The Clathrate Gun Hypothesis -

Methane Driven Climate Change in the Geological Past

Published in the EarthSphere Blog. Cover Image: Lurking Below the Surface by CF Lovelace & WM House (©2022 Archean Enterprises, LLC, Archeanart)

Lightweight, colorless, and odorless, methane is a slippery character in the global warming story. But it is a super-greenhouse gas. While Anthropogenic, man-made sources of methane grab the headlines in climate news, we still don’t fully understand the threat natural methane sources might present for a rapidly warming planet. But some scientists believe our geological history holds examples of past methane disasters.

Understanding these examples requires a bit of knowledge about how methane is generated, stored, and released in nature. Our industrial societies produce a lot of methane, but so does mother nature. The bacterial-driven decay of organic matter in low-oxygen environments, like the swamps and bogs of coastal wetlands, generates over 20 percent of the planet’s natural methane. Bacterial action on or slightly below the ocean bottom also produces methane, known in the oil industry as biogenic gas. Natural sources account for 52 percent of all global annual emissions. Human activities, particularly agriculture and fossil fuel usage, account for the other half.

One of methane’s most notable characteristics from a climate change standpoint is its ability to absorb heat. Over short periods of time, a decade or two, methane’s warming potential is 87 times greater than CO2. Because methane’s lifespan in the atmosphere is short (about 12 years), its long-term heating potential over a century is lower at approximately 28 times that of CO2.

So the rapid release of large quantities of methane into the atmosphere can result in rapid global warming. Human Anthropocene activity is a real-time example of what happens by rapidly releasing greenhouse gases, including methane. But the question remains, can we see examples from the geological past where massive, rapid, and natural methane releases affected global warming? The answer is, perhaps.

Clathrates

Methane (CH4 or natural gas) is a staple of our energy-driven world. We use it for cooking, heating, and producing electricity. We think of it as a gas, but large quantities of methane lie along our deep ocean continental slopes in the form of methane clathrates. These solid formations of methane and water form an icy mixture under high pressure and low-temperature conditions. These conditions occur in our deep oceans, where the weight of the water creates high pressure and the ambient temperature hovers around 4 celsius.

But if ocean temperatures rise, the clathrates revert to methane gas and water. The resulting methane is buoyant and rises as gas bubbles. Some of the gas dissolves into the surrounding seawater, but if enough is released, the methane will eventually reach the ocean surface and enter the atmosphere. The rapid transfer of environmentally significant amounts of methane from the seafloor to the atmosphere is know as the Clathrate Gun Hypothesis.

A lot of methane must enter the atmosphere to affect climate, but the volume of methane trapped in clathrates is large. The USGS estimates the current methane volume trapped in clathrates at between 4,000 and 200,000 times the total amount of natural gas consumed in the US in 2010. There is good reason to believe that similar volumes of clathrates would have been present during past geological intervals when climate conditions were similar to today. Warming oceans could release that trapped methane.

Still, the occurrance of massive clathrate methane releases in the geological past is heavily debated and is by no means proven science. Good arguments can be made on either side of the debate. But there are several periods of rapid global warming where clathrate methane releases, sparked by warming oceans, may have contributed to rapid global warming. The most recent of these periods was a geological interval called the Eemian period.

The Eemian

The Eemian period began 130,000 years ago and continued for about 15,000 years. It was the last interglacial stage of the Pleistocene before the current Holocene period. Global temperatures rivaled temperatures today, and the world’s oceans were about 20 feet higher.

Researchers studying ocean sediment cores on the continental slope of West Africa found two lines of evidence supporting a Clathrate Gun Hypothesis to explain rapid warming during the Eemian period. Both observations come from the chemical analysis of foraminifera shells and heavily rely on analyzing carbon and oxygen isotopes. Isotope evidence is an indirect but commonly accepted way of inferring prevailing ambient temperatures at the time of shell formation. Researchers inferred that mid-ocean temperatures rose by over 6 degrees Celsius during the Eemian. The term mid-ocean references depths between 1300 and 300 meters below sea surface.

Mid-ocean warming moved the Eemian clathrates out of their stability zone, causing large releases of gaseous methane. In addition to the warming, researchers detected an anomalously low δ13C spike in the foraminifera during the Eemian. They believe this spike is indicative of excessive methane in the water from the disintegration of clathrate deposits.

None of this evidence points to methane clathrates as the initiator of rising global temperatures, but it does provide strong evidence that methane clathrates may have supercharged the warming process. Something that is relevant to our situation today.

PETM

A second geological interval where methane clathrates possibly played a role was during the Paleocene Eocene Thermal Maximum (PETM). Average global temperatures reached as high as 27 degrees Celsius, or 13 degrees higher than the average global temperature today. The rate of global temperature rise leading to the PETM was extremely high from a geological perspective but still about 25 times less than the rate of Anthropogenic temperature rise we are currently experiencing. Sea levels were about 300 feet higher than today.

The geological data are clear that atmospheric carbon dioxide (CO2) increased dramatically during the PETM, and also, the earth’s oceans experienced rapid acidification. In total, an estimated 10 trillion tons of carbon entered the atmosphere to create the thermal maximum. However, the causes of atmospheric carbon increase are speculative. One plausible theory holds that massive releases of methane from ocean-bottom clathrates supercharged a short but intense period of warming.

The PETM is characterized by a central temperature spike that saw the planet warm by 4 degrees Celsius over about 20,000 years and then quickly retreat back by 4 degrees. One theory is that the deep oceans warmed until they passed a threshold point where large methane clathrate deposits destabilized and expelled significant amounts of methane into the atmosphere.

The released methane amplified the warming trend causing even more clathrates to decompose. At some tipping-point, the warming and methane release cycle became self-sustaining and continued until all the methane in ocean-floor clathrates was released. This massive release of methane created conditions that amplified climate warming to produce an abnormal temperature spike. The short but intense PETM temperature spike is consistent with an influx of a powerful but short-lived greenhouse gas like methane.

Experts disagree on the potential threat posed today by seafloor clathrates. But the significant volume of methane patiently waiting for release from its dark, cold prison on the ocean bottom should give us pause for thought if global ocean temperatures continue to rise.