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Cameroon's Deadly Lake Bomb Has Been Diffused
Cameroon's Deadly Lake Bomb Has Been Diffused
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Субтитры (277)
0:00
On the morning of August 22, 1986, a local doctor was riding
0:04
his bike towards the village of Nyos in northern Cameroon,
0:07
when he spotted a dead antelope along the side of the road.
0:10
As he pressed on, he found two dead rats, then a dead dog,
0:14
then a whole host of other dead animals.
0:17
At first, he thought they might have
0:18
been killed in a freak lightning strike.
0:20
But as he arrived in Nyos, he soon realised that
0:23
whatever had killed these animals had extended
0:26
its reach much farther than lightning could.
0:28
Among the huts, there were dead bodies strewn everywhere.
0:31
Human bodies. Not a single person was left alive.
0:35
But there was no sign of what had killed everyone.
0:37
The doctor ran for his life.
0:39
Over the next few days, it became clear that
0:41
the people of Nyos and several other nearby
0:44
villages had asphyxiated from breathing in carbon dioxide.
0:47
But figuring out what caused this tragedy took much longer.
0:50
And it would be more than three decades before
0:53
villagers could be sure that the area was safe again.
0:56
This is the story of the Lake Nyos Disaster, and the surprisingly
1:00
low-tech method of preventing a similar tragedy from happening again.
1:05
[♪INTRO]
1:07
Located in Central Africa, Cameroon is a country with a bit of
1:11
everything: desert, savannah, mountains, and tropical rainforest.
1:15
As a result, it’s often described as ‘Africa in miniature’.
1:18
And among that varied landscape, it also has volcanoes.
1:22
The so-called Cameroon line is a nearly straight line of
1:25
volcanic rocks that scientists think formed when the entire
1:29
continent of Africa rotated about 80 million years ago.
1:32
For context, that’s like 30 million years after
1:36
Africa and South America had their major breakup.
1:39
All that rotation failed to rip Africa itself apart, try as it might, but it did
1:44
cause enough magma to spill forth and make a bunch of volcanoes.
1:47
In the ocean, volcanoes along the Cameroon line
1:50
formed several islands, including Sao Tome and Principe.
1:53
And on land, we got the Oku Volcanic field, which extends for
1:57
around 100 kilometers along Cameroon’s northwestern border.
2:00
Within this region, there are a bunch of big volcanoes,
2:03
like Mount Oku, Mount Babanki and Mount Nyos.
2:07
And pretty much the whole landscape
2:09
has been shaped by volcanic activity.
2:11
In particular, there are more than a thousand cinder cone volcanoes,
2:14
and around 50 volcanic lakes called maars,
2:17
which have been blasted out of the bedrock by explosive eruptions.
2:21
One of those maars is Lake Nyos, which is more than
2:24
200 meters deep and covers an area of around 1.5 square kilometers.
2:28
Scientists think a volcanic eruption excavated the lake’s crater in
2:32
the granite bedrock about 12,000 years ago, and the water inside
2:36
it is held back by a rim of material left behind by pyroclastic flows.
2:41
But while things might have seemed quiet at the surface
2:44
over the last few millennia, at the bottom of Lake Nyos,
2:47
a disaster had been brewing.
2:49
See, there’s a network of hydrothermal cracks and
2:52
vents which underlies the entire Oku volcanic field.
2:55
And it turns out that one of those vents
2:57
releases into the waters of Lake Nyos.
2:59
Quietly, these so-called soda springs had been
3:02
leaking carbon dioxide gas out of the lakebed.
3:05
Now normally, that wouldn’t be too much of a problem,
3:07
since the gas can dissolve in water.
3:09
And the pressure of 200 metres of lake on top of it
3:11
is enough to keep it in place at those low depths.
3:14
But that all changed on the night of August 21st, 1986.
3:18
Around 9:30pm, a giant cloud of carbon dioxide was suddenly
3:22
expelled from the lake at up to 100 kilometers per hour.
3:26
It filled the lake basin, extending 100 meters above the crater rim.
3:30
But then, because carbon dioxide is heavier than air, that cloud
3:34
spilled over the northern rim and surged down across the landscape.
3:38
Travelling at up to 50 kilometers per hour, and with
3:41
a thickness of about 50 meters, the cloud of gas filled the valleys,
3:45
flowing through the nearby villages of Nyos, Chah, and Subum.
3:49
And everywhere this concentrated cloud touched, it brought death.
3:54
Normally, CO2 makes up just a tiny fraction of the air we breathe:
3:58
around 400 molecules in every million.
4:01
But when that concentration rises,
4:03
the human body can quickly become overwhelmed.
4:05
We’re constantly making carbon dioxide in our cells, but our circulation
4:09
and respiratory systems work hard to get it out as soon as possible.
4:13
In fact, when we hold our breath, the signals we get from
4:16
our brain are responding to an increase in the amount of
4:18
CO2 in the blood, rather than a lack of oxygen.
