Increase In Solar Luminosity Research Articles

Evolution of the Global Carbon Cycle and Climate Regulation on Earth

AbstractThe existence of stabilizing feedbacks within Earth's climate system is generally thought to be necessary for the persistence of liquid water and life. Over the course of Earth's history, Earth's atmospheric composition appears to have adjusted to the gradual increase in solar luminosity, resulting in persistently habitable surface temperatures. With limited exceptions, the Earth system has been observed to recover rapidly from pulsed climatic perturbations. Carbon dioxide (CO2) regulation via negative feedbacks within the coupled global carbon‐silica cycles are classically viewed as the main processes giving rise to climate stability on Earth. Here we review the long‐term global carbon cycle budget, and how the processes modulating Earth's climate system have evolved over time. Specifically, we focus on the relative roles that shifts in carbon sources and sinks have played in driving long‐term changes in atmospheric pCO2. We make the case that marine processes are an important component of the canonical silicate weathering feedback, and have played a much more important role in pCO2 regulation than traditionally imagined. Notably, geochemical evidence indicate that the weathering of marine sediments and off‐axis basalt alteration act as major carbon sinks. However, this sink was potentially dampened during Earth's early history when oceans had higher levels of dissolved silicon (Si), iron (Fe), and magnesium (Mg), and instead likely fostered more extensive carbon recycling within the ocean‐atmosphere system via reverse weathering—that in turn acted to elevate ocean‐atmosphere CO2 levels.

Half a billion years of good weather: Gaia or good luck?

Earth’s climate has been unusually stable over the Phanerozoic (Greek: visible animal life) eon, the most recent 550 million years of Earth’s history during which organisms have been large and hard enough to form plentiful fossils. For the previous 4000 million years, most of the planet’s lifetime, living things were microscopic, and the climate was much more varied. Oxygen isotope ratios in ancient seashells suggest that, in the Phanerozoic, globally averaged temperatures have generally varied by less than 10 °C from their present value of 15 °C (Veizer et al. 2000). The record of ice-rafted debris preserved in ancient seafloor sediments tells a similar story of a climate that, while showing interesting variations, has rarely differed greatly from that of the present day. This contrasts with the previous 4 billion years of Earth history which is characterized by occasional snowball-Earth episodes (Hoffman et al. 1998), when temperatures were suppressed at least 20 °C, and which also shows evidence of periods when our world was 40 °C warmer than today (Schwartzman 1999). The consistent Phanerozoic temperatures are particularly surprising in the light of the 5% increase in solar luminosity over that time, the 15-fold reduction in atmospheric carbon dioxide concentrations (Berner 1997) and the inevitable variations in Earth-albedo resulting from continental growth and the evolution of land-plants, among other factors. It should be emphasized that there is no problem with understanding why there was significant climatic variability prior to the Phanerozoic. Astronomical, geological and biological processes were constantly altering the solar luminosity, the Earth’s albedo and the greenhouse gas concentrations in our atmosphere and these changes could, in principle, have caused temperature fluctuations even higher than those seen. The real problem is in understanding why temperature fluctuations dropped so dramatically during the Phanerozoic. A widely accepted explanation for long-term climate stability is that chemical-weathering of continental rocks by weakly carbonated rainfall gradually scrubs the greenhouse gas carbon dioxide from the atmosphere and, since this chemical reaction is temperature dependent, the result is a negative-feedback control on climate (Walker et al. 1981). However, while this process can certainly keep the Earth habitable (i.e. it can maintain temperatures allowing the existence of liquid water) it does not seem capable of providing the very fine control on temperature that is characteristic of the Phanerozoic. The coincidence of the emergence of multicelled organisms and of climate stability supports James Lovelock’s Gaia hypothesis that the Earth’s biosphere evolves to maintain optimum conditions for life. Indeed one of the best known Gaia-based models (DaisyWorld, introduced by Watson and Lovelock in 1983) shows how a simple biosphere can control global temperature and enhance climate stability. However, anthropic selection offers another possibility: perhaps climate stability is necessary for complex life and so complex life only evolved because Earth, by chance, provided a stable environment. The probability of such good fortune may seem small but, given the probable number of planets in the universe (>10), it is almost bound to happen on a few lucky worlds. This planetary-level form of anthropic selection, in which we find ourselves living on an unusual planet simply because the conditions necessary for the emergence of intelligent life are rare (Ward and Brownlee 2000), should be distinguished from the more commonly discussed cosmological-level anthropic selection which postulates the existence of multiple universes only some of which have laws of physics which are life-friendly (Barrow and Tipler 1986). Anthropic ideas remain controversial, largely because it is notoriously difficult to provide clear evidence in their favour. It will always be difficult to disentangle feedback from good luck when looking at geological or biological surface processes, but astronomical factors do not have this problem. For example, there is no mechanism whereby the eccentricity of the Earth’s orbit could vary in response to climate change on the Earth’s surface. If our world turns out to have an unusually circular orbit when compared to terrestrial planets around most stars, this is likely to result from anthropic selection. The Moon’s properties too, may provide evidence in support of anthropic selection.

