The Tropical Lake Problem
Lakes in most parts of the world stir themselves regularly, thanks to a remarkable property of water. Pure water grows denser as you cool it, like other substances, but just above freezing—at 4°C to be exact—it starts to expand again. (This is different from its expansion when it freezes into ice, another of water's remarkable properties.) Thus when a lake's surface reaches 4°C, that water sinks and pushes the bottom water upward. Nearly all lakes therefore turn over their waters every autumn and spring. The rare exceptions, called meromictic lakes, usually have dense salt water at depth that resists overturning.
Lakes in the tropics don't get cold enough for this vigorous 4°C overturning, although ordinary cooling or winds do the same thing more gently. But a few deep tropical lakes cannot overturn fast enough to avoid a dangerous buildup of dissolved gases.
Nyos and Monoun
Lake Nyos and Lake Monoun are volcanic crater lakes in Cameroon. Both are fed from below by volcanic groundwater charged with carbon dioxide. A deep dense layer of gas-saturated water slowly builds up on the lake bottom where the high pressure, like that in a corked bottle, allows the water to dissolve large amounts of CO2.
At some point, something happens to release the pressure. It may be a landslide that stirs the water, or a strong windstorm, or an earthquake or even a drought that lowers the water level. Once the first bit of gas-charged water starts to fizz, it rises and releases even more gas, starting a runaway eruption that doesn't stop until all the deep water is stirred and degassed: a lake eruption.
Carbon dioxide is denser than air, so when Lakes Nyos or Monoun erupt they send an invisible flood of choking gas over their rims. It pours downslope and smothers the life out of every animal in its path. Lake Monoun erupted in 1984, suffocating 37 people and an unknown number of wild animals and livestock. Lake Nyos did the same two years later, killing some 1700 people.
Since then, scientists have floated large pipes in both lakes that tap the deep water and vent its gas in harmless, self-sustaining fountains. Both lakes have been defused. That leaves the third lake.
Kivu
Lake Kivu is another deep tropical lake on the other side of Africa, at the border of Rwanda and the Democratic Republic of the Congo. It differs from the Cameroonian lakes in being much larger. Also, it's not a crater lake; rather, it sits in a strand of the Great Rift Valley, where volcanic activity is widespread. And some 2 million people live around it, at risk of a truly gigantic eruption.
Lake Kivu's chemistry is different, too. It has a deep layer of CO2, but in addition there is a layer above that charged with methane gas, the product of anaerobic bacteria that make a living from CO2. Deadly hydrogen sulfide gas is also involved. And salt springs on the lake's bottom, tapping volcanic deposits, add their stabilizing effect to the mix.
The methane has aroused interest as a cheap source of energy in a region that sorely needs it. But if pumping up this saturated water were to trigger a methane eruption, the whole gas cloud could burst into flame. Worse, the underlying CO2 could erupt too to choke whatever hadn't burned.
Fortunately, the lake sediments tell us that eruptions are not common, occurring on the order of every thousand years or so. Unfortunately, the lake is very much larger than Nyos and Monoun and no one knows the best way of controlling it. That will require a concerted program of study with help from the developed nations.
PS: Erupting lakes are rare today, but what about times in geologic history when the Earth was much warmer? There is said to be evidence, for instance, that the ancient lake in Germany in which the Messel Shale fossils were preserved suffered from eruptions. That could not possibly happen there today. Indeed, it's postulated that the very ocean itself could have undergone methane eruptions.
