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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 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. Barbara on the bubble Mikkelson Additional information: Degassing Nyos Degassing Lake Nyos
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