- Late in the evening on August 21st, 1986,
violent explosions rocked Cameroon's Lake Nyos.
A more than 15 meter high wave of water
flattened vegetation along the southern shore,
but there was a second deadly
and nearly invisible wave of carbon dioxide
that blanketed the nearby towns,
suffocating residents and animals.
When it was all over, more than 1,700 people had died.
So how exactly does a lake explode
and shoot out a gigantic wave of deadly carbon dioxide gas?
(upbeat jingle)
Lake Nyos sits on the edge of a dormant volcano
in the country of Cameroon.
Despite its precarious sounding location,
the lake is surrounded by nearby villages.
Its nutrient-rich soil is a huge draw for farmers.
And in 1986, thousands of people lived in the area.
Now, the lake is inside what's known as a maar crater.
These are formed when hot magma
comes in contact with groundwater,
causing it to boil and then explode.
These explosions leave a pit with steep sides
and deposits of rocks around the edges,
which often fill in with water to form a lake.
Just looking at this lake, it seems really peaceful,
but deep below the surface, tens of kilometers down,
there's a magma field,
and that magma is releasing carbon dioxide, or CO2,
that slowly trickles up with groundwater.
And in a lot of lakes,
this might not be a problem
because cool air temperatures
would allow the surface waters
to drop in temperature and sink.
This would mix the layers of the lake
and allow that carbon dioxide to rise to the top
and simply float away into the air.
But the tropical climate in Cameroon
meant the surface waters stayed warm
and acted like a lid.
This was a problem because the carbonated water
at the bottom of the lake stayed at the bottom,
rather than rising to the top,
where it could release the CO2 into the air.
And at 210 meters deep,
the water at the bottom of Lake Nyos
is under extreme pressure,
which also traps that CO2 like the bubbles in a soda can.
Sometimes lots of CO2 dissolved in water
can be a good thing,
specifically in something like your can of soda.
When you crack the seal,
exposing it to lower atmospheric pressure outside,
the carbon dioxide goes from being dissolved in the liquid
to being a gas.
This is what causes all of those bubbles.
But at the bottom of the lake,
the pressurized gas kept building,
kind of like pushing more and more carbon dioxide
into that can without cracking the seal.
And the amount of any gas that will dissolve in water
at a constant temperature
is based on the pressure of the water.
Basically, solubility of a gas in a solution
increases as the pressure of that gas
above the solution increases.
This relationship is known as Henry's law.
You can calculate it by knowing a constant
that describes how that gas will behave
in water under pressure of that gas
and the pressure it's at.
For CO2, this equation looks like
the concentration of CO2 is equal to pressure over K sub h.
K sub h is Henry's constant.
And for CO2, it's 29.41 atmospheres per unit molarity.
You can see that, as pressure increases,
the concentration of CO2
that can be dissolved in a solution also increases.
And as pressure decreases,
so too does the concentration of dissolved gas.
And the carbon dioxide continued to build,
but when the lake couldn't take it anymore, it burst.
This is what's known as a limnic eruption.
It really just means a gas outburst from a lake,
much like highly carbonated soda
bursting out of an overpressurized can.
It was the lake itself,
rather than the underlying volcano, that erupted.
And it wasn't the wave of lake water that was so deadly,
but all of that carbon dioxide,
The scientists still aren't sure exactly
what triggered the eruption that day.
A landslide might've disrupted the waters
or a change in temperature could've
suddenly caused the waters to mix.
But the result was devastating.
Local residents reported two or three eruptions that night.
One resident who lived higher up
reported seeing a fountain of mist coming from the lake,
followed by a huge surge of water
washing up on the southern shore.
An estimated 1.94 million tons of CO2
were released from the lake.
This is equal to over 431,000
Olympic swimming pools full of heavy gas.
If you lined those pools up end to end,
they'd span from the North Pole
all the way past the South Pole.
T make things worse, a valley to the north
created the perfect channel
for this gigantic cloud of gas to flow downhill
and over the nearby towns,
displacing the breathable air
and causing deaths up to 23 kilometers away.
You can see how this might've happened
by looking at the vapor from this bowl of dry ice.
Dry ice is just solid CO2.
And instead of evaporating,
it sublimates, skipping the step
of changing from a solid into a liquid,
and, instead, turning directly into a gas.
While the gas itself is invisible,
it causes water vapor in the air to condense,
allowing us to see its path as a fine white mist.
And because CO2 is 1 1/2 times more dense than air,
you can see that it travels downwards out of the bowl,
just like the wave of gas from Lake Nyos
traveled downwards into the valley.
If you hold a match below the edge of the bowl,
once the gas reaches the flame, the match goes out.
This happens because the more dense carbon dioxide
is actually displacing or pushing out
all of the less dense oxygen the match needs to burn.
And, of course, that oxygen is an important component
of the air we need to breathe.
You see, the air we breathe isn't just oxygen.
Nitrogen makes up the bulk of our air, at around 78%.
The next 21% is oxygen, then argon,
CO2, neon-methane, and the list goes on.
