Antarctica’s Dome C: Where the sun’s rays bounce back better
By Kristan Hutchison, Antarctic Sun staff
Upon reflection, Dome C is as good as it gets for studying what happens when sunlight strikes snow. It’s not just that Concordia, the new French- Italian station there, has chefs from Paris and Milan with a friendly rivalry to produce the best dish each day. It’s not the fresh bread and croissants in the morning, the wine on the table at dinner, or the eleven-course Christmas feast that lasts four hours. The food makes life pleasant for Americans researching at Dome C. But what they like about Dome C is that it is flatter and has less wind than the South Pole.
“It looks like the area around Dome C is the most stable, unchanging surface on our planet,” said Rich Brandt, a University of Washington researcher returning for a third and final season at the 3,200m rise in Wilkes Land, East Antarctica. The regularity of the surface allows Brandt and his fellow researchers to measure precisely how much sunlight is reflecting off the snow at a given location and time.
The climate is affected by how much sunlight is reflected and how much is absorbed. If more sunlight is reflected and lost to space, the world could cool, while retaining more would warm the world. That is one reason the researchers want accurate measurements of the fraction of light reflected, known as the albedo, off the Antarctic Plateau. The plateau represents an area the size of the United States.
They also want to know the albedo so they can calibrate instruments on satellites by pointing them at the Antarctic plateau, said Stephen Warren, the lead investigator for the University of Washington project.
Most of the satellites are designed to monitor the Earth’s surface and atmosphere, looking for changes in the climate. The filters, windows and instrumentation on the satellites slowly degrade and need to be adjusted every few months to stay accurate. The quality of data from one satellite appeared to decrease by six percent a year.
Other parts of the world have been used to calibrate satellites in the past. They have to be places with dry air and no vegetation. The eastern Sahara has been used, but the clouds there aren’t always detectable and may distort the results. Snow is so bright that a slight cloud cover doesn’t matter, Warren said.
When astronauts look at the Earth from space, they see the light being bounced back. About 30 percent of the sunlight directed at the Earth is reflected, almost half of it by clouds. Snow-covered surfaces like Antarctica reflect 70 percent of the light that hits them, but the polar regions don’t have a large impact on the overall albedo of the Earth because the high latitudes get little sunlight to start with.
Snow covering North America and Eurasia in the springtime, as the sun returns in full force, has a much greater effect on the climate, Warren said. In periods when the Earth is thought to have been covered in snow, the planetary albedo may have been as high as 60 percent, Warren said.
“That was a huge change, and that was what allowed the climate to be so cold, even at the equator,” Warren said. “Back then albedo was the dominant control on climate.”
On the other end of the scale, the lowest albedo may have been during the warm Cretaceous period, about 100 million years ago, when lush forests grew over most of the landmasses and if there was any ice left it was a small amount in the interior of Antarctica.
Before going to Dome C, Warren’s research group worked at the South Pole for many years and at Vostok, the Russian station. The snow quality was nearly identical at all three, but the South Pole and Vostok had more sastrugi formed by the winds.
“Much like looking at farmer’s fields once they’re plowed, if you move your head back and forth the brightness changes,” Brandt said. The smoother surface at Dome C made it easier for the researchers to take precise measurements. “We’re getting a lot more data at Dome C,” Warren said. “Much more variety of angles and also we’re covering the whole wavelength spectrum, so we’re doing a pretty thorough study.”
They take the measurements from a 32m-tall tower Brandt built with his French colleague, Delphine Six, two years ago. The tower is little more than an open metal staircase, 2m square. From the top of the tower they have a 230-degree view of pristine snow stretching to the horizon, with Concordia Station blocking the view behind them. It’s a no-drive zone, to keep the snow pure so they can take accurate measurements of the sunlight reflecting off 80 points radiating out from the tower to the horizon.
Last season the researchers had clear weather and ended up with very good measurements. This year they will be repeating the same measurements to corroborate the results, as well as extending the angular range.
“We had a spectacular season last year,” Brandt said. “The more times you measure, it gives you a sense you’ve got it.”
After working at the South Pole, where the sun remains at the same level in the sky day and night, Warren was surprised by the changes in temperature, and the associated formation and evaporation of fog through the day at Dome C. Though the sun never sets, it does move higher and lower on the horizon, changing the slant of light hitting the snow. The temperature varied by 20 degrees, peaking in the afternoon and becoming coldest around midnight.
Measuring the reflections at different times of day and seasons is important, because the albedo changes with the slant of the light.
Out to sea
The snow cover over the Antarctic continent remains year-round, and is unlikely to change very much or very fast, Warren said. Sea ice surrounding the continent is another story completely, shrinking and growing each year. As it does so, the amount of solar energy being absorbed or reflected changes too. A paper Brandt and Warren submitted to the Journal of Climate reports the results of three voyages members of his research team took aboard Australian ships in the Antarctic. They combined data from those voyages with data from satellites and other ships to create a map of the albedo of the sea ice for each season.
When the sea ice starts forming, it is still fairly dark and absorbs most of the sunlight. As it thickens and grows it generally reflects more. Through the year, the sea ice albedo can range from as low as 7 percent reflected to as high as 87 percent.
“It’s much more involved in climate change than the snow surface of the Antarctic,” Warren said. “The sea ice regionally is very susceptible to climate change. In fact, it melts away every summer.”
Though this is the last year for the project, the tower will stay at Dome C as an American contribution to Concordia station. Already, three new projects will use it this year.
