Early in Earth's history, our solar system was a much different place. When the sun was very young, it was faint and provided little heat for the Earth. However, even in its chilly beginnings, the surface of the Earth was ice-free.

For years, scientists have proposed theories for this "faint young sun problem."

Most of these theories are based on the idea that the early Earth must have had extremely high amounts of greenhouse gases like carbon dioxide (CO₂) in the atmosphere in order to warm the planet. According to a team of German scientists, geological evidence of atmospheric CO₂ seems to indicate that levels were "far too low to keep the surface from freezing." However, their new study may provide a new answer to the problem.

The study, under lead author Philip von Paris of the Institut für Planetenforschung (Institute for Planetary Research) at the Deutsches Zentrum für Luft- und Raumfahrt (German Centre for Air and Space Travel) in Berlin, was recently published in the journal Planetary and Space Science.

Classic problem

According the geological record of Earth, liquid water was present on the Earth's surface as early as 3.7 billion years ago (the Earth itself is thought to be about 4.5 billion years old). This means that the average temperature 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 that the planet has been ice-free for most of its history.

However, by looking at sun-like stars of different ages around the universe, astronomers believe that the sun's luminosity 3.7 billion years ago was significantly less than today. If the early Earth's atmosphere was the same as it is now, there wouldn't have been enough sunlight to warm the planet. Temperatures would have been well below freezing up until 2 billion years ago.

Most proposed answers to the "faint young sun problem" involve some degree of greenhouse warming on the early Earth in order to keep it from freezing over. In fact, many scientists believe that warming of the Earth occurred at much higher levels than those seen today due to the presence of gases like carbon dioxide (CO₂), methane, ethane or ammonia. However, there are many uncertainties concerning whether or not each of these gases could have been present on the early Earth.

Impact of asteroids

The new study by the German team is now causing scientists to reconsider the role of CO₂ in warming the early Earth.

They applied a new model to the atmosphere of the early Earth that includes updated information about how radiation could have been absorbed to cause heating. The study also included important parameters concerning the surface albedo (how much light is reflected away by the planet's surface) and the humidity of the atmosphere.

The new model was used to examine interesting points in the history of Earth, such as the end of a period of frequent asteroid impacts known as the Late Heavy Bombardment (3.8 billion years ago), the first evidence of oxygen production by cyanobacteria (2.9 billion years ago) and the first known oxidation event (2 billion years ago).

A warm breath

"Our new model simulations suggest that the amount of CO₂ needed to keep the surface of the early Earth from freezing is significantly less than previously thought," the authors stated in their paper. In fact, the amount of CO₂ might be ten times less than previous studies indicated.

The model showed that a partial pressure of only 2.9 millibars of CO₂ would have been needed during the late Archaean and early Proterozoic periods in order to bring the surface temperature of the Earth above freezing. This result, although contrary to previous studies, agrees with current geological data. For this period of time, the contradictions of the "faint young sun problem" disappear.

The result improves our understanding of how carbon dioxide in 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 most important causes of climate change on Earth. Understanding how carbon dioxide affected the ancient climate of Earth might provide clues about the future of Earth's climate and global biosphere.