2009].

"ocean heat content"

Figure 1: Total Earth Heat Content anomaly from 1950 (Murphy 2009). Ocean data taken from Domingues et al 2008. Land + Atmosphere includes the heat absorbed to melt ice.

What we can appreciate from a study of the Pliocene climate is that equilibrium climate sensitivity may be higher than the consensus view and we may see an unexpected increase once the oceans warm up or equilibrate to the new higher level of carbon dioxide and further that the climate may change states from the current state where we experience an El Nino event every 3-8 years to a permanent El Nino state which may be self sustaining.
To view maps of the locations of continents in the Earth’s past see Chirstopher R. Scotese’s fascinating web site

.
Tony Noerpel

references
Fedorov, A. V., Brierley, C. M., and Emanuel, K., Tropical cyclones and permanent El Nino in the early Pliocene epoch, Nature, Vol. 463, February 25, 2010, 1066-1070.
Govers et al. Choking the Mediterranean to dehydration: The Messinian salinity crisis. Geology, 2009; 37 (2): 167 DOI: 10.1130/G25141A.1
Murdock, T. Q., A. J. Weaver, and A. F. Fanning (1997), Paleoclimatic response of the closing of the Isthmus of Panama in a coupled ocean-atmosphere model, Geophys. Res. Lett., 24(3), 253–256.
Wang, Y.-M., J. L. Lean, J. L., and Sheeley, N. R. Jr , Modeling the sun’s magnetic field and irradiance since 1713, The Astrophysical Journal, 625:522–538, May 20, 2005
Krivova, N. A., Balmaceda, L., and Solanki, S. K., Reconstruction of solar total irradiance since 1700 from the surface magnetic flux, Astronomy and Astrophysics, Volume 467, Number 1, May III 2007, 335 – 346.
Hansen, J., Sato, M., Kharechal, P., Beerling, D., Berner, R., Masson-Delmotte, V., Pagani, M., Raymo, M., Royer, D. L., and Zachos, J. C., Target Atmospheric CO2: Where Should Humanity Aim?, The Open Atmospheric Science Journal, 2008, 2, 217-231.
Murphy, D. M., S. Solomon, R. W. Portmann, K. H. Rosenlof, P. M. Forster, and T. Wong (2009), An observationally based energy balance for the Earth since 1950, J. Geophys. Res., 114, D17107, doi:10.1029/2009JD012105.
Sriver, R., Tropical cyclones in the mix,” Nature, Vol 463, 25 Februrary, 2010, 1032-1033.
http://www.scotese.com/
http://www.archaeologyinfo.com/ardipithecusramidus.htm..
http://www.archaeologyinfo.com/homohabilis.htm
http://www.esd.ornl.gov/projects/qen/pliocene.html
When the Mediterranean Sea dried up lat Miocene
http://www.sciencedaily.com/releases/2009/02/090211122529.htm
early hominids
http://anthro.palomar.edu/hominid/australo_1.htm

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