Cameroon's killer lakes: a rising threat.

In August 1984, little Lake Monoun in the African nation of Cameroon briefly captured international attention when it suddenly belched a cloud of carbon dioxide gas that asphyxiated 37 people. Scientists might have written the event off as a freak occurrence except that it happened again just 2 years later at nearby Lake Nyos. That time, more than 1,700 people perished.

Lake scientists who have studied Monoun and Nyos now say that carbon dioxide is building up in the lakes so quickly that it raises the risk of new catastrophic releases. The gas comes from springs of carbonated groundwater that percolate upward into the bottom of these volcanically formed crater lakes.

"Our recent measurements indicate that the [carbon dioxide] recharge rate is tremendously fast" says George W. Kling, a limnologist at the University of Michigan in Ann Arbor. "If these recharge rates continue, the bottom waters of Nyos will become saturated with CO2 in less than 20 years. In Monoun, it'll be less than 10 years. We didn't have any idea it was that rapid." Kling discussed the recent research results last month at NASA's Goddard Space Flight Center in Greenbelt, Md. The findings will appear in an upcoming issue of NATURE.

Gas accumulates to such dangerous concentrations in Monoun and Nyos because these lakes are naturally stratified into layers that do not mix. A boundary called a chemocline separates freshwater at the surface from deeper, denser fluids containing dissolved minerals and gases.

The Cameroon disasters occurred when some trigger - an earthquake, landslide, or even strong winds - upset the delicate balance within the lakes, creating waves that overturned the water layers. As the deep water rose, dissolved carbon dioxide came out of solution to form bubbles, just as soda fizzes when one uncaps a bottle.

At Nyos, the larger of the lakes, a dense cloud of carbon dioxide spilled over the edges of the crater and sped down a river valley at 20 meters per second (45 miles per hour). Hugging the ground, the gas cloud remained potent enough to kill people up to 25 km downstream from the lake.

Scientists do not know what particular event caused the Nyos and Monoun disasters, but they note that both occurred in August, the coolest and rainiest time of the year in Cameroon. In fact, the 1980s had record low temperatures and high rainfalls, which would have cooled the surface waters and destabilized the lake stratification, Kling says.

Although the lake turnovers in 1984 and 1986 discharged a vast quantity of carbon dioxide, dangerous amounts of gas remained trapped in the lakes even after those releases. By measuring concentrations in 1990 and 1992, Kling and his colleagues determined the rate at which gas seeps into the lake bottoms.

Kling believes that saturation of the lake bottoms will raise the odds of a gas release. "Then, any small movement of the water could trigger it," he says.

In theory, such a trauma need not happen. In 1992, a team of investigators tested a technique for lowering gas concentrations within Lake Monoun by pumping up deep water through a pipe. The scientists and engineers calculated that a system of three pipes, each 14 centimeters wide, could drain Monoun's gas in 3 years.

Nyos presents a more difficult problem. It is deeper than Monoun, contains much more trapped gas, and has a weak natural dam at one end that threatens to collapse. If the dam breaks, the upper 40 meters of the lake will spill into a region inhabited by 10,000 people, says Kling. To make matters worse, the flood would probably trigger a gas release.

Engineers could combat both threats by pumping deep water out of Nyos, thereby draining away gas and lowering the lake below the level of the dam. A conference last fall addressed the topic of reducing the threats at Nyos and Monoun. But so far, Cameroon has not funded this project, nor has any other country volunteered, Kling says.

"It's a social irresponsibility for the world to let that time bomb sit ticking away there," says Daniel A. Livingstone, a lake scientist at Duke University in Durham, N.C.