Early in Earth's history, our solar system was a much differentplace. When the sun was very young, it was faint and provided little heat forthe Earth. However, even in its chilly beginnings, the surface of the Earth wasice-free.
For years, scientists have proposed theories for this"faint young sun problem."
Most of these theories are based on the idea that the earlyEarth must have had extremely high amounts of greenhousegases like carbon dioxide (CO2) in the atmosphere in order to warm theplanet. According to a team of German scientists, geological evidence ofatmospheric CO2 seems to indicate that levels were "far too low to keepthe surface from freezing." However, their new study may provide a newanswer to the problem.
The study, under lead author Philip von Paris of theInstitut f?r Planetenforschung (Institute for Planetary Research) at theDeutsches Zentrum f?r Luft- und Raumfahrt (German Centre for Air and SpaceTravel) in Berlin, was recently published in the journal Planetary and SpaceScience.
Classic problem
According the geological record of Earth, liquid water waspresent on the Earth's surface as early as 3.7 billion years ago (the Earth itselfis thought to be about 4.5 billion years old). This means that the averagetemperature of the early Earth, 3.7 billion years ago, must have been above freezing.Scientists aren't sure how warm the Earth was, but it's generally accepted thatthe planet has been ice-free for most of its history.
However, by looking at sun-like stars of different agesaround the universe, astronomers believe that the sun's luminosity 3.7 billionyears ago was significantly less than today. If the early Earth's atmospherewas the same as it is now, there wouldn't have been enough sunlight to warm theplanet. Temperatures would have been well below freezing up until 2 billionyears ago.
Most proposed answers to the "faint young sun problem"involve some degree of greenhouse warming on the early Earth in order to keepit from freezing over. In fact, many scientists believe that warming of the Earthoccurred at much higher levels than those seen today due to the presence ofgases like carbon dioxide (CO2), methane, ethane or ammonia. However, there aremany uncertainties concerning whether or not each of these gases could havebeen present on the early Earth.
Impact of asteroids
The new study by the German team is now causing scientiststo reconsider the role of CO2 in warming the early Earth.
They applied a new model to the atmosphere of the earlyEarth that includes updated information about how radiation could have beenabsorbed to cause heating. The study also included important parametersconcerning the surface albedo (how much light is reflected away by the planet'ssurface) and the humidity of the atmosphere.
The new model was used to examine interesting points in thehistory of Earth, such as the end of a period of frequent asteroid impactsknown as the LateHeavy Bombardment (3.8 billion years ago), the first evidence of oxygenproduction by cyanobacteria (2.9 billion years ago) and the first known oxidationevent (2 billion years ago).
A warm breath
"Our new model simulations suggest that the amount ofCO2 needed to keep the surface of the earlyEarth from freezing is significantly less than previously thought,"the authors stated in their paper. In fact, the amount of CO2 might be tentimes less than previous studies indicated.
The model showed that a partial pressure of only 2.9 millibarsof CO2 would have been needed during the late Archaean and early Proterozoicperiods in order to bring the surface temperature of the Earth above freezing.This result, although contrary to previous studies, agrees with currentgeological data. For this period of time, the contradictions of the "faintyoung sun problem" disappear.
The result improves our understanding of how carbon dioxidein the atmosphere can affect the global temperature of Earth.
Today, carbon dioxide levels on Earth are increasing,primarily as a result of human activities. This increase is one of the mostimportant causes of climate change on Earth. Understanding how carbon dioxideaffected the ancient climate of Earth might provide clues about the future ofEarth's climate and global biosphere.
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