Too weird not to write about
Anyone paying attention to recent science news will have some idea about the enormous craters mysteriously appearing in Siberia. These holes are roughly cylindrical with round openings at the earth’s surface, and some go more than 160 feet down into the ground. The crater walls are often vertical, giving the appearance of a missile launcher. But the only thing they are launching is tons of soil — spewing it out of the ground and onto the earth’s surface. These projectile incidents appear to be fueled by methane and powered by buoyancy. The physics is cool, but the climate implications are not.
The basic facts
The notion of solid-ground below our feet, suddenly blasting upwards towards the sky, doesn’t promote any warm, fuzzy feelings. It’s just scary. But unless you are wandering around in the vicinity of the Arctic, you are relatively safe from such a fate. Special conditions are needed to generate these amazing methane craters, and a key one is the presence of permafrost.
A thick layer of frozen permafrost has a fair amount of structural integrity, just like thick ice on a river or lake. Air bubbles can collect below the ice, but they won’t normally break through. However, if enough air collects directly below one local area of the ice, then the buoyancy force of the air may be enough to burst through the ice.
Air doesn’t collect below the permafrost, but methane does, and western Siberia contains some of the largest shallow-methane deposits in the world. But from where does this gas come? Is it the source of the methane, which blows holes in the Siberian tundra?
Natural gas, which has methane as a primary constituent, is generally believed to come from two sources — thermogenic and biogenic gas. Biogenic gas is generated from bacteria when they eat up organic materials and poop out methane and CO2. But thermogenic gas comes from heating organic source rocks or existing oil deposits. Enough heat eventually breaks down long hydrocarbon chains into their simplest chemical form, CH4 (methane). Chemical studies of the isotopes in natural methane deposits can distinguish between thermogenic and biogenic gas components.
Oil and gas from the earth
Thermogenic oil and natural gas begin with organic-rich rocks, buried deep underground. Several different types of source-rocks exist, with some generating liquid oil and others prone to producing natural gas. But regardless of the source, temperature and time are required to change source-rocks into hydrocarbons. Temperatures increase with depth, and at about 110 degrees Celsius, the heat starts transforming organic material to oil. By 140 to 150 degrees, the source-rock is at peak oil generation. As temperatures rise towards 200 degrees, oil is baked and broken down into gas. This process is the source of thermogenic methane.
But when organic material or oil is near the ground’s surface, bacteria come into play, eating both oil and soil carbon to produce methane. Bacteria don’t produce liquid hydrocarbons, so the gas they create is called dry gas, almost 100 percent methane.
Back to the permafrost, there are several ways methane accumulates below a permafrost sheet. The permafrost sheet is a solid layer of soil and ice, and it is impermeable, so gas can’t pass through it. A layer of earth that traps gas below is called a “seal” in the oil business. If methane accumulates below the permafrost seal, it must either be rising from deeper in the earth (thermogenic) or bacteria are producing it in situ (biogenic).
Methane: gas or solid
Thus far, we have talked about methane as a gas bubble below the permafrost seal, but methane can also take on a solid form under the right conditions. Low temperatures combined with elevated pressure cause methane and water to combine into an icy structure called a methane clathrate, or methane hydrate. These clathrates form at depths of around 200 meters in permafrost regions.
If either the pressure or temperature conditions change, methane clathrates become unstable, and the methane is then rereleased into a gaseous form. This change is important because methane, in a gaseous state, creates a lot of buoyancy pressure at the base of a seal, whereas methane clathrates don’t.
Let’s put the pieces together. A permafrost layer of soil and ice forms an impermeable seal across large regions of the Arctic. Methane from thermogenic and/or biogenic sources becomes trapped below this seal, either as gas or as a clathrate. A methane crater only forms when the buoyancy force from below overcome the strength of the permafrost seal. There are really only two ways for this to happen. Either the upward pressure from the gas increases or the strength of the seal decreases. One or both of these mechanisms could be responsible for these mysterious craters.
Increasing pressure from below results from more gas, and there are three ways for this to happen. Gas seeps upward from deep reservoirs, bacteria produce it in place, or methane clathrates become unstable and release their methane. We know Arctic temperatures have increased twice as fast as the global average, and higher temperatures cause increased bacterial activity when organic material in the permafrost unthaws. The more they eat, the more methane they expel. Higher temperatures also destabilize methane clathrates — a process resulting in the sudden release of lots of methane and a quick build-up of pressure below the permafrost seal.
Temperature also affects the other side of the equation — weakening of the permafrost seal. As temperatures rise, the permafrost melts, reducing the structural integrity of the seal. At some critical point, a large pocket of gas, previously contained by the seal, exerts enough pressure to breach the frozen cover and explode upward to the surface.
Scientists don’t fully understand how these craters form, but there is general agreement that climate change and global warming are driving the increasing frequency of these explosive events. It is also unclear just how much methane is released into the atmosphere from these explosions. But this much is clear; methane is a super-greenhouses gas, and more methane in the atmosphere exacerbates global warming. The warming Arctic is a positive feedback loop where heat generates more greenhouse gases, which in turn generate more heat.
Scientists worry because methane is a super warmer. The warming potential of various greenhouse gases is measured by comparing them to carbon dioxide (CO2) using a measure called CO2-eq (CO2 equivalent). For example, if actual atmospheric CO2 levels are 410 ppm, but the CO2-eq level is 500 ppm, we then interpret the data to show the warming potential of all gases other than CO2 is equal to a 90 ppm increase in CO2 levels.
The effect of particular greenhouse gases on global warming is a function of two factors: warming potential and lifetime (time in the atmosphere). Methane’s lifetime is short (~12 years) compared to CO2, which lasts for centuries. But methane’s warming potential is up to 85 times greater than CO2 during the first 20 years after it enters the atmosphere. Release a ton of methane into the atmosphere, and over a 20-year period, it has the warming potential of 85 tons of CO2-eq.
Summing it up, methane craters are mechanically and scientifically interesting, but they should scare the crap out of us from a climate change perspective.
Ocean methane hydrates as a slow tipping point in the global carbon cycle (By David Archer, Bruce Buffett, and Victor Brovkin; PNAS)
Why This Monstrous Crater Suddenly Appeared in Russia (By Jennifer Leman; Popular Mechnics)
The More Methane Hypothesis (Source: William House; Medium)