By Richard Black
Environment Correspondent, BBC News website
More work is needed urgently to prevent potentially fatal releases of gas from two lakes in Cameroon, scientists say.
Pressure valve: The Lake Nyos geyser vents CO2 safely to the air
Lakes Nyos and Monoun contain high concentrations of dissolved carbon dioxide. In the 1980s, thousands died when the gas was suddenly released.
Pipes have been installed to remove CO2 from the bottom of the lakes, but new research shows they are not enough.
Scientists warn more pipes must be put in place to avert the danger of further catastrophic releases of gas.
"With one pipe in each lake, we are currently removing more gas than comes in each year," said George Kling, from the University of Michigan, US.
"But in Lake Monoun, the pipe doesn't reach the bottom so we won't be able to tap all the gas.
"In Nyos, the time it will take to remove all the gas remains on the order of decades, and during that time, the danger remains," he told the BBC News website.
In years gone by, the casual observer would have noticed nothing unusual about Nyos and Mounoun, two mountain lakes in the north of Cameroon.
Now, each is home to a spectacular fountain, spouting water 50m into the air in the case of Lake Nyos, and 10m at Monoun..
They come from pipes installed to vent carbon dioxide from the depths.
The Lake Nyos geyser became operational in 2001; Monoun's two years later. The reason why they are there dates back two decades.
In 1984, a sudden release of carbon dioxide from Lake Monoun killed about 40 people by suffocation. Two years later, a similar incident at Lake Nyos resulted in much larger loss of life, estimated at nearly 2,000 people.
This is a volcanic region of Africa; gases released from magma below the lake floor percolate upwards through rock, and some, including carbon dioxide, are dissolved in groundwater, which in turn flows into the lakes.
Once in the lake, CO2 remains near the bottom, because the water in which it is dissolved is denser than the layers above, and so cannot rise and mix.
"Near the lake bottom, the gas pressure is 15 bars - by comparison, what you have in a champagne bottle is about five bars," said Professor Kling.
"The pressure builds up, and when it is eventually released, either spontaneously or through being triggered by something like a landslide, it comes to the surface in bubbles."
This is what happened in the 1980s, with fatal consequences; and what the pipes installed in the two lakes are designed to prevent.
Initially, pumps begin moving the bottom water up the pipe. But as it rises, gas comes out of solution, making the water buoyant and pushing it upwards ever faster.
The process becomes self-sustaining, and the pumps can be turned off.
With the rising water reaching supersonic speeds, it spews out above the lake surface, releasing carbon dioxide harmlessly to the atmosphere.
However, in their paper just published in the journal Proceedings of the National Academy of Sciences, George Kling and his collaborators show that the current installations are not removing gas fast enough.
The pipes were installed using rafts
They recommend lowering the bottom of the existing Lake Monoun pipe and installing an additional pipe there. Lake Nyos, they say, needs a further four pipes.
With these in place, at least 75% of the carbon dioxide would be extracted within five years.
Following the 1986 Lake Nyos incident, the Cameroonian government moved people away from the area; but some have returned, while the shores of Lake Monoun are still home to several villages.
"We estimate the extra cost to degas these lakes is around one to two million (US) dollars," said Professor Kling. "It's not very much considering all the people at risk."