One night in 1986 heavy rain pounded the land around Lake Nyos in Cameroon. A local health care worker named Emmanuel Ngu Mbi took shelter in the village of Wum and slept. The next morning he hopped onto his bicycle and began pedaling. He saw an antelope lying dead along the path, and then rats, dogs, other animals. At the next village he was astounded to see dead bodies everywhere.
Overnight a cloud of carbon dioxide, which is heavier than air, escaped from a fault, a volcanic vent that ran under the lake. The carbon dioxide displaced the air, crept along the ground and in all, some 1746 people and all their livestock were asphyxiated. This was what geologists call a limnic eruption.
Mt. Nyiragongo erupted on 22 May and scientists predict a further eruption which could come, as eruptions often do, with earthquakes. Mt. Nyiragongo looms over the border of the Democratic Republic of Congo and Rwanda at 11,385 feet, abutting the Congolese town of Goma and a short distance from the Rwandan border town of Gisenyi. Perhaps 750,000 people live in the area, and around a million more at the other end of the lake, in the DRC’s Bukavu.
Both Goma and Gisenyi share a pretty shoreline along Lake Kivu, a deep lake at the bottom of which lie layers of carbon dioxide and methane. Should any kind of disruption sufficiently disturb these lower layers, say, an underwater landslide brought on by seismic activity caused by Nyiragongo, accumulated, released gases could create clouds of CO2, like at Nyos. Mt. Nyiragongo is perhaps a dozen miles north of Lake Kivu.
Should that happen, limnologist Sally MacIntyre of the University of California, Santa Barbara says, “it would be completely catastrophic.” Whereas the Lake Nyos eruption released about a cubic mile of carbon dioxide, scientists reckon Lake Kivu contains 300 cubic kilometres.
Meanwhile tens of thousands of residents of Goma and Gisenyi are on the move because the military governor of Congo’s North Kivu province, Lt. Gen. Constant Ndima Kongba said Thursday,
“Based on … scientific observations, we cannot currently rule out an eruption on land or under the lake. And this could happen with very little, or no, warning.”