Gaia or Athena? The Early Faint-Sun Paradox

Abstract. During the Archaean, the Sun's luminosity was 18 to 25% lower than the present day. One-dimensional radiative convective models (RCM) generally infer that high concentrations of greenhouse gases (CO2, CH4) are required to prevent the early Earth's surface temperature from dropping below the freezing point of liquid water and satisfying the faint young Sun paradox (FYSP, an Earth temperature at least as warm as today). Using a one-dimensional (1-D) model, it was proposed in 2010 that the association of a reduced albedo and less reflective clouds may have been responsible for the maintenance of a warm climate during the Archaean without requiring high concentrations of atmospheric CO2 (pCO2). More recently, 3-D climate simulations have been performed using atmospheric general circulation models (AGCM) and Earth system models of intermediate complexity (EMIC). These studies were able to solve the FYSP through a large range of carbon dioxide concentrations, from 0.6 bar with an EMIC to several millibars with AGCMs. To better understand this wide range in pCO2, we investigated the early Earth climate using an atmospheric GCM coupled to a slab ocean. Our simulations include the ice-albedo feedback and specific Archaean climatic factors such as a faster Earth rotation rate, high atmospheric concentrations of CO2 and/or CH4, a reduced continental surface, a saltier ocean, and different cloudiness. We estimated full glaciation thresholds for the early Archaean and quantified positive radiative forcing required to solve the FYSP. We also demonstrated why RCM and EMIC tend to overestimate greenhouse gas concentrations required to avoid full glaciations or solve the FYSP. Carbon cycle–climate interplays and conditions for sustaining pCO2 will be discussed in a companion paper.

Microbial methanogenesis may have been a major component of Earth's carbon cycle during the Archaean eon, generating a methane greenhouse that increased global temperatures enough for a liquid hydrosphere, despite the Sun's lower luminosity at the time. Evaluation of potential solutions to the 'faint young Sun' hypothesis by determining the age of microbial methanogenesis has been limited by ambiguous geochemical evidence and the absence of a diagnostic fossil record. To overcome these challenges, we use a temporal constraint: a horizontal gene transfer event from within archaeal methanogens to the ancestor of Cyanobacteria, one of the few microbial clades with recognized crown-group fossils. Results of molecular clock analyses calibrated by this horizontal-gene-transfer-propagated constraint show methanogens diverging within Euryarchaeota no later than 3.51 billion years ago, with methanogenesis itself probably evolving earlier. This timing provides independent support for scenarios wherein microbial methane production was important in maintaining temperatures on the early Earth.

Radley Horton,1,a Daniel Bader,1,a Yochanan Kushnir,2 Christopher Little,3 Reginald Blake,4 and Cynthia Rosenzweig5 1Columbia University Center for Climate Systems Research, New York, NY. 2Ocean and Climate Physics Department, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY. 3Atmospheric and Environmental Research, Lexington, MA. 4Physics Department, New York City College of Technology, CUNY, Brooklyn, NY. 5Climate Impacts Group, NASA Goddard Institute for Space Studies; Center for Climate Systems Research, Columbia University Earth Institute, New York, NY

CURRENT models for the evolution of the Sun require an increase in solar luminosity by 25% since the formation of the Solar System1. Such an increase in the solar constant should have profound effects on the terrestrial climate, but there is no evidence from the fossil record of a corresponding change in the Earth's global mean temperature2. This apparent conflict cannot be explained by the apparent inability of solar models to account for the low observed neutrino flux3. Even models that are forced to fit the neutrino data require a similar increase in the solar luminosity. As Newman and Rood1 state: “a faint young Sun is one of the most unavoidable consequences of stellar structure considerations”. We discuss here whether CO2–H2O in a weakly reducing atmosphere could have caused this change in the early Earth's temperature by the so-called greenhouse effect.

