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November 16, 2004

SMOTHER NATURE

By Barbara Mikkelson

Claim: An eruption of carbon dioxide from a lake killed hundreds of
people.

Status: True.

Origins: Carbon dioxide is a substance we generally associate with the
happy little bubbles that enliven our favorite brands of soda and
beer. We are not accustomed to thinking of it as dangerous, as most of
our interactions with it are innocuous. Moreover, memories of high
school biology remind us that while we breathe in oxygen and exhale
carbon dioxide, plants operate in the opposite fashion by soaking up
carbon dioxide and exuding oxygen, making for an efficient symbiotic
relationship between people and plants. Carbon dioxide is a natural
part of the world around us, ergo, we don't view it with the same
level of apprehension with which we regard manmade compounds.

Yet carbon dioxide is also a deadly gas. Countless miners laboring
underground have forfeited their lives to "choke damp," the term for
the oxidizing of carbon trapped within coal. When this process takes
place in an enclosed space (such as the depths of a mine), the
resulting carbon Carbon dioxide molecule dioxide cannot dissipate and
forms an invisible deadly cloud. Accounts given by people who
witnessed choke damp in action described deaths that came so quickly
the victims had no chance to escape. One person, recounting the fate
of eight men and one woman who walked into an area where the gas had
accumulated, said they "fell down dead, as if they had been shot."
Another narrative of a different death said the stricken miner was
"without access to cry but once 'God's mercy.'"

Miners not only walked into deadly accumulations of choke damp; they
were also sometimes lowered into them by being let down into mine
shafts on ropes. If they hit pockets of carbon dioxide during their
descents, they would fall from those ropes dead.

While keeping caged canaries or rats in a mine would signal the
presence of "white damp" (carbon monoxide) before it became lethal to
humans, animal warning systems were of little use against "fire damp"
(methane) and "choke damp" (carbon dioxide), the former because its
danger only came to fruition if it came into contact with open flame
(such as a candle or lamp), and the latter because it killed so
quickly as to make such warnings useless. (Carbon monoxide is the gas
utilized by those who commit suicide by piping exhaust fumes back into
their cars or running automobile engines while parked in closed
garages. Methane, sometimes known as "swamp gas" or "marsh gas," is
the major constituent of natural gas.)

Persons not employed in the coal mining trade are unlikely to
encounter deadly masses of carbon dioxide, yet such clouds have been
known to form in the open air and at a cost dear in human life. Which
was indeed the case on 21 August 1986 at Lake Nyos in Cameroon.

Carbon dioxide naturally seeps from geothermal sources below that body
of water and dissolves under pressure in the cold layer at the bottom
of the lake. The water serves to hold the carbon dioxide in place, and
over time the lake becomes infused with the compound. Once the
saturation point is achieved (when the water can absorb no more carbon
dioxide), the lake turns deadly.

On the fateful night of 21 August 1986, the deep waters of the lake
either reached their carbon dioxide saturation point or something
happened to disturb the layer lurking at the bottom of the lake (such
as a rockslide), and without warning the lake "turned over," its
bottom layer shooting to the surface in a violent, frothy eruption of
carbonated water that flew some 250 feet into the sky. The lake waters
turned red as dissolved iron was sucked up to the surface by the
turmoil.

An estimated 100 million cubic metres of gas emerged from the lake in
that explosion, quickly sweeping over the valleys surrounding Lake
Nyos and, being denser than air, sinking to suffocate the inhabitants
below.

Death came quickly. One man living just two hours on foot from the
lake said, "We heard a noise, just like a gunshot." He immediately
checked on his two young daughters, and found them already dead in
their beds.

A total of 1,746 people were smothered in the night, according to the
official casualty toll. The deadly cloud covered an area of up to 12
miles around the lake, killing thousands of cattle as well.

A similar incident in 1984 at Lake Monoun, another crater lake in
western Cameroon, killed 37 people.

Volcanic gases are not usually so toxic. Carbon dioxide is being
vented elsewhere in the region, but because it seeps from the ground,
it releases directly into the air and so poses little danger unless
one is a frog or small rabbit that wanders too close to such
discharges. At Lake Nyos, the gas was released suddenly, at a single
site, and remained highly concentrated.

In the wake of the tragedy, a number of plans have been proposed to
ensure Lake Nyos never erupts again. Pipes have been sunk into the
lake to draw off the carbon dioxide as it accumulates on the lake bed.

Concern over carbon dioxide emissions from coal (which is burned to
power electrical generation plants) has prompted a proposal to pipe
the gas into the North Sea, thereby burying it in the ocean. Carbon
dioxide capture and storage (known as CCS) plans are controversial,
with those on the one side saying it's the best way to rid ourselves
of the gases that are causing global warming, and those on the other
fretting over the potential for danger both to human life and the
environment.

See also: Degassing Lake Nyos

Sources:

Boh, Herbert. "Cameroon Marks 10th Anniversary of Lake Tragedy."
Agence France Presse. 21 August 1996.

Freese, Barbara. Coal: A Human History. Cambridge, MA: Perseus
Publishing, 2003. ISBN 0-7382-0400-5 (pp. 47-48, 238-239).

Jones, Nicola. "The Monster in the Lake." New Scientist. 24 March 2001
(p. 3636).

Krajick, Kevin. "Killer Lakes." Smithsonian. September 2003.

The Economist. "Volcanic Gas; Unsettling." 30 August 1986 (p. 62).