A cold, hard look at a telltale region
SEATTLE — Earth's polar regions are considered the telltales, the canaries in the coal mine, for global warming. As the atmosphere heats up, the poles are expected to feel the effects first.
Yet, even as these effects become apparent, scientists are still trying to uncover the factors that drive them. Now, they're laying the foundation for a two-year international assault on the regions, set to begin in 2007. Researchers will gather data on past and current changes and the processes that govern them. And they will leave behind a network of sensors in key locations that will monitor future changes.
The scientists are driven by the knowledge that polar regions are responsible for up to 30 percent of global climate change. Thus, what happens at the opposite ends of the earth is likely to drive changes at latitudes where most people live.
Dubbed the International Polar Year (IPY), the effort marks the third time in 125 years that scientists have focused their attention exclusively on the top and bottom of the world. And, many say, never has the need been more urgent.
"Climate is changing, and we have every reason to believe it will continue in the future," says Cecilia Bitz, a physicist at the University of Washington's Polar Science Center and a member of the US coordinating committee for the 2007 IPY.
The Arctic, in particular, has been gaining prominence as an early indicator of climate change. Last week, the Arctic Council, an eight-nation group including the United States, published a report describing changes observed in the north. It found that warming in the Arctic during the 20th century has been exceptional compared with the past several hundred years. Over the past 50 years, temperatures in key Arctic regions have risen by up to 7 degrees F. The also noted that summer sea ice has declined dramatically, Arctic glaciers are rapidly receding, permafrost is thawing, and warmer waters from the North Atlantic are reaching farther into the Arctic Ocean basin.
Although the council has projected further melting of the polar ice cap this century, such forecasts are rough estimates. The interplay of ice, ocean, and atmosphere is so complex that there's much that scientists don't yet know. For example, a recently discovered circulation pattern called the Arctic Oscillation suggests that the region is far more dynamic, even over a short period of time, than once believed.
Like other large-scale atmospheric patterns such as El Niño, the Arctic Oscillation cycles from strong to weak and back over the course of several years, as well as on shorter time scales. The results can contribute to changes in temperatures, storms, the location and amount of precipitation and sea ice, and other aspects of the Arctic environment. These in turn can have significant effects on coastal erosion in the Arctic, migration patterns in wildlife that indigenous people rely on for food, and weather patterns at lower latitudes.
Indeed, the Arctic Oscillation appears to be coming out of a strong phase that has lasted roughly a decade, giving researchers a look at a dynamic region once thought to be fairly static.
"You can't tell much about how a system works unless it's moving around a little bit," observes James Morison, a University of Washington oceanographer who has conducted field studies of the region since 1974. "Thank goodness the Arctic Oscillation rattled the box so we could see what was inside."
First described in 1998 by David Thompson and John Wallace, atmospheric scientists at the University of Washington at the time, the Arctic Oscillation is a measure of air-pressure patterns in the Arctic. When pressure over the Arctic is lower than normal, unusually strong westerly winds in the upper atmosphere encircle the globe at high latitudes. When the oscillation enters a phase of air pressure that's higher than normal, the westerlies weaken.
In the Arctic, the changes these swings bring can be profound. One example: the extent and thickness of sea ice.
Over the past decade, as the Arctic Oscillation strengthened, it brought warmer, moister air farther into the Arctic, researchers say. Modeling results suggest that the wind patterns at the surface have flushed much of the oldest, thickest ice out of the Arctic Ocean basin and into the North Atlantic. This leaves more open water in the summer. When ice does reform in the fall, it's thinner - so it freezes quickly, but fails to last long during the next thaw.
At the same time, the strengthening of the Arctic Oscillation appears to have drawn deep, relatively warm North Atlantic water farther than ever into the basin.
In effect, this influx appears to have eroded a thin layer of cold, very salty water that insulates the ice-bearing surface water from the warmer water at depth. Under the right conditions, this warmer water can penetrate the insulating layer to thin the ice from beneath. Gauging the relative influence of such patterns is critical to forecasting the future of the ice cap.
"If the retreat of the ice is mainly wind-driven, then the future of the ice depends on what the winds are going to be doing in the next 20 years - which may or may not favor continued retreats," notes Dr. Wallace, who heads the university's Joint Institute for the Study of the Atmosphere and Ocean.
Several different climate models indicate that global warming - the accumulation of heat-trapping greenhouse gases in the atmosphere as a result of human industrial activities - could strengthen the Arctic Oscillation, although they differ on how much.
An additional aspect of a strong set of westerly winds is the warmer, moist air it draws into the region, which leads to more clouds and precipitation. For years, researchers held that Arctic clouds were better blankets in the winter than in the summer. But after spending a year measuring how heat moves between the ocean, ice, and atmosphere in the Arctic Ocean, researchers found that clouds trapped heat all year long, notes Richard Moritz, an oceanographer who participated in the project, known as SHEBA.
This expanded the melt season for the ice, he says.
Among other objectives, researchers are now trying to figure out what factors drive the Arctic Oscillation to weaken or strengthen. Some modeling experiments suggest that global warming alone can give it extra muscle. Others point to different contributors.
Even then, the Arctic Oscillation doesn't provide a complete explanation for the recent environmental trends in the region, researchers acknowledge. Dr. Moritz notes that over the past two or three years, this large-scale circulation pattern has weakened somewhat, "yet we continue to see these records set for minimum ice extent. Clearly, something's going on that's not just the Arctic Oscillation.... A big chunk of variability is not explained by the Arctic Oscillation."
Researchers say they hope the IPY and other near-term monitoring projects will help solve this mystery.
Indeed, one of the benefits the University of Washington's Dr. Bitz hopes to see from long-term monitoring is a vast improvement in model results. Already, she says, the next report from the Intergovernmental Panel on Climate Change will reflect some "big improvements."
But the efforts have a way to go - largely because of problems modeling the Arctic.
"Climate models have been terrible at climate forecasting in the Arctic, but they've made huge progress," she says. The broad range of potential increases in global average temperatures in current global-warming forecasts "will narrow if we can do better in the Arctic, because the polar regions are responsible for 25 to 30 percent of global climate change."
• Global temperatures have increased by about 1 degree F. since 1900.
• Seven of the 10 warmest years in the 20th century occurred in the 1990s, with 1998 the hottest year since reliable temperature measurements began.
• The global sea level has risen about three times as fast in the past century as in the previous 3,000 years.
• At least 279 species of plants and animals are already responding to global warming, at times changing their geographic range or shifting their spring events earlier by an average of two days per decade.
• The 1986-95 period was not only southern Africa's warmest decade this century, it was also its driest.
Sources: Union of Concerned Scientists; Nature; University of East Anglia