July 30, 2013
Arctic Winters Were Ice-Free Three To Five Million Years Ago
April Flowers for redOrbit.com - Your Universe Online
During the Pliocene Epoch, the Earth was substantially warmer than it is today, despite similar concentrations of carbon dioxide in the atmosphere. A new study, led by the University of Colorado Boulder, suggests year-round ice-free conditions across the surface of the Arctic Ocean could explain the difference. The study was published in a recent issue of Palaeogeography, Paleoclimatology, Palaeoecology.
A new record was marked by instruments at the Mauna Loa Observatory in Hawaii in early May of this year. For the first time in modern history, the concentration of carbon dioxide climbed to nearly 400 parts per million.
Scientists believe the last time the Earth reached this concentration was between 3 and 5 million years ago during the Pliocene, when the planet was 3.5 to 9 degrees Fahrenheit warmer than it is today. During the Pliocene, trees overtook the tundra, growing right to the edges of the Arctic Ocean. The seas swelled and pushed ocean levels 65 to 80 feet higher. This vision of the Pliocene has been cobbled together from fossil records preserved in sediments deposited beneath lakes and on the ocean floor.
"When we put 400 ppm carbon dioxide into a model, we don't get as warm a planet as we see when we look at paleorecords from the Pliocene," said Jim White, director of CU-Boulder's Institute of Arctic and Alpine Research. "That tells us that there may be something missing in the climate models."
Previous research has suggested several hypotheses in the past to explain the warmer climate of the Pliocene. For example, one theory is the formation of the Isthmus of Panama, the narrow strip of land linking North and South America, could have altered ocean circulations during the Pliocene, forcing warmer waters toward the Arctic. So far, however, none of these theories have proven viable.
Ashley Ballantyne, a former CU-Boulder doctoral student who is now an assistant professor of bioclimatology at the University of Montana, led the research team for the current study. The researchers decided to investigate what would happen if they forced the model to assume that the Arctic was free of ice in the winter as well as the summer during the Pliocene. Models operating without these additional parameters show ice-free summers followed by a layer of ice reforming during the sunless winters.
"We tried a simple experiment in which we said, 'We don't know why sea ice might be gone all year round, but let's just make it go away,' " said White, who also is a professor of geological sciences. "And what we found was that we got the right kind of temperature change and we got a dampened seasonal cycle, both of which are things we think we see in the Pliocene."
Because open water surface allows for evaporation, year-round ice-free conditions caused warmer conditions in the Arctic in the model simulations. The heat was a result of the energy needed to cause evaporation, which was stored in the water vapor in the atmosphere. Clouds, which trap heat near the planet's surface, are also created by this water vapor.
"Basically, when you take away the sea ice, the Arctic Ocean responds by creating a blanket of water vapor and clouds that keeps the Arctic warmer," White said.
The team is now seeking to understand what types of conditions could bridge the standard model simulations with the simulations in which ice-free conditions in the Arctic are imposed. If they are able to do so, the new models created could be able to model the transition between a time when ice reformed in the winter to a time when the ocean remained devoid of ice throughout the year.
A model such as this would also offer insight into the potential problems of our future. In our current conditions, approximately 70 percent of the sea ice disappears during the summertime before reforming in winter.
"We're trying to understand what happened in the past but with a very keen eye to the future and the present," White said. "The piece that we're looking at in the future is what is going to happen as the Arctic Ocean warms up and becomes more ice-free in the summertime.
"Will we continue to return to an ice-covered Arctic in the wintertime? Or will we start to see some of the feedbacks that now aren't very well represented in our climate models? If we do, that's a big game changer."