Science News

Greenhouse early-Earth myth

Wed, 31st Mar 2010

Chris Smith

Scientists claim to have solved "the faint Sun paradox", a long-standing scientific mystery highlighted in the 1970s byearth Carl Sagan and George Mullen.

The paradox points out that the Earth's temperatures have been relatively constant during the four and a half billion years that the planet has been in existence, despite the fact that the heat output from the Sun has increased by 30% over this time. The question is, how did the early Earth remain so warm in the face of a fainter Sun? It should have spent at least half its early life frozen solid.

Originally, scientists thought the answer must lie in high levels of a greenhouse gas like ammonia - NH3, the formation of which the low-oxygen environment of the young Earth would have favoured. But then they realised that this breaks down in sunlight, so that couldn't be the answer.

The problem was thought to have been solved when the American scientist, James Kasting, suggested in the early 1990s that CO2 was probably the answer. By acting as a greenhouse gas, and with an atmospheric concentration approaching 30%, he found, this, working together with water vapour, could keep the Earth at a temperature of about 70 degrees celsius.

For a while, everyone was happy with this explanation. But then geochemists unearthed ancient rocks that have shown that the CO2 levels were probably too low to have made this plausible, so the jury was out again. Now, a paper in Nature has produced a plausible explanation.

University of Copenhagen scientist Minik Rosing and his colleagues have analysed compounds of iron found in rocks more than 3.8 billion years old. These so-called banded-iron formations contain two different iron minerals, magnetite and siderite, which form in different ratios according to how much CO2 is around.

Their results indicate that there couldn't have been much more than just three times present-day levels of CO2 in the ancient atmosphere. This is a far cry from the 30% that would be needed if CO2 was the answer.

Instead they suggest that the effect is down to albedo, which is the amount of solar energy reflected off the planet's surface and back into space. On the early Earth, they point out, the continents were much smaller, most of the surface was heat-hungry water, and clouds were made of larger water droplets because there were fewer cloud-forming particles in the atmosphere. These effects meant that far less energy was bounced back into space, keeping the planet warmer.

So why didn't the world warm as the Sun aged and began to produce more heat? Because, the researchers show, the continents grew, life appeared and there were more light-reflective clouds in the sky, which all add up to a balmy, stable temperature.

Why this is also important, say the scientists, is that, contrary to popular belief that CO2 levels in the past have been much higher than they are today, carbon dioxide appears to have remained relatively stable throughout the lifetime of the Earth; this needs to be taken into account in climate models that scientists are attempting to use to predict potential climate change...

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At least over the last 500 million years, Wikipedia indicates much higher carbon dioxide levels. 
http://en.wikipedia.org/wiki/File:Phanerozoic_Carbon_Dioxide.png
http://en.wikipedia.org/wiki/Paleoclimatology

Likely different analysis methods will find some divergence from the findings by Minik Rosing

Assuming the accuracy of the predictions of a warming sun, there are potentially  numerous mechanisms working together that could have kept Earth's temperature moderately stable. 

The "young earth" would have been naturally warmer at the time of formation.  It may take many years for the temperature of the crust to reach equilibrium.  However, a portion of the temperature we experience is due to geothermal heating.
Several long halflife radioactive elements would have been more prevalent in the young earth, thus generating more radioactive decay heat.  Perhaps not a lot, but it is a contributing factor for the surface temperature of Earth. 
If the moon is getting more distant from the Earth, one might also expect the Earth to be getting more distant from the sun through similar mechanisms.  Planetary orbits are also believed to have undergone some major shifts in the past.
Presumably the sun's mass is also slowly decreasing due to solar winds and fusion.  Perhaps it would contribute to the distance of earth from the sun.
CO2, as mentioned, will likely contribute somewhat, even in relatively low concentrations.
Clouds and Albedo as mentioned, as well as changing surface area of the oceans.
Most of the early life was in the marine environment which might be less susceptible to cold temperatures.
There seem to be several factors relating to climate resilience an stability including tropical storms transferring excess heat to the poles, and to the upper atmosphere .  With cooler temperatures, this might not happen as much.  Warmer temperatures could bring more vigorous storms, releasing more energy.  Anyway, there may be some contributing factors and feedback systems to create an overall stability in our climate system.  Certainly the solar output varies a few watts over the course of a decade solar cycle with minimal impact on Earth.
Some plants, including forests seem to do active climate modification including large amounts of water vapor evaporation and rain.  This would have occurred less on the early earth.
Some research indicates that oxygen levels in the atmosphere have increased significantly over time, after the evolution of plant life.  This would have also impacted ozone production, and thus more sunlight is being rejected in the upper atmosphere than would have occurred on the primordial Earth.

Certainly some factors will be more significant than others, or some may not even be important.  However, one should consider multiple factors for previous overall climate stability.  Obviously there have also always been significant climate fluctuations jumping from ice ages to a very tropical climate. CliffordK, Sun, 4th Mar 2012

"University of Copenhagen scientist Minik Rosing and his colleagues have analysed compounds of iron found in rocks more than 3.8 billion years old. These so-called banded-iron formations contain two different iron minerals, magnetite and siderite, which form in different ratios according to how much CO2 is around.

Their results indicate that there couldn't have been much more than just three times present-day levels of CO2 in the ancient atmosphere. This is a far cry from the 30% that would be needed if CO2 was the answer.

Instead they suggest that the effect is down to albedo, which is the amount of solar energy reflected off the planet's surface and back into space. On the early Earth, they point out, the continents were much smaller, most of the surface was heat-hungry water, and clouds were made of larger water droplets because there were fewer cloud-forming particles in the atmosphere. These effects meant that far less energy was bounced back into space, keeping the planet warmer.

So why didn't the world warm as the Sun aged and began to produce more heat? Because, the researchers show, the continents grew, life appeared and there were more light-reflective clouds in the sky, which all add up to a balmy, stable temperature.

Why this is also important, say the scientists, is that, contrary to popular belief that CO2 levels in the past have been much higher than they are today, carbon dioxide appears to have remained relatively stable throughout the lifetime of the Earth; this needs to be taken into account in climate models that scientists are attempting to use to predict potential climate change..."

And what does this idea prove? That they have an idea? And from that they extrapolate into stupi**y.

"Sediment cores from the deep ocean reveal a climate event 55 million years ago that appears to be analogous to the potential global warming climate event in the future. Isotopes of carbon preserved in CaCO3 shells reveal an abrupt release of carbon to the atmosphere-ocean system, which took about 150 thousand years to recover. Isotopes of oxygen show a parallel perturbation, reflecting warming of the climate and the deep ocean in response to the carbon release. Although specifics of the event remain uncertain, such as the source, amount, and release timescale of the CO2, the event confirms the long timescale for recovery from CO2 release, as predicted by the models in this study." (THE PALEOCENE-EOCENE THERMAL MAXIMUM CLIMATE EVENT)

yor_on, Wed, 14th Mar 2012

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