Building a railway across the unstable soil of the Tibetan Plateau was an improbable endeavor from the start, but an army of Chinese government engineers did it anyway.

Now, with the frozen soil disturbed by the process of laying down the rail and a warming climate on the plateau, some scientists question whether the $4-billion rail line will survive as is or require major reconstruction.

Three years after the railway opened in 2006, international research shows that the Tibetan territories are among the fastest warming, and fastest melting, on the planet. The research into the fate of glaciers and the permafrost soils—done by the United Nations, China's scientific agencies, and several independent scientists—is not focused on the railway. But the work raises concerns that the warming ground could lead to a buckling of the railway.

According to a 2007 global outlook from the U.N. Environment Programme (UNEP), the frozen soil of the Tibetan plateau has warmed about 0.3 degree Celsius over the past 30 years—after the poles, faster than anywhere else on the planet. Where human activity has disturbed the soil, such as during the construction of the railway, the rate is double, 0.6 degree C.

That might not seem like much, but it is enough to outpace the rate predicted by railway construction engineers for the landmark rail line, which has carried some six million passengers and five million tons of cargo since opening day. And the news would seem to get worse: UNEP says the permafrost area surrounding the nearby Qinghai–Tibet Highway decreased some 36 percent in size in the 20 years leading up to 1995, the period for which data were recorded. By the end of this century, the report says, China's permafrost (which is almost entirely on the Tibetan plateau) could decrease by half again. By 2050, another U.N. report predicts, the glaciers on the plateau will have shrunk by one third.

Perhaps that outlook can explain the warning offered by the China Meteorological Administration at a meeting in the Tibetan capital of Lhasa in early May that Tibet was warming faster than anywhere else in China. According to an account published by the Xinhua News Agency, China's state media service, the administration's chief, Zheng Guoguang, said that the railway may be in jeopardy and that the region must "tackle" the effects of climate change. Zheng warned that, where the railway crosses plateau regions, the thickness of the permafrost had been decreasing by as much as 25 centimeters each decade.

Melting point
Other research points to the railway itself as a contributing factor. A 2007 paper published by the University of Colorado at Boulder's Institute of Arctic and Alpine Research indicated that the more delicate, warm permafrost areas are more affected by the climate warming because their natural balance has been upset by the construction's disturbance of the land. Whereas it would take about 20 years for the warm permafrost regions to thaw under present climate change conditions, the paper says it could take just five years for that permafrost underneath the disturbed land to reach the melting point. The study, published in Arctic, Antarctic, and Alpine Research, was written in part by Chinese scientists close to the railway project.

That research, however, is general to the region and does not examine the railway itself. Nor does it account for effectiveness of the methods that engineers used to cool the soil while they built the tracks. Methods included using pipes called thermosiphons along the sides of the tracks to refrigerate vulnerable parts of the soil along the highest parts of the plateau, an area that comprises the largest continuous sub-Arctic permafrost region on the planet. These cooling sticks are 7.6-meter-long steel tubes drilled into the soil; they contain ammonia, which draws latent heat out of the soil as it evaporates. The technique has been used along the Trans-Alaska Pipeline and in Siberia, although those projects relied on the cooling sticks to a far lesser extent than the Tibet project and were built in colder climes.

The skeptic
The railway, which connects Lhasa to the China's national railway network, the largest in the world, traverses the Tibetan Plateau 5,182 meters above sea level and crosses about 550 kilometers of some of the most delicate and treacherous frozen soil regions on Earth. Half of the permafrost beneath the rails is within –1 degree C, meaning that it could melt with just a degree of climate change.

Some question whether the engineers who planned the railway made the proper calculations to account for long-term climate change. The project's chief engineers counted on cooling sticks and other tricks to help the rails withstand soil warming of 0.2 degree C and air warming of 2 degrees C on the plateau over the next 50 years. The figures—decided on in early 2003—were the least conservative, and thus the least expensive, to accommodate.

In early 2006, as the five-year construction project finally neared completion, a Chinese Academy of Sciences (CAS) engineer named Wu Ziwang predicted that the rate of change was more serious, concluding that the frozen ground supporting the railway could be soaked with puddles within a decade. A lone voice at the time, the researcher was admonished for his candor, rebuked by his department, and later declined to be interviewed for this story when I visited his CAS office in Lanzhou, located deep in central China.

Out of danger?
Clearly, not everyone believes the railway is in imminent danger. According to Cheng Guodong, one of the project's master scientists, the permafrost directly under the rail bed may be thinning, but it is holding firm, and that bodes well for its ability to withstand further warming. In his 2006 paper published in the journal Cold Regions Science and Technology, he wrote, "The temperature of the permafrost under the duct…decreased remarkably." As a result of this, according to the paper, the frozen layers have spread upward in some places, freezing the dirt infill of the rail bed itself.

Guodong explains that the construction impact posed a greater threat to the plateau permafrost than global warming. "The first two to three years might be the most dangerous period," he wrote in a translated e-mail in May. "If we had not considered the influencing factors well enough, thermal and stress adjustments would tell us."

If the railway has done well this far, as he says it has, then it will likely withstand whatever stresses the climate throws at it next—at least for awhile.

Abrahm Lustgarten is a reporter at ProPublica and is the author of China's Great Train, Beijing's Drive West and the Campaign to Remake Tibet, which was released in paperback in May.