Four months after the reactor explosion at the nuclear power plant in Chernobyl, disaster struck again some 7000 km south-west of the Ukrainian city. This time, however, a lake exploded. The consequences were catastrophic, but they could soon get dwarfed by a similar, even more serious looming threat.
Tragedy at Lake Nyos
August 21, 1986. The sun had long set at picturesque Lake Nyos in western Cameroon. Most of the residents living nearby had retired to their straw huts; some were still enjoying dinner with their families; many were already sound asleep. Then, witnesses recall, there was a deep rumbling noise, like thunder rolling in the distance. If it wasn't for the darkness of night, some of those who were still out and about and close enough to the lake may also have seen the source of the rumbling, but of them barely anyone would ever get a chance to share their story.
Within minutes, at first those living in nearby villages who went out to investigate, noticed their livestock behaving in odd ways, then collapsing onto the ground. Only seconds later they too lost consciousness. With every passing minute, more and more people, living as far as several kilometres from the lake itself, noticed the same strange behaviours in their animals before they themselves collapsed. Most people, though, already in bed and asleep, were entirely unaware that something was terribly wrong; many of them would never wake again.
When residents from surrounding areas arrived several days later, they noticed the water of the lake, normally clear blue, had turned a reddish brown. Trees lining the shoreline had been uprooted and knocked over, and the water level had dropped by close to a metre. Yet, even more harrowing were the scenes that awaited them in the villages. The lifeless bodies of people and animals lay scattered on the ground. Inside their huts entire families had seemingly dropped dead in their tracks. Nearly 1800 people and more than 3500 livestock died that night. It was the worst ever recorded incident of its kind.
What had happened?
Within days of the tragedy, researchers from around the globe arrived at Lake Nyos desperate for answers as to what had happened. Initial suspicions focused on the possibility of a volcanic eruption, largely owing to the lake's proximity to a ridge of known volcanic activity. This hypothesis was also supported by the lack of physical injuries on the victims, suggesting that perhaps they had been poisoned by toxic volcanic gases. However, these assumptions were quickly laid to rest when soil samples taken from the lands surrounding Lake Nyos did not show any traces of such chemicals.
Another clue came in the form of an incident that had occurred only two years earlier. In August 1984, just 100 km south of Lake Nyos at nearby Lake Monoun, 37 people had died suddenly and under eerily similar yet still unresolved circumstances. The victims had been discovered near a short section of road where it passed through a ditch, and they too appeared to have seemingly collapsed and died on the spot. At the time, local authorities and experts were at a loss as to what could have caused the victims’ sudden death. Even an act of terrorism was among the considered explanations. The startling resemblance of the two events, however, now led investigators to question that hypothesis.
While neither victims nor survivors at Lake Nyos showed any signs of physical injuries or remnants of toxic chemicals that could have been indicative of poisoning, medical assessments of the deceased indicated that these people had died of asphyxiation. In their tissues they also found unusually high levels of carbon dioxide (CO2) - a gas that is a natural constituent of the air we breathe and a byproduct of the metabolic processes that occur in every cell of our body. Yet the only circumstance under which it accumulates in our bodies beyond safe levels is due to a condition known as carbon dioxide poisoning.
CO2 poisoning occurs when in the presence of large volumes of highly concentrated CO2. In air its concentration is just 0.04% and, obviously, completely harmless. But at concentrations of over 5%, the human body enters a state called hypercapnia. Mild hypercapnia is characterised by symptoms such as shortness of breath, headaches, confusion, and nausea. Inhalation of CO2 at concentrations exceeding 10% rapidly induces convulsions, loss of consciousness, and sometimes even death. But at even higher concentrations of over 30% these effects are near instantaneous, and death due to suffocation occurs in under one minute.
In addition to its toxicity at high concentrations, CO2 is particularly treacherous because of its physical and chemical properties. As an odourless and colourless gas, it is undetectable without special equipment, and because of its high density which makes it 1.5 times heavier than air, when released, it accumulates on the ground. In such situations, CO2 displaces any lighter gas, such as air, resulting in a deadly layer of concentrated CO2 that covers the ground like an invisible blanket. Unless fanned away or sucked up, as for example in a chemical fume cabinet, the gas remains and poses a serious, potentially deadly, safety hazard. For this reason, incidents of CO2 poisoning rarely occur outside. They are most common in workplaces, such as laboratories, where large quantities of CO2 may be used in confined spaces without adequate ventilation. Because it can neither be seen nor smelled, often neither those initially exposed nor anyone coming to their aid are aware of the present danger.
