The massive flooding that struck the Himalayas near the border between Nepal and China's Tibet was unlike an ordinary flood caused by heavy rain alone. Enormous masses of glacier ice and rock collapsed down the mountainside and poured into rivers, turning into a giant torrent that pushed water, mud and rock forward all at once.
Nepal's disaster management authority put the death toll inside Nepal at at least 1,114 as of the 2nd. The number of missing stood at 3,916, including 590 foreigners. Adding 546 missing in China, including 261 foreigners, and 16 deaths there, the combined toll came to 1,130 dead and 4,462 missing.
Tremors that the U.S. Geological Survey initially detected as an earthquake of about magnitude 5.2 were later assessed to have been caused by the collapse of a large mass of glacier ice and rock. Britain's Guardian reported that part of a glacier broke off a mountain in Nepal's Langtang National Park and poured into a valley, after which ice, rock, water and sediment surged into the Trishuli River system. According to the International Centre for Integrated Mountain Development, river levels in some areas rose by as much as 9 meters in just 30 minutes.
What draws scientists' attention is the possibility that climate change lies behind such disasters. While no single glacier collapse can be attributed directly to global warming, rising temperatures melt glaciers and thaw the permafrost that had held ice and bedrock in place, potentially destabilizing entire high-altitude areas. That raises the risk of what is known as a cascading disaster, in which a glacier collapse spreads into landslides, debris flows and floods.
The Guardian, citing scientists, reported that unusually high temperatures in the region this year may have melted ice and weakened the bonds holding glaciers in place. Reuters reported that Nepal has lost about one-third of its mountain ice over the past three decades and that climate-driven glacier hazards are growing in the Himalayas. A United Nations official said the risk of glacial flooding in the Himalayas could become far more severe in the wake of the disaster.
The warning from Nepal is not confined to the Himalayas. As the climate shifts rapidly, some argue that safety standards for large water infrastructure, built on past rainfall and river flow records, need to be re-examined for whether they still hold.
One prominent example is the Three Gorges Dam on the Yangtze River in China's Hubei province, among the world's largest hydropower and flood-control facilities.
Completed in 2006, the Three Gorges Dam stands 181 meters high and 2,335 meters long, measuring 40 meters wide at the top and 115 meters at the base. Its total storage capacity is about 39.3 billion cubic meters, with 22.15 billion cubic meters reserved for flood control. It was designed against what is called a 1,000-year flood, corresponding to an annual probability of 0.1%. Cao Guangjing, who oversaw the construction, said at the time of completion that the dam was designed to withstand a flood occurring once in 1,000 years and was fully earthquake-resistant, adding that it would not collapse under any conventional weapons attack short of a nuclear strike.
A 1,000-year flood does not mean that humans have observed floods for 1,000 years, nor that a flood of that size occurs exactly once every millennium. It means that statistical analysis of past flood records puts the chance of a flood above a certain scale at 0.1% in any given year.
Because the dam blocks off a vast river, the possibility of an accident has surfaced in news coverage whenever a typhoon or torrential rain hits.
There is no scientific evidence that the Three Gorges Dam is currently at risk of collapse, but scientists say the 1,000-year flood calculation needs to be redone. Conventional flood frequency analysis assumes stationarity — that the statistical properties of past rainfall and floods will not change much in the future. If climate change alters the intensity and frequency of extreme rainfall, however, there is no guarantee that a flood calculated from past records at an annual probability of 0.1% will keep occurring at that same rate.
Research findings supporting such concerns are emerging. In June, the Journal of Hydrology, an international journal specializing in water resources, published research finding that flood magnitudes in the Yangtze basin could increase by about 13% to 16.1% from current levels because of climate change. Operating more than 300 dams in the basin together, including the Three Gorges Dam, would cut flooding by about 21%, the study found, but if climate change intensifies, future flood peaks — the maximum discharge during a flood — could run as much as 8.59% higher than in past records.
Recent research on the Three Gorges reservoir itself points to the same shift. The dam's original 1,000-year design flood was calculated from annual maximum flood data for 1877 to 1990 held by water agencies, along with historical flood records. But recent studies argue that because the Yangtze's hydrological environment has kept changing since the 2000s due to climate change and large-scale dam construction upstream, assessments must reflect non-stationarity — risk that shifts over time — rather than fixed historical statistics alone.
Even so, the glacier collapse in Nepal and the possibility of a Three Gorges Dam failure are not directly linked scientifically. The mechanisms of the two disasters differ, and no sign has been found that the dam is in immediate danger. What the Nepal glaciers and the Three Gorges Dam share, in terms of risk, is warming.
The more fundamental question raised by the Nepal disaster is how far past experience can guarantee future safety. If warming destabilizes glaciers and bedrock and changes the very patterns of extreme rainfall and flooding, the meaning of figures such as a 100-year or 1,000-year flood, calculated from several decades to a century of data, may change as well.
Experts stress that dam safety in an era of climate change cannot rest on the claim that a structure is safe because it was built to withstand a 1,000-year flood. Design floods, reservoir release rules, flood forecasting models and early warning systems must be recalculated continuously in line with a changing climate, they say. The Nepal disaster, in which a glacier collapse escalated into catastrophe, poses the same question to the Three Gorges Dam, one of the world's largest: can the future be judged by the safety standards of the past alone?