4:21
When the carbon dioxide levels outside the body rise,
4:24
you start experiencing some of the same symptoms.
4:27
Like a desperate need to breathe called air hunger
4:30
a fast heart rate, and fast breathing.
4:32
However, at high enough concentrations,
4:34
the carbon dioxide levels in the air match
4:37
the levels coming out of your body.
4:38
There’s no concentration gradient for
4:40
the gas to diffuse out of your blood.
4:42
Meaning you can’t get rid of it.
4:44
According to the CDC, carbon dioxide is immediately
4:47
dangerous to life and health when it hits 50,000 parts per million,
4:51
or around 5% of the air you’re breathing.
4:53
That’s more than 100 times its natural concentration.
4:56
At this point, CO2 levels inside the body keep rising,
5:00
from both internal and external sources.
5:02
Any gas dissolved in your body fluids increases their acidity,
5:06
which has a profound impact on how critical body functions work.
5:09
Enzymes and other proteins lose their shape, so they stop working.
5:13
Blood cells can’t carry oxygen as effectively.
5:15
You start to feel fuzzy and light headed
5:17
as your cellular machinery shuts down.
5:20
Eventually, the neurons in your central nervous system stop firing,
5:24
sending your body into unconsciousness, coma, and potentially death.
5:28
This can happen in mere minutes when
5:30
the concentration hits 10% of the air.
5:32
And of course, if carbon dioxide is the main gas you’re breathing in,
5:37
then you’re not going to be getting the
5:38
oxygen your cells need to keep functioning.
5:40
At Lake Nyos, experts estimated that around 1.2 cubic kilometers
5:45
of carbon dioxide was released from the lake into a single cloud.
5:49
That cloud would have started out at 100% CO2,
5:52
and because of how fast it moved,
5:54
it wouldn’t have had much chance to mix with
5:56
the surrounding air until it was very spread out.
5:59
So the inhabitants of the surrounding villages
6:01
would have had little warning, and no chance to avoid
6:04
the invisible cloud of death that descended from the lake.
6:07
In total, around 1,800 people were killed,
6:10
along with roughly 8,000 livestock animals
6:13
and uncounted numbers of wildlife.
6:15
It took over a day for a rescue group to arrive,
6:18
and nobody is sure of the precise death toll.
6:20
People had already started burying the bodies in mass graves,
6:23
and many survivors had simply fled the area, not knowing what
6:27
had caused the tragedy that had killed their friends and family.
6:30
It was clear that all those people had been killed
6:33
by breathing in carbon dioxide, and experts were
6:36
pretty sure it had something to do with the volcanic
6:38
lake at the center of the disaster zone.
6:41
But they couldn’t agree on exactly how.
6:43
Seven months after that tragedy, in March 1987,
6:46
Cameroon’s president hosted an international science
6:50
conference for 100 experts from a range of disciplines,
6:53
to unravel the geological mechanism of CO2 release.
6:56
And we’ll get to the results of that unraveling,
6:58
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7:47
At the end of the 5-day meeting, the scientists had basically settled
7:51
into two camps: one which thought the carbon dioxide had come from
7:54
a volcanic eruption beneath the lake, and another which blamed the
7:58
gradual buildup and release of dissolved gas in the lake itself.
8:02
The eruption hypothesis fits with what
8:04
we know about how volcanoes work.
8:06
Without warning, hot magma underground can superheat groundwater,
8:10
causing it to flash into steam and explode upwards,
8:13
carrying volcanic minerals and gases into the air without any lava.
8:17
This is what’s known as a phreatic eruption.
8:19
Some survivors reported a sulphurous smell in the air at the time
8:22
of the disaster, while others heard rumbling and saw a surge of water,
8:26
both of which seem to fit with a volcanic eruption.
8:29
But on the other hand, there was no ground movement
8:31
noticed by survivors, or recorded by the
8:33
nearest seismometer 200 kilometers away.
8:36
Plus, later geochemical measurements found
8:38
there were none of the other volcanic minerals
8:41
you’d expect to see left behind in the lake water.
8:43
So instead, the other camp of experts was convinced the
8:46
carbon dioxide had been gradually seeping into the lake
8:50
and saturating the bottom layers with the gas,
8:52
until some external force triggered its release.
8:55
At the lake’s maximum depth of around 200 meters,
8:58
water can hold up to 15 times its own volume of carbon dioxide.
9:02
And it’s been estimated that every liter of water in the lower parts
9:06
of Lake Nyos contained between one and five liters of CO2.
9:10
The weight of water above this saturated layer would have acted like
9:13
a cap on a soda bottle, keeping all of that carbon dioxide inside.
9:17
But if the status quo is ever disturbed,
9:20
if something displaces the gas-rich layer at the lake bottom,
9:24
that layer rises up and suddenly experiences less pressure.
