A limnic eruption occurs when carbon dioxide (CO2) trapped in deep lake water erupts to the surface, displacing the lake water, and possibly causing a tsunami. As a result of the CO2 released, any wildlife or humans in the vicinity die through asphyxiation, CO2 poisoning, or through drowning in the resultant tsunami. (Limnic means of or pertaining to fresh water.)
Limnic eruptions, also known as lake overturns (since the bottom water comes to the top) are so rare that only two are known to have certainly occurred. In 1984 a limnic eruption occurred in Lake Monoun, Cameroon, in Africa, which killed 37 people, and in 1986 another occurred in Lake Nyos, also in Cameroon, which killed between 1700 and 1800 people. In both cases wildlife and fish were also killed in the eruptions. These lakes, along with Lake Kivu, are known as exploding lakes.
Limnic eruptions are so rare because they require a number of conditions to be met.
1. The lake must be stable and deep or have sections of deep, cold, water. This is because CO2 only dissolves at high pressure and cool temperatures such as those of the cold water at the bottom of a deep lake.
2. There must be a high concentration of CO2 in the water so that the water at the bottom of the lake becomes saturated with CO2. This means there must be a source of excess CO2, such as volcanic activity in the lake.
3. There must be little turnover of water between the bottom and top layers of water. (This means that temperate lakes such as those in the US and Europe cannot have limnic eruptions, since there is turnover of the water at different levels.)
4. There must be a trigger for the eruption, such as a landslide, volcanic eruption, explosion, or even a severe storm. The trigger causes the water saturated with CO2 at the bottom of the lake to rise. As the water rises the CO2 is released as bubbles. This has a snowball effect with more and more saturated water rising and more and more CO2 being released. The effect is like shaking a fizzy soft drink can and then opening it.
When the CO2 is released from the surface of the lake it sits like a cloud that does not rise because it is denser than air. The cloud of CO2 asphyxiates any living trapped on the ground through displacing the oxygen and also causes CO2 poisoning that is quickly fatal. Any wildlife or humans may also be drowned by the resulting tsunami, which in the case of Lake Nyos was five metres high.
One way to avoid limnic eruptions is to degas the lake by creating a fountain that brings the CO2 saturated water to the surface in a controlled way. This is only possible however in small lakes.
Another lake in Africa, Lake Kivu (on the border between Rwanda and the Democratic Republic of the Congo), is also suspected to have had limnic eruptions in the past since there have been local mass extinctions around every thousand years, and it may be approaching another eruption. There is a large build-up of CO2 and methane in the lake, but it is too big to be degassed. Approximately two million people live in the vicinity of the lake.