Primeval 22 – Eocene Optimum

 

The Eocene Epoch contained a wide variety of different climate conditions that included the warmest climate in the Cenozoic Era and ended in an ice-house climate. The evolution of the Eocene climate began with warming after the end of the PETM at 56 million years ago to a maximum during the Early Eocene Climatic Optimum (EECO) at around 49 million years ago. During this period, little to no ice was present on Earth with a smaller difference in temperature from the equator to the poles. Following the maximum was a descent into an ice-house climate from the EECO to the Eocene-Oligocene transition at 34 million years ago. During this decrease ice began to reappear at the poles, and the Eocene-Oligocene transition is the period where the Antarctic ice sheet began to rapidly expand.

The Arctic Ocean During the Eocene

The Arctic Ocean During the Eocene

One of the unique features of the Eocene’s climate was the equable and homogeneous climate that existed in the early parts of the Eocene called the EECO. A great deal of evidence support the presence of a warmer equable climate being present during this period. This evidence includes the presence of fossils native to warm climates, such as crocodiles, located in the higher latitudes,(1)L.Cirbus Sloan and D. K. Rea (1995) “Atmospheric carbon-dioxide and early Eocene climate: a general circulation modeling sensitivity study.” Paleogeo. Paleoclim. Paleoeco,. Volume 119, Pages 275-292 the presence in the high-latitudes of frost-intolerant flora such as palm trees which cannot survive during sustained freezes, and fossils of snakes found in the tropics that would require much higher average temperatures to sustain them.(2)Matthew Huber (2009) “Snakes tell a torrid tale.” Nature, Volume 457, Pages 669-671 Using isotope proxies to determine ocean temperatures indicates sea surface temperatures in the tropics as high as 35°C (95°F) and, relative to present day values, bottom water temperatures that were 10°C (18°F) higher.(3)M. Huber and R. Caballero (2011) “The early Eocene equable climate problem revisited.” Climate Past Discuss, Volume 6, Pages 241-304

Modelling this environment is difficult.(4)L. Cirbus Sloan and Eric J. Barron (1990) “Equable” climates during Earth history?” Geology, Volume 18, Pages 489-492. Using all different ranges of greenhouse gasses that occurred during the early Eocene, models were unable to produce the warming that was found at the poles and the reduced seasonality that occurs with winters at the poles being substantially warmer. The models, while accurately predicting the tropics, tend to produce significantly cooler temperatures of up to 20°C (36°F) underneath the actual determined temperature at the poles. This error has been classified as the “equable climate problem”. To solve this problem, the solution would involve finding a process to warm the poles without warming the tropics.

Polar stratospheric clouds over the Arctic.

Polar Stratospheric Clouds Over the Arctic

Several theories have been proposed, however there are some facts known. Diverse geochemical and paleontological proxies indicate that at the maximum of global warmth the atmospheric carbon-dioxide values were at 700 to 900 parts per million.(5)Paul N. Pearson and Martin R. Palmer (2000) “Atmospheric carbon-dioxide concentrations over the past 60 million years” Nature, Volume 406, Pages 695-699 For comparison the current atmospheric carbon-dioxide levels are at 400 ppm.(6)David Shukman (May 10, 2013) “Carbon-dioxide Passes Symbolic Mark” BBC News Science & Environment Prior to the Industrial Revolution carbon-dioxide levels are believed to have been at 280 ppm.(7)NOAA (May 10, 2013) “Carbon Dioxide at NOAA’s Mauna Loa Observatory reaches new milestone: Tops 400 ppm” NOAA Earth System Research Laboratory Methane was another greenhouse gas that had a drastic effect on the climate. In comparison to carbon-dioxide, methane has a much higher impact on temperature, as methane has around 34 times more effect as a greenhouse gas per molecule than carbon-dioxide on a 100-year scale.(8)G. Myhre et al. (2013) “Anthropogenic and Natural Radiative Forcing” In T. F. Stocker et al. editors Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change The majority of the methane released to the atmosphere during this period would have been from wetlands, swamps, and forests.(9)L. Cirbus Sloan et al. (1992) Possible methane-induced polar warming in the early Eocene. Nature, Volume 357, Page 1129-1131 Due to the warmer climate and sea level rise associated with the early Eocene, more wetlands, forests, other environmental sources would have been available for methane release. Comparing the early Eocene production of methane to current levels of atmospheric methane, the early Eocene would be able to produce triple the amount of current methane production.

