San Francisco, Calif. -- A Penn State meteorologist has a single theory that could solve both the Faint Young Sun problem and the Snowball Earth problem if it proves to be correct.
"I looked at the climate problems in the Archean, 3.8 to 2.5 billion years ago, where the environment was very warm with a sun that is much cooler than today's," says Dr. Gregory S. Jenkins, assistant professor of meteorology. "Then I looked at the late Proterozoic, where some researchers suggest episodes of snowball Earth where the entire globe ices up and I realized that the same solution could solve both problems.
Jenkins of Penn State's College of Earth and Mineral Sciences explains his solution in two papers presented today (Dec. 14) and tomorrow (Dec. 15) at the Fall Meeting of the American Geophysical Union in San Francisco. The papers are "The high obliquity solution to understanding Archean climate and the faint-young sun paradox," and "Climate model solutions to late Proterozoic glaciation," written with Steven R. Smith, a recent Penn State meteorology graduate.
According to Jenkins, tilt is the key to both events in Earth's past. Many scientists believe that the collision of a large planetoid with Earth created the moon and some believe that collision also altered the Earth's tilt.
"If the Earth were tilted to 70 degrees from vertical, then both the faint young sun problem and the snowball Earth problem would be solved," says Jenkins.
The faint young sun problem arises because young stars are 20 to 30 percent less bright, and consequently less warm, than older stars. Geological evidence shows that the Earth was much warmer in the early Precambrian than it is today, despite a weaker sun. Most solutions to this problem suggest that levels of greenhouse gases like carbon dioxide and methane were much higher then and these gases where what warmed the Earth.
Jenkins used a global climate model of the Earth during the Archean with only slightly elevated carbon dioxide, but with tilt to 70 degrees and geography mirroring that 3.8 to 2.5 billions years ago.
"The Earth was 95 percent ocean then," says Jenkins. "Water has a very high heat capacity and because of the tilt, the Earth would have remained warm."
Tilting the earth 70 degrees gives large parts of each hemisphere 24 hours of sunlight for three months. If this sunlight were to fall on land, temperatures would be hotter than 125 degrees Fahrenheit in summer, but, because land cools very rapidly, during the three-month, 24-hour night, the temperatures would plummet. Twenty-four hours of sunlight for three months on water does not get as hot, but also never cools below the freezing point of ocean water.
"The climate model shows that the 70-degree tilt could produce the temperatures expected during the Archean," says Jenkins.
The snowball Earth problem is different but has the same solution. During the Proterozoic 2.5 billion to 544 million years ago, evidence exists for glaciation at the equator. Researchers suggest that total global glaciation occurred three different times.
"Three glaciations would mean three extinctions for large numbers of species," says Jenkins. "To suggest that life made it through really hard times three times is really hard for me to believe." The global climate model for this period uses an Earth with somewhat more land mass, the 70-degree tilt and shows glaciation occurring at the equators, while the poles remain ice free. This scenario, where some ocean and land remain unfrozen, would allow species to survive the equatorial glaciations.
Although a collision can be cited as the source of the 70-degree tilt, the question of what untilted the planet remains because today's tilt is about 23 degrees from vertical.
"George Williams, department of geology and geophysics, University of Adelaide, originally proposed the tilt and suggested that if a large enough mass built up on the pole, it could untilt," said Jenkins. "At the end of the Precambrian, most of the Earth's land mass was centered around the South Pole." Recent studies suggest that with enough mass, it would take about 100 million years to return the earth to a tilt between 20 and 30 degrees. "Based on what we know about the climate then, this is a viable solution," Jenkins said.
EDITORS: Dr. Jenkins may be reached at 814-865-0478 or sj1@psu.edu