Global warming mystery: Are North and South really polar opposites?

Two studies, one about plants covering previously frozen landscapes in the Arctic, the other about expanding winter sea ice in Antarctica, appear to say different things about global warming.

The amount of land in the high Arctic covered by trees and upright shrubs could increase by as much as 52 percent by midcentury, warming the region to levels climate scientists had previously not expected to see there until 2100.

That's the take-home message from a new study that looks at statistical ties between climate and vegetation types to estimate how the Arctic's landscape could change with global warming. The impact of the vegetation changes on the region's climate not only would be felt at lower latitudes through changing atmospheric circulation patterns, researchers say. The changes also would affect the range and types of wildlife in the area and the livelihoods of the Inuit who rely on the wildlife for food.

The results are appearing just as a new study from the bottom of the world offers an explanation as to why warming in Antarctica might appear to some people to be on hold, given a 20-year trend of expansion in winter sea ice.

Taken together, the two studies highlight the ways in which human-triggered warming averaged over the entire planet can play out in unique ways in specific regions of the globe – in this case, two regions that play a critical role in Earth's climate system as "sinks" for heat generated in tropics.

At the top of the world, warming at the surface has occurred at nearly twice the rate of warming as the world as a whole. Some studies indicate that the winter temperatures have been rising at least four times faster than the summer temperatures. This warming has brought trees and woody, upright shrubs to areas once dominated by tundra.

Previous studies of the impact of a greener Arctic on the region's climate indicated the trend would reinforce warming.

On the one hand, a green canopy could shade soils once the snow melts, keeping them cooler than they otherwise would be and slowing the release of CO2 from soils. But a darker canopy also would capture and reradiate heat – warming the air earlier in the spring and slowing the return of cold temperatures in the fall. In addition, during the growing season, trees give off water vapor, a potent greenhouse gas, through a process known as evapotranspiration. This also would tend to reinforce warming in the region.

Earlier studies had suggested that the factors that reinforce warming would win out, contributing 0.66 to 1.8 degrees Celsius (about 1.2 to 3 degrees Fahrenheit) to Arctic warming. But modelers had to make a best guess on how much additional land would be covered by trees and upright shrubs. They settled on an increase of about 20 percent by 2100.

A team led by Richard Pearson of the American Museum of Natural History in New York took a different approach. It used statistical tools to determine the climate conditions each of 10 broad vegetation types could tolerate. Then it used climate models to explore the range of conditions the models projected for the Arctic by 2050. The two sets of results allowed the team to estimate the new ranges for the vegetation types. Some, such as trees and upright shrubs, migrated north. Other types, in coastal regions with nowhere farther north to go, vanished.

The approach has been used for other regions, notes Scott Goetz, a senior scientist at the Woods Hole Research Institute in Woods Hole, Mass., and a member of the team performing the study. But, he adds, it's the first time anyone has applied the technique to the Arctic.

Overall, the team found that if climate-induced shifts in plant types were patchy, the changes would affect 48 to 69 percent of the Arctic regions it studied above 60 degrees north latitude. Those regions spanned northern Russia, northern Alaska, and northern Canada.

If the takeover was unrestricted by geography or other conditions, between 57 and 84 percent of the study regions would experience shifts in plant types.

For trees and upright shrubs in particular, patchy expansion would spread them across 12 to 33 percent more land than they occupy today. Unrestricted expansion would spread them over 24 to 52 percent more land.

Overall, the changes to the landscape darkened it by between 5 and 18 percent, Dr. Goetz says.

"Those are pretty significant changes," he adds.

Beyond effects on regional climate, the vegetation changes also would have a profound effect on wildlife, notes Dr. Pearson.

“For example, some species of birds seasonally migrate from lower latitudes and rely on finding particular polar habitats, such as open space for ground-nesting,” he said in a prepared statement.

Overall, the team found that the assumptions earlier modeling studies had made about changes in the distribution of plant types and their impact on Arctic warming were "realistic," but that the effects could appear 50 years sooner than the earlier studies had suggested.

The team formally reported its results on Sunday in the journal Nature Climate Change.

Meanwhile, at the globe's southernmost continent, scientists have been trying to explain why, in the face of global warming, the extent of Antarctica's winter sea ice has been expanding and sea-surface temperatures have been cooling. Some skeptics have seized on this as evidence that the planet as a whole really isn't warming. But if a new study is correct, the main reason for the expanding sea ice in winter is melting ice shelves in the summer.

Some scientists have attributed the trend in expanding winter sea ice to a long-term intensification of the westerly winds that travel unimpeded around the globe over the Southern Ocean. These have intensified over the past 20 years, in effect making it harder for circulation patterns at lower latitudes to transfer heat to the southern polar region. Other scientists have attributed the ice increase to falling sea-surface temperatures.

But a team led by Richard Bintanja, a researcher with the Royal Netherlands Meteorological Institute, used modeling studies to suggest that the main culprit for sea-ice expansion appears to be meltwater from Antarctica's ice shelves – floating outlets on the continent's coast for glaciers fed by the continent's two vast ice caps.

The shelves are melting from the undersides as relatively warm water at depths between about 300 and 1,000 feet comes into contact with them. That water has come from lower latitudes where global warming has warmed ocean temperatures, the team notes. By some estimates, Antarctica is loosing ice mass at a pace of about 250 billion tons a year.

As the ice melts, the team found that the fresh, buoyant meltwater forms a layer on the sea surface. The freshwater layer mixes with the salty seawater, notes Dr. Bintanja in an e-mail. But it still retains a higher freezing point than the saltier water below.

As the austral autumn begins, cooling air temperatures have a new season's worth of relatively fresh surface water to freeze. Farther down, the mixture of meltwater and seawater also serves as a thick lid, making it difficult for warmer, denser layers below rise and release heat to the atmosphere.

Although the sea ice virtually vanishes in the austral summer, the expanded winter sea ice still plays a key role in reflecting sunlight back into space in the spring and fall, Bintanja says.

Ironically, the expanded ice also may contribute to the loss of ice from the continent, the team suggests. Aside from cooling the springs and falls by reflecting sunlight back into space, the extended sea ice also prevents seawater from evaporating. And the ice keeps air temperatures colder than they might otherwise be, reducing the amount of moisture the overlying atmosphere can hold.

These may be combining to reduce the amount of moisture in the atmosphere available for snow over the continent – snow that would help replenish the continent's ice caps.

The results appear in Sunday's issue of the journal Nature Geoscience.