Warming the early earth—CO2 reconsidered
Introduction
Geological evidence has shown that liquid water was present on the Earth's surface earlier than 3.7 Gy ago (e.g., Mojzsis et al., 1996, Rosing and Frei, 2004) which implies average temperatures on the surface above 273 K. Some authors have even argued for a hot Archaean climate , based on oxygen (Knauth and Lowe, 2003) and silicon (Robert and Chaussidon, 2006) isotope analysis of seawater cherts. However, as pointed out by, e.g., Kasting and Howard (2006) and Shields and Kasting (2007), these isotopic signatures changes might not only be caused by temperature effects. Sleep and Hessler (2006), for example, deduced a more moderate surface temperature below 300 K, based on quartz weathering records in paleosols. Nevertheless, it is generally accepted that the Earth has been ice-free throughout most of its history.
Observations of several solar-type stars of different ages and virtually all standard models of the solar interior have shown that the total solar luminosity has increased since the ZAMS (zero age main sequence) by about 30% (Gough, 1981, Caldeira and Kasting, 1992). Had the composition of the Earth's atmosphere been the same then as today, the reduced solar flux would have resulted in surface temperatures below 273 K prior to 2.0 Gy (Sagan and Mullen, 1972). This apparent contradiction between solar evolution models, climatic simulations and geological evidence for liquid water and moderately warm temperatures on Earth has been termed the “faint young Sun problem”.
Numerous studies have attempted to solve this problem. For example, Minton and Malhotra (2007) explored the hot early Sun scenario for a non-standard solar evolution. Shaviv (2003) showed that moderate greenhouse warming in combination with the influence of solar wind and cosmic rays on climate could resolve the problem. Jenkins (2000) assumed high obliquities in a General Circulation Model to account for high Archaean temperatures. However, the most accepted scenario involves a much enhanced greenhouse effect (GHE) on the early Earth compared to modern Earth. Today, the GHE produces around 30 K of warming, raising the mean surface temperature of the Earth to about 288 K. Increased abundances of greenhouse gases such as carbon dioxide (Kasting, 1987), methane (Pavlov et al., 2000), ethane (Haqq-Misra et al., 2008) or ammonia (Sagan and Mullen, 1972, Sagan and Chyba, 1997) will strengthen the GHE, hence potentially resolve the problem. However, all of these studies faced some form of contradictions or large uncertainties, either from geological data on atmospheric conditions or from atmospheric modeling. The formation and destruction of ammonia is highly dependent on UV levels in the atmosphere (Sagan and Chyba, 1997, Pavlov et al., 2001). The hydrocarbon haze necessary to allow higher hydrocarbons to accumulate in the atmosphere depends critically on the CO2/CH4 ratio (Pavlov et al., 2003). The high values of methane required to heat the surface of the early Earth depend on estimates of the early biosphere and volcanic activity, which is not well determined. Past CO2 concentrations required by atmospheric models (Kasting, 1987) to reach surface temperatures above 273 K are in conflict with inferred concentrations from the sediment data (Hessler et al., 2004, Rye et al., 1995). In this work, the role of CO2 in warming the early Earth is reconsidered. We used a one-dimensional radiative–convective model, including updated absorption coefficients in its radiation scheme for thermal radiation. The model was applied to the atmosphere of the early Earth in order to investigate the effect of enhanced carbon dioxide on its climate. Additionally, we investigated the effect of two important parameters, namely the surface albedo and the relative humidity (RH), upon the resulting surface temperature.
Our results imply that the amount of CO2 needed to warm the surface of early Earth might have been over-estimated by previous studies. Furthermore, the results show that the contradiction between modeled CO2 concentrations and measured values might disappear by the end of the Archaean.
Section 2 describes the model and Section 3 the runs to validate the new radiation scheme. The runs performed for this work are explained and summarized in Section 4. In Section 5, the results are presented and discussed. Section 6 gives the summary of the results.
Section snippets
Atmospheric model
We used a one-dimensional radiative–convective model based on the climate part of the model used by Segura et al. (2003) and Grenfell et al., 2007a, Grenfell et al., 2007b. Our model differs in upgrades of the radiation scheme to calculate the thermal emission in the atmosphere. The model calculates globally, diurnally averaged atmospheric temperature and water profiles for cloud-free conditions. We will first state some basic characteristics of the model (Section 2.1). Then we will describe
General remarks
MRAC has been tested in two different ways.
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Case 1: k distributions. The calculated k distributions, i.e., the model input data, have been validated against published values to show that the algorithm creating the k distributions works correctly.
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Case 2: Earth atmosphere temperature profiles. Temperature profiles of an Earth-like test atmospheres (composition: N2 0.77, O2 0.21, Ar 0.01, CO2 ) calculated with MRAC and RRTM have been compared. This was done since RRTM has been extensively
Absorption cross sections
The absorption cross sections used in the runs performed for this work (summarized in Table 3, Table 4) were calculated assuming a N2–CO2-background atmosphere, consisting of 95% molecular nitrogen and 5% carbon dioxide. According to Kasting and Ackerman (1986) and Toth (2000), the foreign broadening coefficient for water is enhanced by a factor of 2 with respect to air for CO2 as a broadening gas and by a factor of 1.2 for N2 as a broadening gas. Similarly, for carbon dioxide the foreign
Examples of thermal structure and water profiles
Fig. 9 shows the temperature profiles for run 3 (plain line) and run 4 (dotted line). Run 3 () considered 20 mb CO2 partial pressure, run 4 () roughly 3 mb (see Table 3). These two runs were chosen because they represent interesting points in time, such as the advent of cyanobacteria (run 3) and the first oxidation event (run 4).
Clearly, in the troposphere the temperature differences for the two runs are rather small. In the lower to middle stratosphere (from around 10 to 25 km), run 4
Summary
In this work, we addressed the “faint young Sun problem” of the ice-free early Earth. In order to do this, we applied a one-dimensional radiative–convective model to the atmosphere of the early Earth. Our model included updated absorption coefficients in the thermal radiative transfer scheme.
The validations done for the new radiative transfer scheme have been described. The new scheme is found to perform significantly better than a previous scheme under conditions deviating from the modern
Acknowledgments
We are grateful to Jim Kasting and Eli Mlawer for useful discussion while creating the new radiation scheme. Furthermore, we are grateful to Viola Vogler for help in doing some of the plots in this paper.
We thank the two anonymous referees for their constructive remarks which helped to improve and clarify this paper.
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