Methane (CH4) is a very potent greenhouse gas, 20-30 times more powerful than carbon dioxide (CO2) on a century timescale. Fortunately, it only occurs in very low concentrations in the atmosphere - about 0.3 to 0.4 parts per million during ice ages and 0.6 to 0.7ppm during warmer periods.
In 1750 at the start of the Industrial Revolution, the concentration was about 0.7ppm. By 2010 it had reached more than 1.85ppm and is now at its highest level in more than 500,000 years. This is largely due to human activities such as keeping large herds of cattle and flocks of chickens, the destruction of forests, and the mining and burning of fossil fuels. Methane only stays in the atmosphere for 9-15 years during which time it oxidizes into CO2.
Large amounts of methane are produced by bacteria in the sediments below the seabed and by chemical action in organic material buried at greater depths. When this methane comes into contact with very cold or deep water, it bonds with water to form an ice-like substance called a clathrate. Clathrate remains stable provided it remains beneath deep water where there is high pressure or in shallower, very cold water such as that found in Polar Regions. When it melts, clathrate yields 164 cubic meters of methane per cubic meter of solid clathrate. Like Savoir Faire, Clathrates are seemingly everywhere
Clathrate is found in the Antarctic and particularly in the Arctic where large quantities are found in the relatively shallow though very cold seabed of the vast continental shelves which almost encircle the Arctic Ocean. It is also found in the seabed of warmer water deep enough to exert sufficient pressure on the clathrate to prevent it from becoming unstable and releasing methane.
Over millions of years, methane clathrate has accumulated below the seabed. Billions of tonnes of it now lie dormant beneath permafrost, in the pores of sandstone or shrouded in silt. As long as it remains under pressure (covered by deep water) or in cold conditions (below 0°C) it remains stable and does not release methane.
We know that in the past there have been sudden changes resulting in global warming and that these changes have been associated with releases of greenhouse gases. These releases occurred very quickly, they were very large and were characterized by an unusually low amount of carbon isotope 13 and by large extinction (30-50%) of water breathing animals, particularly those living on or near the seabed. Most recently, this occurred at the time of the Paleocene-Eocene Thermal Maximum (PETM) about 55.8 million years ago. This was a time when average global temperatures increased by around 6°C and by over 10°C at the poles.
The world at the approximate time of the PETM (courtesy Christopher Scotese)
It is believed that the PETM was started by natural changes in the way the Earth goes round the sun and the angle at which it leans towards the sun. As a result this increased the amount of solar radiation reaching the Earth, particularly at its polar regions ( Sexton et al, 2011). Over many thousands of years, this warmed the Arctic causing ocean warming and melting of permafrost sufficient to let clathrates release large amounts of methane into the atmosphere. These releases were characterized by low levels of ∂13C and caused more rapid and extensive melting of deeper clathrate deposits - the feedback effect.
A major difference between the PETM (Natural) and present day (Anthropogenic) global warming is that Natural global warming was probably started by increased exposure to solar radiation causing methane feedback and rapid global warming. Anthropogenic global warming is being caused by the much faster release of CO2 caused by on-going burning of fossil fuels. Each year this releases larger amounts of CO2 into the atmosphere, increasing sea and atmospheric temperatures in the Arctic and starting the release of methane from clathrate, swamps and tundra.
Natural global warming is self-correcting: by the slow weathering of rock which can absorb carbon from the atmosphere and by the gradual return of the Earths orbit to a state where solar radiation reaching the Earth's surface is reduced, particularly at the poles. The result is cooling oceans able to gradually absorb and lower atmospheric CO2 and a cooler atmosphere. This enables gradual increase in the area and duration of ice and snow cover, increasing the amount of solar energy radiated back into space. This explains why global temperature is quick to rise to a maximum but slower to fall.
There is no "natural" way of reducing anthropogenic global warming since it is started by the very rapid and sudden (in geological terms) release of greenhouse gases into the atmosphere, now at their highest level in 15 million years. The release of greenhouse gases as a result of human activity is both continuous and accelerating. The only way of correcting this is to stop emission of CO2 and reduce its concentration in the atmosphere to levels which do not stimulate carbon feedbacks - that is, to a safe level, widely held to be 350ppm or less.
I know what you're thinking: Was it one shot or two?
How does global warming begin? Carozza et al (2011) undertook a study which found that Natural global warming occurred in 2 stages. First, global warming of 3° to 9° C occurs. This is accompanied by the release of a large amount of organic carbon caused by the burning of biomass on the Earth's surface (Kurtz et al, 2003) over a period of about 50 years. Second, there is a catastrophic release of methane from clathrate over a 50 year period - those released at depth oxidizing in the water column and entering the atmosphere as CO2, while shallow clathrate are released as CH4.
