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Dr. Brian Toon, University of Colorado Boulder – Faint Young Sun Paradox


In today’s Academic Minute, Dr. Brian Toon of the University of Colorado Boulder reveals how a weaker sun could have supported early life on Earth. 

Brian Toon is a professor of atmospheric and oceanic sciences at the University of Colorado Boulder. His research group studies radiative transfer, aerosol and cloud physics, atmospheric chemistry and parallels between the Earth and other planets. He has also helped conceive, develop and lead many NASA airborne field missions. He has published more than 300 papers and he holds a Ph.D. from Cornell University.

About Dr. Toon

Dr. Brian Toon – Faint Young Sun Paradox

The faint young Sun paradox asks the question, how did the Earth remain warm despite the fact that the Sun was 20 to 25 percent less luminous in the distant past?  Evidence from ancient rocks clearly indicates that the early Earth was a warm and habitable planet, with life emerging more than three-and-a-half billion years ago.  However, long established models of the evolution of stars predict that the young Sun was weak.

This apparent paradox was first recognized by Carl Sagan in the early 1970s, and has lingered without satisfactory resolution ever since.  However, recently we have applied state of the art three-dimensional climate models to the problem.  Note that these are the same advanced models used for predictions of present day climate change. 

Our results indicate that it may not be so difficult to keep the early Earth warm.  With about 40 times the present day level of carbon dioxide and 500 times the present day level of methane, we can create a strong enough greenhouse effect to overcome a 20 percent reduction in the Sun’s brightness, yielding surface temperatures much like today.  While these amounts may sound a bit extreme, they fall perfectly within the estimates determined by geologists from analyzing ancient soil samples, known as paleosols.

Furthermore, there are other controls on climate that make warming the early Earth even easier.  In our model, we have tested recent hypothesis regarding changes to clouds, land, and the total atmospheric pressure on Earth during its earliest days.  Given plausible assumptions for the latter, only 10 times the present day level of carbon dioxide along with 50 times the present level of methane may be all that is needed to keep the early Earth warm.  The faint young Sun paradox may be no more.

Production support for the Academic Minute comes from Newman’s Own, giving all profits to charity and pursuing the common good for over 30 years, and from Mount Holyoke College.

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