Why early Earth was no snowball: Illuminating the ”faint young Sun paradox”

12/17/2012 - In the early history of planet Earth, the Sun was up to 25 per cent less luminous than today. Yet there is strong evidence that the Earth’s oceanic surface was not completely frozen. High concentrations of warming greenhouse gases like carbon dioxide (CO2) seem to be the most obvious solution to this famous “faint young Sun paradox”. A team of scientists from the Potsdam Institute for Climate Impact Research (PIK) analyzed in computer simulations how much CO2 in the atmosphere was necessary to prevent the early Earth from falling into a “snowball state”. They found the critical amount to be significantly higher than previously assumed, according to their study now published in Geophysical Research Letters. This sheds light on the environment on early Earth during a time when life first appeared on our planet.
Why early Earth was no snowball: Illuminating the ”faint young Sun paradox”

“For the first time, we present comprehensive 3-dimensional computer simulations of the Archean climate”, says first author Hendrik Kienert. In Earth’s history of 4.54 billion years, the Archean (3.8-2.5 billion years ago) was an era where only small continents existed on a planet mostly covered by oceans and illuminated by a much fainter Sun. “According to our results, CO2 levels in the early Archean must have been 1,400 times higher than in pre-industrial times to prevent early Earth from being a snowball with a completely frozen surface”, Kienert says. This is considerably more than estimated by previous studies, which indicated the critical CO2 level to be only 200 times higher.

“In contrast to previous simulations which focused on the effects of greenhouse gas amounts on temperature, our model also includes processes like the sea-ice albedo feedback and the higher rotation rate of the early Earth”, co-author Georg Feulner says. This is important because sea-ice reflects sunlight back into space, thus reducing Earth’s energy intake. Furthermore, the planet’s rotation rate affects heat transport from the equator to the poles and thus the extent of sea ice. Previous research based on 1-dimensional simulations neglected these aspects.

“Even if we consider the uncertainties like, for example, the locations and sizes of the continents, the critical CO2 amounts for the Archean are clearly above those derived from previous models”, co-author Vladimir Petoukhov says. In the light of these results, it will be more difficult to resolve the faint young Sun paradox for the late Archean. For this time period, other greenhouse gases like methane must have provided additional warming because recent geochemical data indicate an upper limit to the CO2 concentration in the atmosphere.

The current study is part of the PIK project “Ancient Climates on Earth (ACE)” which explores puzzling paleoclimate problems in earlier epochs of Earth’s history. “The faint young Sun paradox has been one of the major open questions in paleoclimatology since it was first pointed out four decades ago”, Feulner says. “Our study gives us a better understanding of the climate on early Earth and is thus a crucial step towards a solution to this problem.”

Article: Kienert, H., Feulner, G., Petoukhov, V. (2012): Faint young Sun problem more severe due to ice-albedo feedback and higher rotation rate of the early Earth. Geophysical Research Letters (doi:10.1029/2012GL054381)

Weblink to the article: http://dx.doi.org/10.1029/2012GL054381

Weblink to a recent review paper on the faint young Sun problem: http://dx.doi.org/10.1029/2011RG000375

Weblink to PIK project ACE: http://www.pik-potsdam.de/research/earth-system-analysis/projects/flagships/ace

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E-Mail: press@pik-potsdam.de