Nevertheless, no functional models have yet been simulated that can explain the Eocene Optimum. Another theory that does help explain what was happening, but not why, is the theory that the Arctic and Antarctic had constant polar stratospheric clouds.(10)L. Cirbus Sloan and D. Pollard (1998) “Polar stratospheric clouds: A high latitude warming mechanism in an ancient greenhouse world” Geophysical Research Letters, Volume 25, Pages 3517-3520 Polar stratospheric clouds are clouds that occur in the lower stratosphere at very low temperatures. Polar stratospheric clouds have a great impact on radiative forcing. Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and traps outgoing longwave radiation. Different types of polar stratospheric clouds occur in the atmosphere: polar stratospheric clouds that are created due to interactions with nitric or sulfuric acid and water (Type I) or polar stratospheric clouds that are created with only water ice (Type II). Methane would have been an important factor in the creation of the primary Type II polar stratospheric clouds in the early Eocene. Since water vapor is the only supporting substance used in Type II polar stratospheric clouds, the presence of water vapor in the lower stratosphere is necessary, however in modern times the presence of water vapor in the lower stratosphere is rare. When methane is oxidized, a significant amount of water vapor is released. Another requirement for polar stratospheric clouds is cold temperatures to ensure condensation and cloud production. Polar stratospheric cloud production, since it requires the cold temperatures, is usually limited to nighttime and winter conditions. With this combination of wetter and colder conditions in the lower stratosphere, polar stratospheric clouds could have formed over wide areas in Polar Regions.

65 Million Years of Climate Change

65 Million Years of Climate Change

To test the polar stratospheric clouds effects on the Eocene climate, models were run comparing the effects of polar stratospheric clouds at the poles to an increase in atmospheric carbon-dioxide. The polar stratospheric clouds had a warming effect on the poles, increasing temperatures by up to 20°C in the winter months. A multitude of feedbacks also occurred in the models due to the polar stratospheric clouds’ presence. Any ice growth was slowed immensely and ice that did form would melt quickly. Only the poles were affected with the change in temperature and the tropics were unaffected. Due to the warming of the troposphere from the increased greenhouse effect of the polar stratospheric clouds, the stratosphere would cool and would potentially increase the amount of polar stratospheric clouds.

While the polar stratospheric clouds could explain the reduction of the global temperature gradient and the increased temperatures at the poles during the early Eocene, there are a few drawbacks to the idea of maintaining polar stratospheric clouds for an extended period. Separate model runs were used to determine the sustainability of the polar stratospheric clouds.(11)D. B. Kirk-Davidoff and J. F. Lamarque (2008) “Maintenance of polar stratospheric clouds in a moist stratosphere” Climate of the Past, Volume 4, Pages 69-78 Methane would need to be continually released and sustained to maintain the lower stratospheric water vapor. Increasing amounts of ice and condensation nuclei would be need to be high for the polar stratospheric cloud to sustain itself.

Therefore a method could have existed to maintain a globally warm environment, however it is difficult to envision as a naturally occurring phenomena. If in fact the PETM was artificially induced either by either surviving dinosauroids or extraterrestrials, then it is likely that the following Eocene Thermal Maximums were induced by the same species, and ultimately the EECO could have been an attempt to create a stable long term biosphere that was artificially warmed via the constant production of methane.

References   [ + ]

1. L.Cirbus Sloan and D. K. Rea (1995) “Atmospheric carbon-dioxide and early Eocene climate: a general circulation modeling sensitivity study.” Paleogeo. Paleoclim. Paleoeco,. Volume 119, Pages 275-292
2. Matthew Huber (2009) “Snakes tell a torrid tale.” Nature, Volume 457, Pages 669-671
3. M. Huber and R. Caballero (2011) “The early Eocene equable climate problem revisited.” Climate Past Discuss, Volume 6, Pages 241-304
4. L. Cirbus Sloan and Eric J. Barron (1990) “Equable” climates during Earth history?” Geology, Volume 18, Pages 489-492.
5. Paul N. Pearson and Martin R. Palmer (2000) “Atmospheric carbon-dioxide concentrations over the past 60 million years” Nature, Volume 406, Pages 695-699
6. David Shukman (May 10, 2013) “Carbon-dioxide Passes Symbolic Mark” BBC News Science & Environment
7. NOAA (May 10, 2013) “Carbon Dioxide at NOAA’s Mauna Loa Observatory reaches new milestone: Tops 400 ppm” NOAA Earth System Research Laboratory
8. G. Myhre et al. (2013) “Anthropogenic and Natural Radiative Forcing” In T. F. Stocker et al. editors Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
9. L. Cirbus Sloan et al. (1992) Possible methane-induced polar warming in the early Eocene. Nature, Volume 357, Page 1129-1131
10. L. Cirbus Sloan and D. Pollard (1998) “Polar stratospheric clouds: A high latitude warming mechanism in an ancient greenhouse world” Geophysical Research Letters, Volume 25, Pages 3517-3520
11. D. B. Kirk-Davidoff and J. F. Lamarque (2008) “Maintenance of polar stratospheric clouds in a moist stratosphere” Climate of the Past, Volume 4, Pages 69-78