Limnic eruption

Causes

For a lake to undergo a limnic eruption, the water must be nearly saturated with gas. CO₂ was the primary component in the two observed cases, Lake Nyos and Lake Monoun. In Lake Kivu's case, scientists, including lake physicist Alfred Johny Wüest, were also concerned about the concentrations of methane. CO₂ may originate from volcanic gas emitted from under the lake, or from decomposition of organic material.

Before a lake becomes saturated, it behaves like an unopened carbonated soft drink: the CO₂ is dissolved in the water. In both lakes and soft drinks, CO₂ dissolves much more readily at higher pressure due to Henry's law. When the pressure is released, the CO₂ comes out of solution as bubbles of gas, which rise to the surface. CO₂ also dissolves more readily in cooler water, so very deep lakes can dissolve very large amounts of CO₂ since pressure increases, and temperature decreases, with depth. A small increase in water temperature can lead to the release of a large amount of CO₂.

Once a lake is saturated, it is very unstable but a trigger is needed to actually set off an eruption. In the case of the 1986 Lake Nyos eruption, landslides were the suspected triggers, but a volcanic eruption, an earthquake, or even wind and rain storms can be potential triggers. Limnic eruptions can also be caused by gradual gas saturation at specific depths triggering spontaneous gas development. Regardless of cause, the trigger pushes gas-saturated water higher in the lake, where the reduced pressure is insufficient to keep gas in solution. The buoyancy from the resulting bubbles lifts the water even higher, releasing yet more bubbles. This process forms a column of gas, at which point the water at the bottom is pulled up by suction, and it too loses CO₂ in a runaway process. This eruption discharges the gas into the air and can displace enough water to form a tsunami.

Limnic eruptions are exceptionally rare for several reasons. First, a CO₂ source must exist; regions with volcanic activity are most at risk. Second, the vast majority of lakes are holomictic (their layers mix regularly), preventing a buildup of dissolved gases. Only meromictic lakes are stratified, allowing CO₂ to remain dissolved. It is estimated only one meromictic lake exists for every 1,000 holomictic lakes. Finally, a lake must be very deep in order to have sufficiently pressurized water that can dissolve large amounts of CO₂.

Consequences

Once an eruption occurs, a large CO₂ cloud forms above the lake and expands to the surrounding region. Because CO₂ is denser than air, it has a tendency to sink to the ground, forcing out breathable air and causing asphyxiation. CO₂ can make human bodily fluids highly acidic and potentially cause CO₂ poisoning. As victims gasp for air, they actually accelerate asphyxia by inhaling CO₂.

At Lake Nyos, the gas cloud descended into a nearby village where it settled, killing nearly everyone; casualties as far as 25 km (16 mi) were reported. A change in skin color on some bodies led scientists to hypothesize the gas cloud may have contained dissolved acid such as hydrogen chloride, though this hypothesis is disputed. Many victims were found with blisters on their skin, thought to have been caused by pressure ulcers, which were likely caused by low blood oxygen levels in those asphyxiated by carbon dioxide. Nearby vegetation was largely unaffected, except any growing immediately adjacent to the lake. There, vegetation was damaged or destroyed by a 24-metre-high (79 ft) tsunami caused by the violent eruption.

Degassing

Efforts are underway to develop a solution for removing the gas from these lakes and to prevent a build-up which could lead to another disaster. A team led by French scientist Michel Halbwachs began experimenting at Lake Monoun and Lake Nyos in 1990 using siphons to degas the waters of these lakes in a controlled manner. The team positioned a pipe vertically in the lake with its upper end above the water surface. Water saturated with CO₂ enters the bottom of the pipe and rises to the top. The lower pressure at the surface allows the gas to come out of solution. Only a small amount of water must be mechanically pumped initially through the pipe to start the flow. As saturated water rises, the CO₂ comes out of solution and forms bubbles. The natural buoyancy of the bubbles draws the water up the pipe at high velocity resulting in a fountain at the surface. The degassifying water acts like a pump, drawing more water into the bottom of the pipe, and creating a self-sustaining flow. This is the same process which leads to a natural eruption, but in this case it is controlled by the size of the pipe.

Each pipe has a limited pumping capacity and several would be required for both Lake Monoun and Lake Nyos to degas a significant fraction of the deep lake water and render the lakes safe. The deep lake waters are slightly acidic due to the dissolved CO₂ which causes corrosion to the pipes and electronics, necessitating ongoing maintenance. There is some concern that CO₂ from the pipes could settle on the surface of the lake forming a thin layer of unbreathable air and thus potentially causing problems for wildlife.

In January 2001, a single pipe was installed by the French-Cameroonian team on Lake Nyos, and two more pipes were installed in 2011 with funding support from the United Nations Development Programme. A pipe was installed at Lake Monoun in 2003 and two more were added in 2006. These three pipes are thought to be sufficient to prevent an increase in CO₂ levels, removing approximately the same amount of gas that naturally enters at the lake bed. In January 2003, an 18-month project was approved to fully degas Lake Monoun, and the lake has since been rendered safe.

There is some evidence that Lake Michigan in the United States spontaneously degasses on a much smaller scale each fall.

Lake Kivu risks

Lake Kivu is not only about 1,700 times larger than Lake Nyos, but is also located in a far more densely populated area, with over two million people living along its shores. The part within the Democratic Republic of the Congo is a site of active armed conflict and low state capacity for the DRC government, which impedes both studies and any subsequent mitigating actions. Lake Kivu has not reached a high level of CO₂ saturation yet; if the water were to become heavily saturated, a limnic eruption would pose a great risk to human and animal life, potentially killing millions.

Two significant changes in Lake Kivu's physical state have brought attention to a possible limnic eruption: the high rates of methane dissociation and a rising surface temperature. Research investigating historical and present-day temperatures show Lake Kivu's surface temperature is increasing by about 0.12 °C per decade. Lake Kivu is in close proximity to potential triggers: Mount Nyiragongo (an active volcano which erupted in January 2002 and May 2021), an active earthquake zone, and other active volcanoes.

While the lake could be degassed in a manner similar to Lake Monoun and Lake Nyos, due to the size of Lake Kivu and the volume of gas it contains, such an operation would be expensive, running into the millions of dollars. A scheme initiated in 2010 to use methane trapped in the lake as a fuel source to generate electricity in Rwanda has led to a degree of CO₂ degassing. During the procedure for extracting the flammable methane gas used to fuel power stations on the shore, some CO₂ is removed in a process known as catalyst scrubbing. It is unclear whether enough gas will be removed to eliminate the danger of a limnic eruption at Lake Kivu.