9:28
Like opening that soda bottle, the dissolved carbon dioxide
9:32
starts to come out of solution and forms bubbles.
9:34
Those bubbles rise turbulently, pulling in more gas-rich water
9:38
and forming ever more bubbles, and so on and so on.
9:41
So the final result is a huge outburst of carbon dioxide at the surface.
9:46
All that fits with the observations from Nyos as well:
9:49
the rumbling and surging of water could have come
9:51
from gas bubbling violently out of the lake surface.
9:53
Plus, the lake level had dropped by about
9:55
a meter after the event, which could be explained
9:58
by the loss of all that gas from the lake’s bottom waters.
10:01
Today, most scientists have come to agree that this is
10:04
the most likely explanation for the Lake Nyos disaster.
10:07
And it’s not the only one.
10:09
Two years earlier, 37 people died from carbon dioxide expelled from
10:13
another maar lake, Lake Monoun, less than 100 kilometers away.
10:17
The term limnic eruption, literally “an eruption from a lake”,
10:21
was coined to describe these two carbon dioxide emission events.
10:25
They are, however, the only limnic eruptions
10:28
in history that scientists are aware of.
10:30
What still remains unknown is what triggered the
10:33
release of gas from Lake Nyos’s bottom in the first place.
10:36
It may have been a landslide, since a relatively recent scar
10:39
was spotted at the edge of the lake during aerial surveys,
10:42
but nobody knows exactly when it happened.
10:44
Alternatively, it may have been an influx of
10:47
water from a ton of recent rainfall.
10:49
The evening of the disaster was reportedly a rainy one,
10:52
and the cold, fresh water could have sunk to
10:54
the bottom and disturbed the lake’s lower layers.
10:57
Or there could have been no external trigger at all!
10:59
The bottom layers could have just become oversaturated,
11:02
and any further carbon dioxide was forced
11:04
to form bubbles despite the pressure.
11:06
Without any certainty about causes or triggers,
11:09
the surviving population around Lake Nyos
11:11
were understandably cautious.
11:13
Fifteen thousand survivors fled the area, and over 4,000 villagers
11:17
were resettled to temporary camps in the first week after the disaster.
11:21
Eventually, they were relocated to permanent
11:23
sites far from the potentially deadly lake.
11:26
But these villagers had lost their ancestral homes,
11:29
along with prime agricultural and pastoral land
11:32
that they depended on for their lives and livelihoods.
11:35
So at that 1987 conference, a long term solution was sought
11:39
to ensure a disaster like this would never happen again.
11:42
The solution turned out to be a bunch of flexible
11:45
plastic pipes and a controlled recreation of the
11:48
process that caused the eruption in the first place.
11:50
It was nicknamed the Lake Nyos Organ Pipes project,
11:53
and tested throughout the 90s. Permanent installations started in 2001.
11:57
Essentially, the project featured long plastic pipes installed
12:00
vertically into the waters of both Lake Nyos and Lake Monoun.
12:04
Using a small pump, operators could trigger
12:06
the creation of bubbles at the bottom of the pipe.
12:09
These bubbles would expand and rise, pushing the gas-water mixture
12:13
up the pipe and pulling more carbon dioxide-rich water into the bottom.
12:17
This created a self-siphoning process
12:19
that continued without any external input.
12:22
Up at the surface, the gas and water was expelled from
12:25
the pipe with such force that it created a continuously flowing jet,
12:29
basically a fountain, in the middle of the lake.
12:32
When this permanent siphoning began,
12:34
powered by high gas concentrations,
12:36
the jet reached a height of 46 meters above Lake Nyos’s surface.
12:40
But over time, the gas concentration and therefore the jet height
12:44
decreased until it was just a spray less than a meter high.
12:48
So we had a clear visual indicator that
12:50
at least some progress was being made.
12:52
And measurements of the gas concentration at the bottom of
12:54
the lake confirm the success of the degassing project.
12:57
In 2001, water at the bottom of lake Nyos started out
13:00
containing 10 liters of carbon dioxide for every liter of water.
13:04
That’s worryingly close to the saturation point.
13:07
But by 2018, this had been reduced to
13:09
just 1.35 liters of CO2 per liter of water.
13:13
Lake Nyos and Lake Monoun won’t ever be carbon dioxide-free.
13:16
The volcanic gas leaking in from the
13:18
Oku Volcanic Field will make sure of that.
13:20
But now, the hazardous levels have been reduced,
13:23
and the carbon dioxide levels can’t rise higher because any excess
13:26
will immediately be degassed through the permanent organ pipes.
13:29
With just a few plastic pipes in each lake, and a clever bit of physics,
13:33
the threat of another limnic eruption has been eliminated.
13:36
Most natural disasters are unexpected and uncontrollable.
13:40
But just this once, it seems scientists and
13:44
engineers have triumphed over the Earth.
13:52
[♪OUTRO]