Evolution of a habitable planet

■ Abstract Giant planets have now been discovered around other stars, and it is only a matter of time until Earth-sized planets are detected. Whether any of these planets are suitable for life depends on their volatile abundances, especially water, and on their climates. Only planets within the liquid-water habitable zone (HZ) can support life on their surfaces and, thus, can be analyzed remotely to determine whether they are inhabited. Fortunately, current models predict that HZs are relatively wide around main-sequence stars not too different from our sun. This conclusion is based on studies of how our own planet has evolved over time. Earth’s climate has remained conducive to life for the past 3.5 billion years or more, despite a large increase in solar luminosity, probably because of previous higher concentrations of CO2 and/or CH4. Both these gases are involved in negative feedback loops that help to stabilize the climate. In addition to these topics, we also briefly discuss the rise of atmospheric O 2 and O3, along with their possible significance as indicators of life on other planets.

On climate response to changes in the cosmic ray flux and radiative budget

We examine the results linking cosmic ray flux (CRF) variations to global climate change. We then proceed to study various periods over which there are estimates for the radiative forcing, temperature change and CRF variations relative to today. These include the Phanerozoic as a whole, the Cretaceous, the Eocene, the Last Glacial Maximum, the 20th century, as well as the 11‐yr solar cycle. This enables us to place quantitative limits on climate sensitivity to both changes in the CRF, and the radiative budget, F, under equilibrium. Under the assumption that the CRF is indeed a climate driver, the sensitivity to variations in the globally averaged relative change in the tropospheric ionization ℐ is consistently fitted with μ ≡ − (dTglobal/dℐ) ≈ 7.5 ± 2°K. Additionally, the sensitivity to radiative forcing changes is λ ≡ dTglobal/dF = 0.35 ± 0.09°KW−1m2, at the current temperature, while its temperature derivative is undetectable with (dλ/dT)0 = −0.01 ± 0.04 m2W−1. If the observed CRF/climate link is ignored, the best sensitivity obtained is λ = 0.54 ± 0.12°KW−1m2 and (dλ/dT)0 = −0.02 ± 0.05 m2W−1. Note that this analysis assumes that different climate conditions can be described with at most a linear function of T; however, the exact sensitivity probably depends on various additional factors. Moreover, λ was mostly obtained through comparison of climate states notably different from each other, and thus only describes an average sensitivity. Subject to the above caveats and those described in the text, the CRF/climate link therefore implies that the increased solar luminosity and reduced CRF over the previous century should have contributed a warming of 0.47 ± 0.19°K, while the rest should be mainly attributed to anthropogenic causes. Without any effect of cosmic rays, the increase in solar luminosity would correspond to an increased temperature of 0.16 ± 0.04°K.

Evolution of a Habitable Planet

▪ Abstract Giant planets have now been discovered around other stars, and it is only a matter of time until Earth-sized planets are detected. Whether any of these planets are suitable for life depends on their volatile abundances, especially water, and on their climates. Only planets within the liquid-water habitable zone (HZ) can support life on their surfaces and, thus, can be analyzed remotely to determine whether they are inhabited. Fortunately, current models predict that HZs are relatively wide around main-sequence stars not too different from our sun. This conclusion is based on studies of how our own planet has evolved over time. Earth's climate has remained conducive to life for the past 3.5 billion years or more, despite a large increase in solar luminosity, probably because of previous higher concentrations of CO2and/or CH4. Both these gases are involved in negative feedback loops that help to stabilize the climate. In addition to these topics, we also briefly discuss the rise of atmospheric O2and O3, along with their possible significance as indicators of life on other planets.