The faint young sun (FYS) was introduced as a paradox by prominent astronomer Carl Sagan dan George Mullen. This paradox posits that the luminosity of the sun 4.6 billion years ago (when it came into existence) was less than 30% of its current luminosity. The variation in solar radiance might have facilitated the Origin of Life (OOL) on Earth by influencing prebiotic chemistry, especially in the formation of prebiotic polyester. Polyester, synthesized from alpha hydroxy acids(ɑHAs), serves as a model studied as a framework that facilitates OOL. Studies indicate that polyester gels can form from dehydration reactions of ɑHAs in wet-dry cycles, potentially initiating prebiotic life. Moreover, various investigations have demonstrated that ultraviolet UV light has been known to initiate prebiotic chemical reactions, as well as produce polyester. However, experiments have been done, but none has truly explored broadband light from the FYS. Furthermore, prebiotic polymerization induced by FYS has not been shown. In this paper, we will discuss about FYS, some research conducted regarding prebiotic reactions induced by UV light, and the prospects of how FYS’ broadband light might be beneficial for prebiotic polymerization involving polyester.

The mean surface temperature on Earth and other planets with atmospheres is determined by the radiative balance between the non-reflected incoming solar radiation and the outgoing long-wave black-body radiation from the atmosphere. The surface temperature is higher than the black-body temperature due to the greenhouse warming. Balancing the ice-albedo cooling and the greenhouse warming gives rise to two stable climate states. A cold climate state with a completely ice-covered planet, called Snowball Earth, and a warm state similar to our present climate where greenhouse warming prevents the total glaciation. The warm state has dominated Earth in most of its geological history despite a 30% fainter young Sun. The warming could have been controlled by a greenhouse thermostat operating by the temperature control of the weathering process depleting CO2 from the atmosphere. This temperature control has permitted life to evolve as early as the end of the heavy bombardment 4 billion years ago.

We study the interactions between the geochemical cycles of carbon and long-term changes in climate. Climate change is studied with a simple, zonally averaged energy balance climate model that includes the greenhouse effect of carbon dioxide explicitly. The geochemical model balances the rate of consumption of carbon dioxide in silicate weathering against its release by volcanic and metamorphic processes. The silicate weathering rate is expressed locally as a function of temperature, carbon dioxide partial pressure, and runoff. The global weathering rate is calculated by integrating these quantities over the land area as a function of latitude. Carbon dioxide feedback stabilizes the climate system against a reduction in solar luminosity and may contribute to the preservation of equable climate on the early Earth, when solar luminosity was low. The system responds to reduced land area by increasing carbon dioxide partial pressure and warming the globe. Our model makes it possible to study the response of the system to changing latitudinal distribution of the continents. A concentration of land area at high latitudes leads to high carbon dioxide partial pressures and high global average temperature because weathering of high-latitude continents is slow. Conversely, concentration of the continents at low latitudes yields a cold globe and ice at low latitudes, a situation that appears to be representative of the late Precambrian glacial episode. This model is stable against ice albedo catastrophe even when the ice line occurs at low latitudes. In this it differs from energy balance models that lack the coupling to the geochemical cycle of carbon.

Earth’s atmosphere has a central role in keeping the environmental conditions on Earth suitable for life. When considering the evolution of the composition of the atmosphere, attention should be paid specifically to greenhouse gases and oxygen. The most important greenhouse gases are water vapor, carbon dioxide and methane. Atmospheric water vapor is always near its condensation temperature and cannot act as an independent climate forcer. Carbon dioxide and methane, in turn, are greenhouse gases, with a high potential for significant variations in their concentrations. For understanding Earth’s climate in the past, it is essential to estimate the concentration history of these two atmospheric components. Oxygen is highly reactive gas, which is in strong disequilibrium with minerals and rocks of the planetary surface. Direct analyses from atmospheric gases exist only for the last 800,000 years. For the earlier geological history, compositional estimates are derived from proxy variables and elemental cycle modeling. The proxy records and carbon cycle modeling indicate that Earth’s history is characterized by generally increasing carbon dioxide levels. In the past, higher contents of carbon dioxide were required to keep the planet from freezing as a result of the reduced power output from the faint young Sun. According to the standard solar model, immediately after the formation of the planet, the luminosity of the Sun was only 70% of the corresponding present-day value. The early atmosphere was anoxic, and as a result, the contents of methane in the atmosphere may have been several orders of magnitude higher compared to the contents in the modern atmosphere. About 2,400 Ma ago the atmosphere experienced a major shift from an anoxic to an oxic state, which may have been one of the most severe environmental changes ever having faced the planet. The most convincing evidence for the shift in the oxidation state is provided by the disappearance of mass independent sulfur isotope fractionation effects in sedimentary sulfur minerals.