The description of Stage 2: Very rapid and massive release of carbon with low ∂13C does put one in mind of the Methane Gun hypothesis. This hypothesis states that rising temperature and/or lower sea level causes clathrate at shallow depth to begin melting which in turn causes warming to increase, melting even larger and deeper clathrate deposits. The result: the relatively sudden, massive and catastrophic release of methane into the atmosphere - the firing of the Methane Gun. The recent discovery by Davy et al (2010) of kilometer-wide (ten 8-11 kilometer and about 1,000 1-kilometer-wide features) eruption craters on the Chatham Rise seafloor off New Zealand adds further ammunition to the Methane Gun hypothesis.
It has been known for many years that methane is being released from Siberian swamplands which until recently have been covered by permafrost, trapping an estimated 1,000 billion tonnes of methane. Permafrost on land is now seasonally melting and with each season melting it at greater depth and over a wider area. The result is that each year the methane released into the atmosphere from this source increase.
Methane clathrate has accumulated over the East Siberian continental shelf where it is covered by sediment and very shallow seawater usually no more than 50 meters deep. An estimated 1,400 billion tonnes of methane is stored in these deposits. By comparison, total human emissions of CO2 from 1750 - 2005 add up to some 350 billion tonnes.
Onshore, clathrate is buried deep beneath the surface and would take centuries to melt. However, large releases of methane can occur when permafrost covering swampland is thawed by a warmer atmosphere and this is no occurring. Methane released from swampland and from offshore clathrate at shallow depth bubbles to the surface with little or no oxidation. It therefore enters the atmosphere as CH4. This powerful greenhouse gas increases local then Arctic atmospheric and ocean temperature resulting in deeper and larger deposits of clathrate melting.
Methane released from deeper clathrate deposits such as those found off Svalbard has to pass through a much thicker water column (more than 300 meters) before reaching the surface. As it does so much of it oxidizes to CO2 and in the process takes oxygen from the seawater. Some of it dissolves, helping to make seawater more acidic, before reaching the surface. Once reaching the surface, it then enters the atmosphere, largely as CO2, which causes far slower warming than CH4.
A very large release of methane due to melting of the vast deposits trapped by permafrost and clathrate in the Arctic would result in massive loss of oxygen, particularly in the Arctic Ocean and possibly in the atmosphere. This could cause large extinctions, especially of water breathing animals, which is what we find at the PETM.
Shakhova et al (2010) reports that the continental shelf of East Central Siberia (ECS), with an area of over 2 million square kilometers, is emitting more methane than all other ocean sources combined. She calculates that methane being released from the ECS is now in the order of 8 million tonnes per annum and increasing. This is equal to about 200 million tonnes/annum of CO2, more than the combined CO2 emissions of Scandinavia and the Benelux countries in 2007.
However, measurements by Shakhova indicate that emissions of methane from the ESA may be as high as 3.5 billion tonnes per annum (if verified, this amount is enough to trigger abrupt climate change), or about 3 times the total mankind artificially adds into the carbon cycle yearly. This methane release is likely coming from sediments previously covered by thin and now melting permafrost at the sea bed as well as melting clathrate, or some combination of both. Even more troubling, the ESA is estimated to hold 1,000 billion tonnes of methane in the form of clathrates alone...
Methane released from the ECS and onshore swamplands is already helping to raise temperatures in the Arctic, with the resultant warming at more than twice the rate of increase in average global temperature. The rate of release from onshore sources alone is conservatively predicted to reach 1.5 billion tonnes of carbon per annum before 2030, helping to make climate change even faster. This could result in the amount of ice being lost from the Greenland Ice Sheet to double every ten years, causing it to collapse (Hansen et al, 2011. This would produce a possible sea level rise of about 5 meters before 2100, according to Hansen et al.
Evidence supports the theory that sudden and massive releases of greenhouse gases, including methane, caused decade-scale climate changes - with consequent species extinctions - culminating in the Holocene Thermal Optimum.
'Ware the Kraken
In summary, immense quantities of methane clathrate have been identified in the Arctic. If a fraction of these were to melt, the result would be massive release of carbon, initially as CH4. This would produce global warming sufficient to cause deeper clathrate to melt and oxidize, adding CO2 to the atmosphere. Were this to occur, it would greatly worsen global warming.
While natural global warming during the ice ages was started by increased solar radiation due to cyclic changes in the way the Earth orbits the sun, there is no evident way of correcting Anthropogenic Global Warming over the next several centuries.
The latter has already begun producing methane and CO2 in the Arctic, starting a feedback process which may lead to uncontrollable, very dangerous global warming, akin to that which occurred at the PETM.
This extremis we ignore - to our peril.
More on Global Climate Change
Even More Methane Found Leaking From Arctic Seafloor
Potent Greenhouse Gas on the Rise: Atmospheric Methane Levels Increasing Again