The ultraviolet history of the terrestrial planets — implications for biological evolution

A radiative transfer model is employed to investigate the comparative surface ultraviolet (UV) radiation histories of Earth, Mars and Venus from 4.5 Ga to the present and thus their comparative theoretical photobiological histories. Earth probably began with a period of higher ultraviolet radiation fluxes during the anoxic Archean. During the early Proterozoic UV fluxes declined as oxygen partial pressures and thus ozone column abundance rose, but the ozone column became subject to stochastic depletion events caused principally by impact events and possibly large-scale volcanism and less frequently, close cosmic events such as supernovae. In contrast Mars has been subject to a history dominated by a slow increase in solar luminosity and a reduction in partial pressures of CO 2, both of which have resulted in an increase in UV flux. The UV radiation history of Venus has been dominated by the greenhouse effect through which high partial pressures of CO 2 made the surface UV radiation environment clement. These distinct histories influence the potential comparative evolutionary photobiology of the three planets. On Earth, life transitioned from the Archean, when tolerance to UV radiation, particularly for exposed organisms, must have been high to a more photobiologically clement era. In this latter era the predominant evolutionary selection pressure is one that allows for tolerance of sudden and unpredictable increases in UVB radiation above seasonal and diurnal maxima caused by exogenous perturbation of the ozone column. In the case of Mars, the UV radiation flux has increased over time. Today the biologically effective irradiances to DNA are not considerably different from those that are calculated for Archean Earth. If the planet suffered an atmospheric collapse then it may have been subject to an ultraviolet crisis at some point in its past when DNA-weighted irradiance would have increased three to five-fold. Venus transitioned into a photobiologically clement era soon after late bombardment. The lifeless surfaces of Mars and Venus, when in the former case DNA-weighted irradiances are not much greater than Archean Earth and in the latter case, insignificant, are testament to the unimportance of UV radiation as an evolutionary selection pressure when other physical factors, particularly lack of liquid water, become limiting to life. Understanding the comparative evolutionary differences in surface UV flux of the terrestrial planets can help us understand the influence, and lack of influence, of UV radiation in determining their suitability as abodes for life at different stages in their past.

Evolution of terrestrial proto-CO 2 atmosphere coupled with thermal history of the earth

The effect of volatile exchange between surface reservoirs and the mantle on the evolution of proto-CO 2 atmosphere on the Earth is investigated using a global carbon cycle model coupled with thermal evolution of the mantle. Carbon is assumed to circulate among five reservoirs (atmosphere, ocean, continents, seafloor and mantle) and the carbon flux of each reservoir is calculated under varying conditions, such as an increase in solar luminosity, continental growth, and a decrease in tectonic activity with time. We consider processes such as continental weathering, carbonate precipitation in the ocean, carbonate accretion to the continents, metamorphism of carbonates followed by CO 2 degassing through arc volcanism, carbon regassing into the mantle, and CO 2 degassing from the mantle. The degassing rate of volatiles from the mantle is assumed to be proportional to the volatile concentration in the mantle multiplied by the mantle degassing volume. The mantle degassing volume is determined by the seafloor spreading rate and the melt generation depth in the mantle. We use a parameterized convection model to calculate the thermal evolution of the mantle, from which we estimate the seafloor spreading rate and the melt generation depth in the mantle. Numerical simulations suggest that the amount of surface carbon at the present time would be in a steady state. This is because the response time of the carbon cycle system against the perturbation for surface carbon is short, being estimated at about 900 million years under the present conditions. Thus the present amount of surface carbon would not be affected by the initial amount of surface carbon. The seafloor spreading rate would have been almost constant throughout the Earth's history to explain apparent constancy of the carbon isotope ratio in the mantle after about 3.5 billion years ago. The surface carbon might probably have been circulated between surface reservoirs and the mantle once or twice after the Archean period. In any event, CO 2 in the proto-atmosphere on the Earth is suggested to have decreased with the growth of continents, resulting in stabilization of the terrestrial environment against the increase in solar luminosity.

