The Lake Nyos gas disaster

Abstract

The Lake Nyos disaster, which killed an estimated 1250 people on the 21st August 1986, was caused by the release of a toxic aerosol of water and carbon dioxide.

Lake Nyos is stable, under normal circumstances, despite being highly charged with carbon dioxide, as it was both before and, to a lesser extent, after the disaster. This is a consequence of an increase in water density with depth, due to dissolved carbon dioxide (and other solutes), which is greater than the density decrease due to the thermal gradient.

During each wet season a slightly dense but metastable layer of cold water builds up on the lake surface and the prevailing winds can cause this layer to migrate and thicken towards the southern part of the lake. What disturbed the lake’s stable stratification in 1986 can never be known, for certain, but if the winds were more steady and persistent than usual it is possible that the surface layer thickened more than usual, became unstable, and sank. A large volume of cold water sinking in the southern part of the lake could have created a circulation cell within the upper part of the lake and cause gas-rich water to rise in the north-eastern part of the lake.

Raising carbon dioxide rich water started to release gas as the hydrostatic pressure decreased. As the exsolved bubbles rose, they increased the convective flow and helped to drag more gas oversaturated water towards the surface. The rising water was cooled, partly by the exsolution of gas and partly by the adiabatic expansion of that gas, and sank to progressively greater depths as the rate of gas release, and consequent cooling, increased. The sequence thus fed on itself and led rapidly to a runaway degassing of part of the lake. At the surface the vigorous release of gas generated a wave of water which swept into the valleys to the south. The gas itself was not only cold, when it was released, but some of the water accompanying it was transformed into a fine mist, thus generating the cold dense aerosol of water and carbon dioxide which swept down the valleys to the north, through Nyos and on to Subum, Cha and Fang, leaving death and devastation in its wake.