Journal Article

No climate paradox under the faint early Sun

Bird, Dennis K., Sleep, Norman H., Bjerrum, Christian J., Rosing, Minik T.

2010

Overview

The cool of the Sun The faint early or 'young' Sun paradox, raised by Carl Sagan and George Mullen in 1972, points out that solar luminosity during the Archaean was about 70% of today's, so it would — the theory goes — have been too cold for liquid oceans to survive on Earth. Yet the geological record shows that liquid water was present. This is usually explained as the consequence of a greenhouse effect due to a high concentration of atmospheric carbon dioxide and/or methane. Minik Rosing et al . suggest that there is no need to invoke greenhouse warming — and no climate paradox. They demonstrate that the mineralogy of Archaean sediments is inconsistent with high greenhouse gas concentrations and the metabolic constraints of the methanogens of the time. They hypothesize that the low albedo of the early Earth, with little in the way of continents, and a preponderance of dark heat-absorbing ocean, together with a lack of biologically induced cloud condensation nuclei, were sufficient to maintain temperatures above freezing. It has been inferred that, during the Archaean eon, there must have been a high concentration of atmospheric CO 2 and/or CH 4 , causing a greenhouse effect that would have compensated for the lower solar luminosity at the time and allowed liquid water to be stable in the hydrosphere. Here it is shown, however, that the mineralogy of Archaean sediments is inconsistent with such high concentrations of greenhouse gases. Instead it is proposed that a lower albedo on the Earth helped to moderate surface temperature. Environmental niches in which life first emerged and later evolved on the Earth have undergone dramatic changes in response to evolving tectonic/geochemical cycles and to biologic interventions 1 , 2 , 3 , as well as increases in the Sun’s luminosity of about 25 to 30 per cent over the Earth’s history 4 . It has been inferred that the greenhouse effect of atmospheric CO 2 and/or CH 4 compensated for the lower solar luminosity and dictated an Archaean climate in which liquid water was stable in the hydrosphere 5 , 6 , 7 , 8 . Here we demonstrate, however, that the mineralogy of Archaean sediments, particularly the ubiquitous presence of mixed-valence Fe( II – III ) oxides (magnetite) in banded iron formations 9 is inconsistent with such high concentrations of greenhouse gases and the metabolic constraints of extant methanogens. Prompted by this, and the absence of geologic evidence for very high greenhouse-gas concentrations 10 , 11 , 12 , 13 , we hypothesize that a lower albedo on the Earth, owing to considerably less continental area and to the lack of biologically induced cloud condensation nuclei 14 , made an important contribution to moderating surface temperature in the Archaean eon. Our model calculations suggest that the lower albedo of the early Earth provided environmental conditions above the freezing point of water, thus alleviating the need for extreme greenhouse-gas concentrations to satisfy the faint early Sun paradox.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/106/694 / 704/2151/525/870 / Albedo / Analysis / Atmosphere / Atmosphere - chemistry / Atmospheric carbon dioxide / Carbon dioxide / Carbon Dioxide - analysis / Climate / Cloud condensation nuclei / Clouds / Earth / Earth (Planet) / Earth sciences / Earth, ocean, space / Ecosystem / Environmental aspects / Environmental conditions / Exact sciences and technology / Fractionation / Freezing / Freezing point / Geochemical cycles / Geologic Sediments - chemistry / Geology / Greenhouse Effect / Greenhouse gases / Growth models / History, Ancient / Humanities and Social Sciences / Hydrogen - analysis / Hydrosphere / Isotopes / letter / Luminosity / Mineralogy / Models, Theoretical / multidisciplinary / Paradoxes / Partial Pressure / Science / Science (multidisciplinary) / Stratigraphy / Sun / Sunlight / Surface temperature / Water - analysis / Water - chemistry