On August 21, 1986, seventeen hundred people who lived around Lake Nyos in Cameroon suffocated overnight – victims of an extremely rare and silent natural disaster.
I’m going to begin this blog post with a little detour into lake hydrology. Lakes can be classified into three distinct types: holomictic lakes, meromictic lakes, and amictic lakes.
Most lakes in the world are holomictic. The waters within the lake intermingle, so the whole body of water cycles around and has a similar chemical composition. They may mix all the time or only in certain seasons, but over the course of a year there is some kind of commingling between the surface and the depths. In meromictic and amictic lakes, however, mixing mostly doesn’t happen. Amictic lakes are frozen on the top. Meromictic lakes have aquatic strata.
By aquatic strata, I mean that meromictic lakes have layers of water, and these layers do not interact. Because the waters of the lake bottom do not mix with the surface waters, they tend to lose all their oxygen over time and become a kind of dead zone. And because they do not mix, sometimes they can become even deadlier.
Consider this scenario: somewhere underneath a lake there is geothermal activity. Carbon dioxide seeps up into the lake, effectively carbonating it. In a holomictic lake, this isn’t a problem: the CO2 diffuses from the surface into the atmosphere and is gone. But if the lake is meromictic… well, the carbonation builds up unchecked and unreleased in that bottom layer.
The carbon dioxide can build up for years, until something triggers a sudden mixing. It could be an earthquake, Or a landslide. And when that happens, the carbon dioxide is released all at once, as if a bottle of soda had been opened.
We know of three lakes in the world that fit such a scenario: Lake Kivu between Rwanda and the Democratic Republic of the Congo; Lake Monoun in Cameroon; and Lake Nyos, also in Cameroon.
August 21, 1986. A landslide breaks the seal on the bottom layer of Lake Nyos. Hundreds of thousands of tons of CO2 rush up from the depths, shoot into the air at 100 km/h, and then settle like a heavy fog on the land around the lake. People begin to die, suffocating in their sleep as the carbon dioxide supplants all the air.
All in all, the Lake Nyos disaster killed 1746 people; thousands more fled the area. Since that time a pipe has been installed in the lake to siphon off the carbon dioxide and prevent it from reaching such terminal levels again. But Lake Nyos isn’t the problem now: Lake Kivu is.
Lake Kivu is much larger than Nyos, 2700km2. It’s a meromictic lake, and underneath the surface huge amounts of methane and carbon dioxide are building up because of volcanic activity. Venting a lake that large would be extremely expensive, and while there have been some experiments most of the gas remains.
Two million people live on or close to the shores of Lake Kivu. That explosive venting of noxious gas is thought to occur in Lake Kivu once every thousand years or so. I hope we come up with a solution before the next one is triggered.