Long-term stability of the earth's climate

Earth's climate has remained reasonably temperate for at least the last 3.5 billion years, despite a large increase in solar luminosity with time. The increase in solar flux has probably been offset by a decrease in atmospheric CO 2 concentration caused by a negative feedback in the carbonate-silicate geochemical cycle. The same feedback mechanism implies that an Earth-like planet could remain habitable (i.e. posses liquid water) out to at least the orbit of Mars. The initial atmospheric CO 2 concentration may have been much higher than the amount required to offset the lower solar output, in which case the Earth may have originally been much hotter than it is today. However, once the initial accretion period was over, Earth should have been stable against either a runaway greenhouse, that is, complete evaporation of the oceans, or against rapid loss of water. Long-term climatic evolution has thus far been studied only with one-dimensional, globally-averaged climate models. Although such models can provide a qualitative understanding of climate history, they rely on number of assumptions that may not have been valid in the past. Some problems that deserve to be investigated with more sophisticated climate models are discussed.

Long-term stability of the earth's climate

Earth's climate has remained reasonably temperate for at least the last 3.5 billion years, despite a large increase in solar luminosity with time. The increase in solar flux has probably been offset by a decrease in atmospheric CO 2 concentration caused by a negative feedback in the carbonate-silicate geochemical cycle. The same feedback mechanism implies that an Earth-like planet could remain habitable (i.e. posses liquid water) out to at least the orbit of Mars. The initial atmospheric CO 2 concentration may have been much higher than the amount required to offset the lower solar output, in which case the Earth may have originally been much hotter than it is today. However, once the initial accretion period was over, Earth should have been stable against either a runaway greenhouse, that is, complete evaporation of the oceans, or against rapid loss of water. Long-term climatic evolution has thus far been studied only with one-dimensional, globally-averaged climate models. Although such models can provide a qualitative understanding of climate history, they rely on number of assumptions that may not have been valid in the past. Some problems that deserve to be investigated with more sophisticated climate models are discussed.

The geologic record of climatic change

This paper reviews the principal results from paleoclimate studies and includes background material slanted toward climate modelers. The inferred temperature history of the last 4.6 billion years indicates major changes in the components of the earth's climate system. A secular change in global insolation receipt is due to a 20–30% increase in solar luminosity since the formation of the earth. A CO2‐H2O greenhouse effect may have offset the lower luminosity during early earth history. Inferred fluctuations of global temperature have occurred over a broad range of time scales. On time scales of 106–108 years, paleogeographic factors (e.g., continental drift and sea level changes) have contributed significantly to temperature changes associated with transitions between nonglacial and glacial states. Preliminary modeling efforts indicate that additional factors (e.g., CO2, changes in atmospheric circulation) must also be considered in order to explain the origin of nonglacial climates. The origin of polar ice caps may result from ocean circulation changes that were caused by plate tectonic processes. Fluctuations of ice volume on a time scale of 10³–105 years correlate with insolation variations caused by orbital perturbations. Feedback interactions within the land‐sea‐air‐ice system (e.g., ocean circulation changes and bedrock dynamics) have been responsible for a significant modulation of the orbital signal. Ice ages may be due to orbitally induced temperature changes superimposed on a global cooling of terrestrial origin.

Direct Measurement of Solar Luminosity Variation

Two rocket flights of an absolute pyrheliometer, separated by 30 months, indicate an increase in solar luminosity (solar constant) of 0.4 percent. The significance of this result is considered in light of the instrument performance during the rocket flights and of pre- and postflight intercomparisons with independently maintained pyrheliometers. There is a high probability that the measured difference is real. Additional observations are required to determine whether the difference results from random fluctuations in solar luminosity, a nonrandom change of short duration, or a sustained change that has climatological significance.

Enhanced CO2 greenhouse to compensate for reduced solar luminosity on early Earth

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.

Glazed Lunar Rocks: Origin by Impact

The glassy coating of lunar rock 12017 is enriched in 15 trace elements relative to the crystalline interior. It apparently consists chiefly of shock-melted rock, somewhat richer in rare earth elements and alkali metals than rock 12017 itself. The glass has been contaminated by about 0.5 percent carbonaceous-chondrite-like material or, alternatively, by a mixture of 0.06 to 0.3 percent fractionated meteoritic material and approximately 10 to 15 percent local soil. The glazing seems to represent molten material splashed from a nearby meteorite impact and not in situ melting by a sudden increase in solar luminosity.