Part of the plotline of “The Blue Angels” concerns
itself with natural disasters, so I set myself the task of researching
different types. We can all name some: earthquakes and tsunamis, volcanic
eruptions, tornadoes and other types of storm, forest fires, landslips… the
list goes on. The chances are that most of you will
remember seeing reports of many of these on television, or in the newspapers,
even if the exact date or location eludes you. Most of the world’s population
lives in the shadow of at least one type of natural disaster and with climate
change tightening its grip, that proportion is increasing all the time.
Of all the ways in which Mother Earth can kill, however,
the phenomenon of the limnic eruption is one of the rarest - there have only
been two in recorded history. What might be surprising then, is that both of
these were within living memory, and both occurred within a short distance of
each other. In a limnic eruption, vast volumes of gas are suddenly released from
a body of water, suffocating surrounding animals and people. In 1984, Lake Monoun in Cameroon erupted, which killed 37 people. In 1986, neighbouring Lake Nyos erupted too, killing between 1700 and 1800
people. The second of these eruptions is known to have produced 80 million cubic metres
of carbon dioxide. So, what is it about these two lakes that makes them so
dangerous? Could the same thing happen at Windermere, Huron, Como, Baikal or
Victoria?
What Makes a Lake “Explode”?
For a limnic eruption to occur, the lake must be saturated with dissolved gas. In Lakes Monoun and Nyos the gas in question was carbon dioxide,
but at Lake Kivu (a limnically active lake that lies on the border between
Rwanda and the DRC), methane is also a worry. The dissolved gas may come from
volcanic sources under the lake bed, or from the decomposition of organic
material, or it may be dissolved in the lake’s inflow. While the lake is building
up to this point, it behaves like an unopened soda bottle: gas under higher
pressure dissolves much more readily than gas at lower pressure. This is why
bubbles in a soda can only form once the can is opened, as the release of
pressure forces excess gas out of solution. In a lake, pressure increases
steadily with depth, reaching a maximum at the lake bed. Gases such as carbon
dioxide also dissolve more readily in cooler water, which is typically found at
the lake bottom. Hence, a large deep lake can dissolved huge quantities of gas.
Once the lake is saturated with gas, it becomes
unstable, only requiring a suitable trigger event to set off an eruption. In
the case of Lake Nyos in 1986, landslides are suspected but a volcanic
eruption, an earthquake or even wind and rain could trigger similar events in
susceptible bodies of water. The trigger event displaces some of the
gas-saturated water upwards, lowering its pressure and forcing it to release
some of its dissolved gas as bubbles. These bubbles rise, pushing more water
ahead of them, which in turn is forced to surrender some of its gas and so on.
In this way, a column of rising gas quickly forms that pulls water and sediment
from the bottom of the lake by suction, causing a runaway process. The released
gas and sediment-rich water erupts onto the lake’s surface, giving it a boiling,
dirty appearance. If the released gas is heavier than air (as is the case with carbon
dioxide) then it cannot rise further and instead begins to spread out over the
lake’s surface and ultimately onto surrounding land.
Fortunately, this type of eruption is rare. Firstly,
there must be a source of gas, predisposing regions with volcanic activity.
Secondly, lakes in the world’s temperate regions undergo regular “turn over” in
response to seasonal surface temperature changes. These changes encourage water
from different depths to mix, causing gases to be released much more gradually
and preventing saturation. Finally, the lake must be deep in order to offer the
high pressure, low temperature conditions that are favourable to limnic
activity. Together, these factors imply that limnic eruptions are only possible
in deep, stable, tropical volcanic lakes.
What are the Consequences of a Limnic Eruption?
As the released gas flows across the lake’s surface
and onto neighbouring land, it pushes breathable air upwards. Carbon dioxide
acidified body fluids and is toxic even at relatively low concentrations.
Unlike other tasteless, colourless gases such as nitrogen, high levels in the
blood causes the respiratory system to gasp, in an attempt to secure the
required oxygen, thus speeding up its toxic effects.
At Lake Nyos, the gas cloud descended from the
lake into a nearby village where it settled, killing nearly everyone.
Fatalities were recorded as far as 16 miles from the lake. Some of the bodies
exhibited a change in skin color, leading scientists to believe that the gas
cloud may also have contained a dissolved acid, but this is uncertain. Many
victims were found with blistered skin. This is believed to have been caused by
low blood oxygen. At the lake’s edge, vegetation was damaged or destroyed by a five
metre tsunami. Vegetation further away was largely unaffected, as would be
expected from a temporary glut of carbon dioxide.
In one final twist, many victims were also found
with what appeared to be frost bite: the gas released from the lake bottom would
have been very cold. Victims would have experienced a sudden and prolonged wind
of bitter cold “air”. Some survivors reported a smell of rotten eggs and feeling
warm before passing out. This can be explained by the fact that, at high
concentrations, carbon dioxide acts as a sensory hallucinogenic.
What Can Be Done?
In 1990, a team of French scientists began to experiment with a controlled degassing of Lakes Monoun and Nyos. Under this scheme, a long tube is inserted vertically into the lake, with its upper end standing clear of the lake’s surface. A small amount of gas-saturated water is then pumped upwards, causing it to give up its gas. The gas then rises as bubbles, pushing water ahead of it as before. In open water, this effect spreads outwards as well as vertically, to create the runaway reaction described above. Inside the pipe, however, this outward spread is not possible, and so fresh water is drawn into the bottom of the pipe to create a kind of siphon. Gas and water emerge from the top of the pipe and gas concentrations in the lake are slowly lowered, reducing the risk of a further eruption.
Degassing Lake Nyos |