So investigators had identified how the people and animals had died. But where could an amount of CO2 large enough to cover several square kilometres of ground and suffocate hundreds of people around Lake Nyos (and to a smaller extent near Lake Monoun) have come from? In search of the answer, and after much deliberation as to the possible origin of the gas, researchers eventually began to probe the lake’s water to assess its chemical composition. What they found was nothing short of terrifying. When they pulled up the first of several bottles deployed for collecting water from the lake’s deep water layer, its lid popped off like the cork of a champagne bottle. According to their measurements, the water contained absurdly high levels of dissolved CO2. But there was more. When several follow-up measurements showed this level was slowly but steadily increasing even further, not only did it become clear what had likely caused the event at hand, but also that it could easily happen again.
Today, geologists refer to Lake Nyos and Lake Monoun as limnically active (from Greek limne, meaning lake, and relating to freshwater bodies). They share four distinguishing features which make them both extremely rare and, at the same time, mimic ticking time bombs capable of exploding in what geologists call a limnic eruption.
Both are crater lakes of volcanic origin, exceptionally deep (>100 m), located in the tropics and in proximity of volcanic activity. Owing to their volcanic origins, their lake beds are made up of cooled volcanic rock (magma). Magma contains large amounts of trapped CO2 which in these cases it slowly releases into the cooler and deeper waters of the lakes. High water pressures present at those great depths allow an enormous amount of gas to go into solution, leading to a build-up of CO2 gas-pressure akin to that in a champagne bottle.
But how come this does not seem to be an issue for other lakes of volcanic origin around the globe? In most lakes where CO2 is being released into the water, the amount of dissolved CO2 never grows so large that gas-pressures high enough to cause an eruption build up. This is because the vast majority of such lakes are either not very deep or they are located in regions where temperatures fluctuate throughout the year. As a result, warm surface waters intermittently cool (i.e., with the changing of the seasons) and mix with cool deep water thus allowing CO2 that had accumulated in deeper water to escape uneventfully.
In the cases of Lake Nyos and Monoun, however, year-round warm air temperatures, owing to their equatorial location, heat the lakes’ surface waters which results in the formation of a year-round thermocline - a cold water-warm water boundary caused by temperature-related density differences - which acts like a barrier between the warm surface water and the cool deep water. Without seasonal disruption of the thermocline, surface and deep water never mix and dissolved CO2 in the deep water cannot escape. Eventually, this results in the lakes’ deep water to become supersaturated with dissolved CO2; the lakes are now ticking high pressure time-bombs.
Returning to the example of the champagne bottle, the warm surface water and the resulting thermocline are similar in function to the bottle’s cork. As long as it remains closed, the internal pressure remains high, keeping the dissolved CO2 - which is what makes the champagne fizzy - in solution. But as soon as the air-tight seal of the cork is broken, air trapped inside the bottle rapidly escapes, resulting in a sudden pressure drop. Without the pressure necessary to keep the CO2 in solution, gas bubbles instantly form and force their way up in a powerful foamy column.
Under normal circumstances, the CO2 dissolved in a supersaturated - limnically active - lake remains in solution because the thermocline remains in place year-round. But events such as landslides, earthquakes, volcanic activity, or possibly even rain that slightly cools the surface water, can cause some vertical mixing of the separated layers and thus temporarily break through the surface water ‘cork’. When this happens, small amounts of supersaturated deep water may breach the thermocline and reach shallower depths where decreasing water pressure allows dissolved CO2 to come out of solution and form bubbles which float to the surface. These bubbles pull more and more deep water with them, setting off a chain reaction that causes an avalanche of gas bubbles and supersaturated water to push towards and burst through the surface. The amounts of gas and water released in the violent eruption that follows can be so enormous that it triggers a huge tsunami-like wave and noticeably lowers the water level of the lake.
At Lake Nyos, the upwelling of deep water to the surface was also responsible for the colour change from clear blue to reddish brown. In addition to CO2, the volcanic lake bed also released high amounts of iron into the lowly oxygenated deep water. When this deep water - now high in dissolved iron - reached the highly oxygenated surface, the iron oxidised, giving the water a reddish tint. This is the same phenomenon that makes oxygenated arterial blood appear red compared to the blueish tint of non-oxygenated venous blood.
Once expelled from the lake, CO2 sinks back to the ground where it may travel to lower lying areas, much in the same way that water follows a river bed. The extent of the affected area depends on the volume of expelled gas. The larger and deeper the lake, the bigger the potential amount of accumulated and expelled CO2.
Based on the size and depth of Lake Nyos, combined with the measurements taken from the lake’s deep water, researchers estimated that the amount of expelled CO2 must have been between 100000 and 300000 tons - enough to fill roughly 270 to 810 football stadiums. The column of water, foam and mist that rose from the lake during the eruption was estimated to have been 100 m high, and it would have triggered a wave up to 25 m high that smashed into the shoreline, thus explaining the many knocked over trees and destroyed vegetation, as well as the lowered water level. The CO2 layer that formed from such a huge amount of gas is thought to have been up to 50 m thick, and, based on the timing reported by those who heard and survived the eruption in villages as far as 25 km away, it must have travelled at speeds of up to 95 km/h - suffocating hundreds near Lake Nyos within minutes.
