Epistemic status: a preliminary look at a possible cause area
TL;DR: Lake Kivu could erupt and kill 2 million people around it. But we could prevent this by installing oblique pipes, which will slowly and safely release gases and generate energy.
Large lake Kivu on the border between Rwanda and Democratic Republic of the Congo has a lot of gases dissolved near its bottom and it can erupt as lake Nyos did in 1986 when 1700 people were killed, but Kivu eruption could be 2000 times stronger.
A future overturn and gas release from the deep waters of Lake Kivu would result in catastrophe, dwarfing the historically documented lake overturns at the much smaller Lakes Nyos and Monoun. The lives of the approximately two million people who live in the lake basin area would be threatened.
An experimental vent pipe was installed at Lake Nyos in 2001 to remove gas from the deep water, but such a solution for the much larger Lake Kivu would be considerably more expensive. The approximately 510 million metric tons of carbon dioxide in the lake is a little under 2 percent of the amount released annually by human fossil fuel burning. Therefore, the process of releasing it could potentially have costs beyond simply building and operating the system. Wiki
A report Gas emissions from lake Kivu claims that lake Kivu has erupted in the past with a periodicity of 1000 years and could do it again in 100-200 years.
Lake Kivu has 65 cubic miles of methane (this is around 140 Mt and will produce additional methane concentrations increase of 28 ppb if released in the atmosphere, adding to the current level of around 1900 ppb). The lake also has 260 cubic miles of CO2.
Mitigation
Vertical pipes allow slowly extraction of gases from the lake and the collection of methane for energy use. Commercial extraction already started, but it is slow.
There are risks related to gas extraction via pipes:
Risk 1: what if the pipes will destabilise the lake?
Risk 2: releasing CO2 will contribute to global warming. Methane is even worse as a greenhouse gas, but it could be collected with profit. CO2 actually may be used too for fracking and for chemical and food production.
Concerns about these risks are slowing down current methane extraction. However, if the lake erupts, all methane and CO2 will go into the atmosphere and will be equal to several years of the Earth’s emissions, mostly because of short-term methane’s greenhouse effects. But a part of methane will be combusted in eruption.
The strong methane fire may take the form of an explosion which will contribute to gas release and to the devastation around the lake (total methane energy in the lake is around 1 gigaton TNT). However, explosive methane fire will prevent CO2 accumulation on lower grounds near the shores and CO2 suffocating effects will be smaller because gases will mix with the atmosphere quickly.
The project to reduce gases in the lake is ongoing, but its impact is not clear to me: it may not be enough to stop the increase of the gases’ concentration, which is still increasing, but could be enough to create risks of the eruption is something goes wrong (a pin and balloon effect). Especially because it is made for profit which creates an incentive to take higher risks. A much larger and simultaneously safer project is needed to prevent the eruption of the lake.
As gas concentrations rise in Kivu’s depths, so does the risk. Wüest and colleagues found that from 1974 to 2004 the concentration of CO2 increased by 10%. But the bigger concern at Kivu is the methane concentration, which rose 15-20% during the same period. BBC
Oblique pipes
I think that the pipes should be built in a way that excludes increasing risk of eruption, maybe they should be very oblique. Vertical pipes could become seeds of degassing, as degassing already happens inside them, and if a tube is vertically raptured, the degassing stream of bubbles could expand. If the pipe is very oblique, almost horizontal, its rapture (presumably) will not create a vertical stream of gas bubbles. I guess that oblique venting pipes could cost tens of millions of USD.
Without pipes, the eruption is inevitable and has a 0.5-1 per cent yearly probability.
Therefore, the cost-effectiveness of the suggested project is around 10-100 dollars per saved life and may be more effective, as the lake catastrophe will be damaging to the whole of Central Africa, and safe methane extraction could generate profit. But we should also include in the calculation uncertainties and remote timing of the possible eruption.
What needs to be done:
- design and test tubes which can’t become seeds of degassing.