So CO2 surrounds us every day.
We are constantly inhaling
and exhaling it each time we breathe.
But in the air around you,
typical CO2 levels hover around 0.04%.
At higher concentrations, carbon dioxide can suffocate you.
It's colorless, odorless, dense,
and people often don't even realize
they're breathing dangerously high concentrations of it
until it's too late.
Even at CO2 concentrations as low as 10% in the atmosphere,
severe health problems can start minutes after exposure,
and lead to unconsciousness and, eventually, death.
As concentrations of CO2 continue to increase,
you can lose consciousness almost immediately,
and your lungs might stop working in just about a minute.
Without oxygen, our cells aren't able
to efficiently produce ATP,
the fuel they need to survive.
This can cause almost immediate damage
in some of our most energy hungry cells in our body,
our brain cells.
And carbon dioxide can also be a toxicant,
leading to terrible effects,
even at concentrations too low to suffocate.
Excessive CO2 in your body
can make your blood become acidic.
Anxiety, blurred vision and confusion
are all symptoms, and can make people seem drunk.
The few survivors spoke of coughing up blood
and losing consciousness.
Many of the dead were found
with blood around their nose and mouth.
In addition to the more than 1700 people who died,
over 3,500 livestock were lost,
and thousands of people were forced
to evacuate from their homes.
Scientists arriving in the region
had to piece together what happened.
It soon became clear that the deaths
were due to CO2 suffocation,
but even then they weren't sure
where all this gas had come from.
Did the Co2 come from magma below the lake?
Was it decomposing plant material in the lake?
Or, could it have been from chemicals dumped in the lake?
To figure this out,
scientists used radioactive carbon dating
to look at the CO2 in the water
and show that this deadly cloud of carbon dioxide
came from the volcanic source deep below Lake Nyos.
How?
Well, radioactive carbon dating is a method
of figuring out how old something is
by measuring the relative amount
of different isotopes of carbon in it.
Works like this.
Every living thing on this planet contains carbon.
Most carbon is stable carbon-12,
which gets its name from the sum of the six protons
and six neutrons in its nucleus.
But living things also contain
a radioactive isotope of carbon called carbon-14,
with six protons and eight neutrons.
It's produced in the atmosphere,
and everything above the surface of the earth
contains a bit of this radioactive isotope.
Plants and trees take it in by photosynthesis,
and animals get it from the vegetation they eat.
But once an organism dies,
it stops incorporating new carbon-14.
Now, carbon-14 is unstable,
and about every 5,700 years,
half of it decays, turning into nitrogen-14.
This is known as a half-life.
So the older the decaying organism gets,
the less carbon-14 it will contain.
So scientists can measure the amount of carbon-14
and carbon 12 in an object to see how old something is.
But this does have a limitation,
because after about 50,000 years,
all the carbon-14 in something is gone,
and you can't measure it with carbon dating any longer.
However, magma deep in the Earth's core
isn't exposed to the radioactive carbon-14
in the atmosphere at all,
so it contains none of that isotope.
When scientists examined the carbon dioxide
in the waters of Lake Nyos, they found it contained
only 1% of the carbon-14 you would expect
if the CO2 came from the atmosphere,
which meant that the CO2 build-up in the lake
was coming from underground sources
rather than the decomposition
of any living material from the surface.
So scientists determined it was from the magma deep below.
Now, while these kinds of limnic eruptions
don't happen that often,
another eruption actually happened in this same region
just two years earlier, and only 100 kilometers away,
at Lake Monoun, killing 37 people.
And just because Lake Nyos erupted
didn't mean things were over.
The volcanic carbon source continued
to pressurize the lake, building towards a new disaster.
But scientists created a solution
to prevent a future disaster
by creating a sort of pressure release valve for the lake
that could keep residents safe.
Think of it like shaking up a bottle of soda,
but then cracking the lid a little at a time.
This way, you're releasing the gas slowly,
instead of in a giant eruption.
To do this, a pipe 203 meters long
was installed in the lake to degas
the deep pressurized waters.
The pipe was then attached to a raft,
creating a self-sustaining floating fountain of CO2.
And while these two limnic eruptions in Cameroon
were the first recorded of their kind,
other regions around the world
show evidence of past eruptions,
and some sites even have the potential for future eruptions.
Scientists have found evidence of a limnic eruption
that may have happened in the fossil pits of Messel, Germany
over 30,000,000 years ago.
And Lake Kivu, which sits along the border
between Rwanda and the Democratic Republic of the Congo,
has the potential for an even larger disaster
because it's estimated to have
over 1,000 times more CO2 than Lake Nyos,
and is way too big to be degassed the same way.
However, a nearby power plant
extracts methane from the lake.
Removing this gas helps
to decrease the chance of an eruption,
and acts as a power source.
After 30 years, the people of the Lake Nyos region
returned to their homes.
And thanks to constant monitoring and degassing of the lake,
the people near Lake Nyos
are hopefully safe from future disasters.
(gentle music)