October 4, 2013
The Paradox Of The Young Sun And Earth’s Early Climate
Brett Smith for redOrbit.com - Your Universe Online
When life originated on Earth between 3.8 and 2.4 billion years ago during the Archean era, our sun was only functioning at about 75 percent of its current power. With this ‘low wattage’ sun, the Earth should have been covered in glaciers, yet scientists have found no evidence of this taking place.
"During the Archean, the solar energy received at the surface of the Earth was about 20 to 25 percent lower than present," explained study author Ray Burgess, from the University of Manchester's School of Earth, Atmospheric and Environmental Sciences in a statement. "If the greenhouse gas composition of the atmosphere was comparable to current levels then the Earth should have been permanently glaciated but geological evidence suggests there were no global glaciations before the end of the Archean and that liquid water was widespread."
If greenhouse gas levels, which regulate Earth's climate, were considerably higher during the Archean than they are today, then it may have prevented widespread glaciation.
"To counter the effect of the weaker Sun, carbon dioxide concentrations in the Earth's atmosphere would need to have been 1,000 times higher than present," said study author Bernard Marty, from the CRPG-CNRS University of Lorraine. "However, ancient fossil soils – the best indicators of ancient carbon dioxide levels in the atmosphere – suggest only modest levels during the Archean.”
“Other atmospheric greenhouse gases were also present, in particular ammonia and methane, but these gases are fragile and easily destroyed by ultraviolet solar radiation, so are unlikely to have had any effect,” Marty added.
In the study, scientists wanted to test another theory – that the amount of atmospheric nitrogen could have been high enough that it amplified the greenhouse effect of carbon dioxide, keeping the Earth relatively ice-free.
The team was able to analyze tiny samples of air trapped in bubbles of quartz from a region of northern Australia that is known to contain extremely old and exceptionally well-preserved rocks.
"We measured the amount and isotopic abundances of nitrogen and argon in the ancient air," Marty said. "Argon is a noble gas which, being chemically inert, is an ideal element to monitor atmospheric change.”
“Using the nitrogen and argon measurements we were able to reconstruct the amount and isotope composition of the nitrogen dissolved in the water and, from that, the atmosphere that was once in equilibrium with the water,” he added.
The team discovered that the partial pressure of atmospheric nitrogen during the Archean was similar, maybe even a bit lower, than today. This finding essentially eliminated nitrogen as one of the main contenders for solving the paradox.
"The amount of nitrogen in the atmosphere was too low to enhance the greenhouse effect of carbon dioxide sufficiently to warm the planet,” Burgess said. “However, our results did give a higher than expected pressure reading for carbon dioxide – at odds with the estimates based on fossil soils – which could be high enough to counteract the effects of the faint young Sun and will require further investigation."