Limnic eruptions, or lake overturns, are a rare natural disasters where large quantities of carbon dioxide are released from deep lake water resulting in a deadly cloud of CO2 which suffocates all in its path.
Thirty-seven people were killed as the result of an overturn on the shore of Lake Monoun in Cameroon on August 15, 1984 and 1,800 people and 3,500 livestock animals were killed in villages near Lake Nyos in Cameroon during a limnic eruption on August 21, 1986. After these events another lake in Africa, Lake Kivu, was determined to have the same potential for an eruption with over two million at risk. Lake Kivu is also in central Africa on the border between the Democratic Republic of the Congo and Rwanda.
Exploding lakes have certain features in common. These lakes are positioned over pools of magma released from a nearby volcano. Gases seep up through the bottom of these crater lakes and dissolve into the water as HCO3, bicarbonate. The bicarbonate is converted to CO2 by acidity as sulfur, also present in a volcanic setting, oxidizes.
These lakes must have a layer of deep, cool water that is not overturned by seasonal temperature changes. This lack of mixing allows this deep layer to become saturated with CO2. The water must also be deep enough to maintain a level of pressure sufficient to keep a large quantity of gas in solution.
In order for an event to occur, there must be two thermal layers with different levels of saturated gas. Saturated water, rising for any reason, must encounter conditions of lower pressure and less saturation for a violent CO2 release. If the water is evenly saturated throughout its depth any disturbance would not cause the gas to explode out of the water. It would, instead, convert to gas at the lake surface and release steadily into the atmosphere.
Once the deep layer is saturated the condition of the lake is critical. Any nearby volcanic activity, a landslide or an explosion can act as a trigger. It is thought that even violent wind and rainstorms can trigger an overturn although if the water is 100% saturated the water can begin to bubble on its own without an external trigger.
As soon as the deep layer is agitated with saturated water rising, the pressure lessens and the gas comes out of solution and begins to erupt with force. A column of gas begins to rise, pulling even more water up which also gives up CO2 until there is a full eruption. This process also causes a tsunami that washes well beyond the usual borders of the lake adding to the devastation.
These events have been compared to the eruption of soda from an agitated container. Shaking the can or bottle causes the CO2 in a carbonated beverage to release into the liquid and once the pressure is released by opening the cap, soda fountains out of the container.
Because of this model pipes have been placed in Lake Nyos and Lake Monoun to try and provide a system of steady release and prevent a critical situation. First tested by a team from France in 1990, followed by the involvement of the U.S. Office of Foreign Disaster Assistance, pipes have been placed in both lakes. Once some water is pumped out, the natural action of saturated water rising begins, releasing CO2 and pulling water after it to form a column of water from the top of the pipe. In this way, CO2 is released continually.
Some exciting work is being done at Lake Kivu. The plan is to not only release enough gas to prevent a lake overturn but to capture the gases, in this case methane as well as CO2, in order to use it. A small local brewery has successfully used some of the methane to run its boilers but the new project will be much larger in scope.
This method will most likely work at Lakes Nyos and Monoun as well, both averting disaster to the local inhabitants as well as providing an energy source for these same inhabitants.
Addressing the other conditions of these erupting lakes is much more difficult if not impossible. Since only three lakes have been identified with the particular set of conditions needed to cause a limnic eruption these methods seem to be the best solution to prevent future disasters.