- gain the support of local communities and founders
- remove excess levels of gases
Lake Kivu lies on the border between Rwanda and Congo, and there is an active war there now. War could prevent any efforts to mitigate risks from the lake; moreover, war can cause eruption if thing lake depth charges will be used. A single sinking ship may create a column of bubbles which then will become self-reinforcing.
Thanks for thinking and writing about this!
I'm an undergraduate Geography student and we've talked about these lakes a few times in several courses. Below is a summary of what we've discussed in lessons about this topic.
Lake Kivu, just like Lake Nyos, is a lake in the African Rift where magma flows below the surface degasses. The gases themselves are not toxic, but as they release very suddenly in massive amounts, they push away all the oxygen, causing all life in the vicinity to suffocate. But this only happens if the gases released are 1) heavier than air/oxygen and 2) have nowhere to go i.e. the lake is in a valley. This is the case for both Lake Kivu and Lake Nyos.
To reiterate, the main risk is caused by all the gases releasing at once. These events don't happen out of the blue. The water in the lake needs to saturated by the gases (but I guess one gas works too). The amount of gas that can be dissolved in the water also depends on the depth (see Figure 8 in the report linked in the original post). Whilst it is not at critical levels yet, it won't happen overnight.
The water in Lakes Kivu and Nyos are 'layered' meaning that the water in the lake is very stable and doesn't circulate a lot, if at all. This allows for gas saturation to occur in the lower layers. This gas will not release spontaneously. A trigger is needed. For Lake Nyos, this was a landslide that pushed water from the bottom to the top, causing it to degas. Lake Kivu is surrounded by hills too, but I'm not confident a small landslide will cause the entire lake to degas. There are several volcanoes in the vicinity, which could erupt and cause landslides, which in turn cause internal waves or turbulence and therefore degassing. Finally, a large amount of gas that gets inserted from the lake bed could also cause the saturation threshold to be crossed.
But even if this saturation threshold gets crossed by some water layers, it won't cause the entire lake to degas. Only the gas molecules above the saturation threshold will form bubbles and rise through the surface. So for the lake to degas to such an extent that all the gas gets released, all the water in the lake needs to be basically turned upside down.
The report doesn't mention any likelihood of any of these events happening. It's also uncertain about the amount of gas that could be released and how much gas is added to the bottom of the lake.
Finally, I'd like to address the risks you identified:
As explained above, the pipes need to destabilise in such a way that causes turbulence with vertical movements of up to 130m. As the pipes are hollow and inserted vertically, I think it is extremely unlikely that this will happen. (For a fun at-home experiment: try inserting a straw in a glass of water with some liquid colouring to visualise the turbulence caused.). I think that most of the turbulence it does cause will cause some parts of the upper layers to move downwards, but not necessarily the other way.
It seems like there is currently not really a way around this. Either the CO2 or CH4 gets released through the degassing explosion, or more controlled by the vertical pipes and turned into CO2 by burning the methane. So regardless of whether the amount extracted by current power plants is higher or lower than the amount added, we're still turning it into CO2 . If we want to expand the amount of gas that we extract, we probably need to add some sort of carbon capture and storage system to the power plants. I have absolutely no clue how viable this is given current technology and the war that's going on there, but I think it's rather difficult
As there's so much uncertainty about the amounts of gas and how much more degassing capacity is needed, if any at all, I'd say it's quite difficult to estimate how much this would cost and therefore it's difficult to give a cost-effectiveness analysis. On the other hand, if a solution is already known and it's relatively cheap to implement it, I don't see any reason why we shouldn't.
For those who'd like to know more, the report linked above goes into quite some detail and includes many equations to calculate stuff related to it that is beyond my knowledge. The technical term for these kinds of eruptions is limnic eruptions.
Thanks for these details!
May be you discuss a different report than I read. The one I read says:
Sorry for my late reply, but thanks for mentioning. I edited my reply to remove that section.