Investigating the early Earth faint young Sun problem with a general circulation model

Solar luminosity on the early Earth was significantly lower than today. Therefore, solar luminosity models suggest that, in the atmosphere of the early Earth, the concentration of greenhouse gases such as carbon dioxide and methane must have been much higher. However, empirical estimates of Proterozoic levels of atmospheric carbon dioxide concentrations have not hitherto been available. Here we present ion microprobe analyses of the carbon isotopes in individual organic-walled microfossils extracted from a Proterozoic ( approximately 1.4-gigayear-old) shale in North China. Calculated magnitudes of the carbon isotope fractionation in these large, morphologically complex microfossils suggest elevated levels of carbon dioxide in the ancient atmosphere--between 10 and 200 times the present atmospheric level. Our results indicate that carbon dioxide was an important greenhouse gas during periods of lower solar luminosity, probably dominating over methane after the atmosphere and hydrosphere became pervasively oxygenated between 2 and 2.2 gigayears ago.

The long‐term extent of the Earth system response to anthropogenic interference remains uncertain. However, the geologic record offers insights into this problem as Earth has previously cycled between warm and cold intervals during the Phanerozoic. We present an updated compilation of surface temperature proxies for several key time intervals to reconstruct global temperature changes during the Cenozoic. Our data synthesis indicates that Earth’s surface slowly cooled by ca. 9°C during the early Paleogene to late Neogene and that continent‐scale ice sheets developed after global temperature dropped to less than 10°C above preindustrial conditions. Slow cooling contrasts with the steep decrease in combined radiative forcing from past CO2 concentrations, solar luminosity, and ocean area, which was close to preindustrial levels even as Earth remained in a much warmer state. From this, we infer that the Earth system was less sensitive to greenhouse gas forcing for most of the Cenozoic and that sensitivity must have increased by at least a factor of 2 during the Plio‐Pleistocene. Our results imply that slow feedbacks will raise global surface temperatures by more than 3°C in the coming millennia, even if anthropogenic forcing is stabilized at the present‐day value (2 W/m2), and that their impact will diminish with further warming.

The recent publication of the summary for policy makers by Working Group I of the Intergovernmental Panel on Climate Change (IPCC) [1] has injected a renewed sense of urgency to address climate change. It is therefore timely to review the notion of preventing 'dangerous anthropogenic interference with the climate system' as put forward in the United Nations Framework Convention on Climate Change (UNFCCC). The article by Danny Harvey in this issue [2] offers a fresh perspective by rephrasing the concept of 'dangerous interference' as a problem of risk assessment. As Harvey points out, identification of 'dangerous interference' does not require us to know with certainty that future climate change will be dangerous—an impossible task given that our knowledge about future climate change includes uncertainty. Rather, it requires the assertion that interference would lead to a significant probability of dangerous climate change beyond some risk tolerance, and therefore would pose an unacceptable risk.

Stellar evolution theory predicts that the luminosity of the Sun has increased by ∼30% over the past 4,000 Myr. Yet geological and biological evidence indicates that the climate of the Earth between 3,000 and 4,000 Myr ago was as warm as, or warmer than, today. This apparent contradiction, the ‘faint Sun paradox’, has been resolved by invoking the greenhouse effect of radiatively active gases in the early Earth atmosphere. Sagan and Mullen1 first suggested that the concentration of ammonia in the early atmosphere was around 10–100 p.p.m., sufficiently high to counteract the reduced luminosity. However, because ammonia photodissociates readily and has a short atmospheric residence time2,3, such a concentration could be maintained only by a large continuous ammonia source. For this reason, carbon dioxide is now considered to have been the radiatively active gas4–7. Some atmospheric ammonia is, nevertheless, required to provide conditions conducive to the origin of life8. We now show that, if the early Earth's atmosphere contained high concentrations of CO2, as suggested above, then the chemical conditions required for life to begin can be maintained by very low ammonia partial pressures, rather similar to those observed today.

A warm or a cold early Earth? New insights from a 3-D climate-carbon model

Can non-shivering thermogenesis in brown adipose tissue following NA injection be quantified by changes in overlying surface temperatures using infrared thermography?