Defusing deadly lakes
When researchers had identified the causes for the disasters, anxiety grew that they might happen again. Various solutions to safely remove CO2 from the deep water were proposed, and after several trials, permanent degassing pipes were installed at Lake Nyos in 2001 and 2011, and at Lake Monoun in 2003. These pipes intentionally pierce through the thermocline and allow small amounts of supersaturated deep water to push to the surface. This way, excess CO2 can escape from the deep water in a steady and controlled way.
But, as researchers and authorities were dealing with the known dangers at Lakes Nyos and Monoun, attention soon shifted to another lake that was found to also accumulate gas in its depths: Lake Kivu, located several thousand kilometres south-east of Cameroon at the border between Rwanda and the Democratic Republic of the Congo. The reason for their alarm? While the Lake Nyos disaster was devastating, Kivu holds the potential to make it look like a mere blip on the measuring stick of limnic eruptions.
Compared to Lakes Monoun and Nyos, Kivu is huge. For reference, at about 2 km long, 1.2 km wide, and a maximum depth of 208 m, Lake Nyos holds approximately 0.15 cubic km of water - enough to trap several hundred thousand tons of CO2 and kill hundreds of residents in the lake's vicinity when it overturned. In contrast, Lake Kivu, at 89 km long, 48 km wide, and 480 m deep at its deepest point, holds a total of 550 cubic km of water - more than 3000 times the volume of Lake Nyos. Given this immense size, Kivu is thought to harbour terrifyingly large amounts of dissolved CO2. And as if that were not perilous enough, in addition to trapped CO2, Lake Kivu's unique geology - it is located directly over an area of volcanic activity where magma pushes up through the earth's crust, causing the overlying tectonic plates to drift apart - also leads to an accumulation of methane (CH4) and hydrogen sulfide (H2S).
Geologists believe that these conditions make Lake Kivu particularly vulnerable to limnic eruptions triggered by volcanic activity. They fear that given the lake's size, such an event could have catastrophic consequences with the potential to wipe out the entire fauna in the areas surrounding it. Their warning is fuelled further by the lake's proximity to two active volcanoes - one of which erupted as recently as 2002. Indeed, analyses of core samples taken from Lake Kivu's sediments suggest that such local extinction level events may have occurred several times over the past few thousand years.
The gravity of the existential threat that a limnic eruption at Lake Kivu poses to virtually all life surrounding it becomes particularly clear when one considers the number of people who live in its vicinity. At the time of the Lake Nyos disaster, only about 14000 people lived in the areas surrounding the lake; more than 10% of them were killed. Lake Kivu is home to around two million people, and the amounts of CO2 and CH4 trapped inside it would be enough to suffocate every single one of them.
Surprisingly, people have known about the presence of methane and hydrogen sulfide in Lake Kivu going back far longer than when the conditions causing limnic eruptions were understood. For generations the local peoples grew up and lived near the lake well aware that occasionally toxic fumes rise from the water and may kill people, animals and even plants. Their traditional language - Swahili - even has a word describing this very phenomenon: Mazuku they call it - Evil Wind. Nonetheless, up until the tragic events of Lake Nyos and Monoun, and researchers understanding what made them possible, there was little concern about the situation present at Lake Kivu. Since then, however, researchers and authorities have been racing to find solutions that could defuse the looming threat. Unfortunately, the enormous amounts of gas alone make it difficult to find strategies that are both safe and affordable.
One approach to address the accumulation of the combustible methane is its extraction for the purpose of power generation. Kivu is estimated to contain at least 65 cubic km of methane, equating to ~100 GW - or 750000 homes worth - of power. To this end, several energy companies, including the British ContourGlobal who established the KivuWatt Powerplant in Kibuye on the eastern shore of the lake, and US-based Symbion Power who operate the Kivu 56 Thermal Power Station at Nyamyumba on the north-eastern shore, generate a combined 75 MW of power by burning the extracted and purified methane.
To extract methane from the lake, supersaturated deep water is pumped to the surface via floating extraction platforms. Once the water reaches the surface, the gases form bubbles and can easily be collected. Unfortunately, although this serves as a valuable and efficient means to generate power for the people living in the area, the amounts of gas removed from the lake are far from sufficient to eliminate the threat of a limnic eruption. But large scale undertakings to achieve this are mired with complications, mainly stemming from the sheer amount of gas they would need to evacuate and the financial burden and constructive effort it would take to realise them.
Perhaps it comes as a blessing then that a more recent investigation of Lake Kivu's chemistry and the associated likelihood of a limnic eruption to occur, concluded that previous assessments of the present conditions and risks likely overestimated the probability of the threat.
Let's hope they are right.
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