A Carbonyl Sulphide Blanket
Chemistry researchers uncover why the Archean world was not frozen solid
A perfect greenhouse gas
"The young Sun was approximately 30 percent weaker than it is now, and the only way to prevent Earth from turning into a massive snowball was a healthy helping of greenhouse gas," Associate Professor Matthew S. Johnson of the Department of Chemistry explains. He has found the most likely candidate for an Archean atmospheric blanket is carbonyl sulphide, a product of the sulphur disgorged during millennia of volcanic activity.
"Carbonyl sulphide is and was the perfect greenhouse gas. Much better than carbon dioxide. We estimate that a blanket of carbonyl sulphide would have provided about 30 percent extra energy to the surface of the planet, and that would have compensated for what was lacking from the Sun," says Johnson.
"There is really no process in the rocky mantle of Earth that would explain this distribution of isotopes," says Johnson. "You would need something happening in the atmosphere."
Painstaking experimentation helped the researchers find a likely atmospheric process. By irradiating sulphur dioxide with different wavelengths of sunlight, they observed that sunlight passing through carbonyl sulphide gave them the wavelengths that produced the weird isotope mix.
Life caused Ice Age
As life emerged on Earth, it produced increasing amounts of oxygen. With an increasingly oxidizing atmosphere, the sulphur emitted by volcanoes was no longer converted to carbonyl sulphide. Instead it was converted to sulphate aerosols, which are powerful climate coolants.
Johnson and his co-workers created a computer model of the ancient atmosphere. In conjunction with laboratory experiments, the model suggests that the fall in carbonyl sulphide and rise of sulphate aerosols taken together would have been responsible for creating Snowball Earth, the planet-wide Ice Age hypothesised to have taken place near the end of the Archean era 2.5 billion years ago.
The implications to Johnson are alarming. "Our research indicates that the distribution and composition of atmospheric gasses swung the planet from a state of life-supporting warmth to a planet-wide Ice Age spanning millions of years. I can think of no better reason to be extremely cautious about the amounts of greenhouse gasses we are currently emitting to the atmosphere."