SAN FRANCISCO - Rising seas, melting polar ice caps and strange weather tend to grab headlines as Earth's climate grows warmer. But there are other dramatic outcomes that scientists are only beginning to grasp and which could damage structures in northern areas, reconfigure towering mountains and alter biology.
As winters get milder, changes occur underfoot and go largely unnoticed until critical thresholds are reached. Railroad tracks are deformed. Rocky peaks crack apart and spill into ravines. Whole mountainsides lose footing, creating flows of ice and mud that move as fast as a BMW on the Autobahn.
Some 24 percent of land area in the Northern Hemisphere is underlain by perennially frozen ground. Scientists call this permafrost. Another 57 percent -- extending down into much of the United States and Europe -- freezes seasonally.
But these numbers are changing rapidly, scientists reported here last week at a meeting of the American Geophysical Union.
Seasonally frozen areas in the Northern Hemisphere decreased by 15 to 20 percent during the 20th Century, said Tingjun Zhang of the University of Colorado at Boulder. "In the last 20 years, the decrease is more dramatic," he said.
In locations across the former Soviet Union, where long-running observations are starting to generate meaningful results, the warm-up has been documented as a 1-degree increase in the average temperature of soil 16 inches (40 centimeters) below the surface.
"The change is real," Zhang said. "It is happening."
The effect is not just in the far north. Some 80 percent of U.S. soil freezes every winter. Change to the cycle will affect crops, native plants and even how much carbon is exchanged between Earth's surface and atmosphere, Zhang and others say.
There is "widespread evidence" that global warming is responsible for the observed changes in seasonally frozen soil and permafrost, said Frederick Nelson, a geographer at the University of Delaware.
Nelson examines what happens below the surface.
Permafrost exists at depth, and the surface layer above it freezes seasonally. When the seasonal freezing is of shorter duration, owing to climate warming, the seasonal thaw runs deeper and extends into the former permafrost, Nelson told LiveScience. The active layer -- freezing and thawing each year -- grows deeper.
Because water in the soil expands when frozen and loses volume upon melting, it causes uneven movements in the ground surface. Under sustained climatic warming, the consequences of disappearing permafrost could be "very severe" for structures, Nelson said.
Thawing permafrost can render railroad tracks useless, as seen in this photo from the northern Tibetan Plateau taken in the early 1960s. Credit: Tingjun Zhang
The problem could be particularly acute for urban and suburban places in the far North, such as Barrow and Fairbanks, Alaska. Nelson notes, however, the problem can be mitigated if engineers look ahead.
Zhang is helping the builders of an ambitious Tibetan railroad do just that.
The Qinghai-Xizang railroad will be 695 miles (1,118 kilometers) long when completed in 2007. Most of it is above 13,000 feet (4 kilometers), and about half of it is being built on permafrost, much of which is likely to melt in coming years, Zhang said.
So Zhang has helped the engineers devise an insulation system -- a thick layer of crushed rock over the permafrost.
All of Nature can't be insulated, however.
Antoni Lewkowicz of the University of Ottawa has studied several northern landslides and rockslides that he says can be at least partially attributed to thinning and weakening of ice or permafrost caused by climate warming. In one case, an earthquake broke off a weakening glacier in the Yukon. About 500,000 tons of ice raced down a mountain.
"By the time it reached the bottom it would have been going about 140 mph," Lewkowicz said.
At other remote catastrophe sites, Lewkowicz has documented a bizarre situation in which thin permafrost sits atop unfrozen sand containing groundwater under pressure. The system is stable until the icy overlay gets slushy. The whole mess then gives way.
Some of these events expose a layer of earth -- perhaps a very salty layer -- on which nothing can grow for years, resulting in "profound ecological effects," Lewkowicz said.
And landslides like this could become common if the climate grows warmer, as many scientists expect it will.
Charles Harris of Cardiff University in the United Kingdom documented rockslides high in the Swiss Alps that, again, were related to thawing permafrost. During 2003, the warmest summer on record in the Alps, the slushy active layer of the permafrost moved down from its long-term average depth of 15 feet (4.5 meters) to 29 feet (9 meters).
"There is likely to be an increase in rockfalls and landslides" at high-altitude sites, Harris said.
More research is needed, the scientists agree, to understand exactly what is happening globally, what the future holds, and what might be done to mitigate certain problems.
Many parts of the planet haven't been closely examined. And there are several causes and effects that haven't been explored. Heavy rainfall, for example, could be a contributing factor to some of the landslides and rockslides, and other studies predict heavier rainfall is one possible result of climate warming.
Nelson, the University of Delaware geographer, says thawing permafrost will "profoundly affect" biological activity in ways that are not fully known.
"In the first instance, climatic warming might be expected to degrade permafrost, but the relationship may not be quite so straightforward," Nelson said. "A warming climate may also increase the number and density of shrubby plants that shade the surface, which could ultimately help to protect the permafrost. The jury is still out on a lot of this."