Methane (CH4) is an extremely potent greenhouse gas, 20-30 times more powerful than carbon dioxide (CO2) on a century timescale. Fortunately it normally occurs in very low concentration in the atmosphere – about 0.3 to 0.4ppm during glacial periods and 0.6 to 0.7ppm during warmer periods.
In 1750 the concentration was ~0.7ppm. By 2010 it had reached >1.8ppm, and is now at its highest level in 500,000 years. This is largely due to human activity, particularly the keeping of large herds of cattle and flocks of chickens and the production of fossil fuels. Methane has a relatively short life in the atmosphere where it oxidizes into CO2 over a period of 9-15 years.
Large amounts of methane are produced in anaerobic conditions by bacterial activity in the sediments below the seabed as well as by chemical transformation of organic matter at greater burial depths. Methane hydrates are formed by bonding with water to make an ice-like substance in certain temperature/pressure conditions that can be found at shallow water depths in polar regions. It yields 164 m3 of CH4 per m3 of solid clathrate.
Clathrate occurs in the Antarctic and particularly in the Arctic where it is abundant in the relatively shallow though very cold seabed of the vast continental shelves which almost encircle the Arctic Ocean. It also occurs in the sea bed of warmer waters where they are of sufficient depth to enable it to remain stable.
Methane clathrate has accumulated below the seabed over millions of years. Billions of tons of it lie dormant beneath permafrost, in the pores of sandstones or shrouded in silt. As long as it remains under pressure or in cold conditions (below 0°C) it is stable and does not release methane.
We know that in the past there have been sudden changes in global warming associated with releases of greenhouse gases. These rapid, massive releases were characterised by unusual deficiency in carbon isotope 13 (∂13C ) and large extinction (30-50%) of water breathing animals, particularly Benthic species, most recently at the time of the Paleocene-Eocene Thermal Maximum (PETM) about 55.8 million years ago.
The world at the approximate time of the PETM (courtesy Christopher Scotese)
It is believed that the PETM was likely initiated by changes of the orbital parameters of the Earth (eccentricity, obliquity and precession of axis) causing an increase in the intensity and distribution of solar radiation reaching the earth (Sexton et al, 2011). This in turn, over many thousands of years, triggered natural climate change, amplified by CH4 releases characterised by a ∂13C deficiency.
A major difference between the PETM (Natural) and present (Anthropogenic) global warming is that the former was likely initiated by increased exposure to solar radiation causing carbon feedbacks and rapid global warming. The latter, geologically sudden increase is primarily caused by the on-going burning of fossil fuels, which yearly inject a massive bolus of CO2 in the atmosphere, initiating further carbon feedbacks.
Natural global warming is self-rectifying either by slow chemical weathering processes responsible for mineral sequestration of carbon or by gradual return of Earth’s orbital parameters to what they were before the onset of global warming, thereby significantly reducing the amount of solar radiation reaching the Earth’s surface. The result is cooling oceans able to gradually absorb and lower atmospheric CO2, enabling restoration of albedo at higher latitude/altitude, producing further slow global cooling. This explains why post-maximum temperatures are slow to fall. The mechanism for reducing anthropogenic global warming, initiated through radiative forcing of greenhouse gases, is to stop emissions and reduce their concentration in the atmosphere to levels which do not stimulate carbon feedbacks.
Carozza et al (2011) find that natural global warming occurred in 2 stages: First, global warming of 3° to 9° C accompanied by a large bolus of organic carbon released to the atmosphere through the burning of terrestrial biomass (Kurtz et al, 2003) over approximately a 50-year period; second, a catastrophic release of methane hydrate from sediment, followed by the oxidation of a part of this methane gas in the water column and the escape of the remaining CH4 to the atmosphere over a 50-year period.
The description of Stage 2: Very rapid and massive release of carbon deficient in ∂13C, does put one in mind of the Methane Gun hypothesis. It postulates that methane clathrate at shallow depth begins melting and through the feed-back process accelerate atmospheric and oceanic warming, melting even larger and deeper clathrate deposits. The result: A relatively sudden massive venting ofmethane - the firing of the Methane Gun. 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 emitted from Siberian swamplands hitherto covered by permafrost, trapping an estimated 1,000 billion tons of methane. Permafrost on land is now seasonally melting and with each season melting it at greater depth, ensuring that each year methane venting from this source increases.
Methane clathrate has accumulated over the East Siberian continental shelf where it is covered by sediment and seawater up to 50 meters deep. An estimated 1,400 billion tons of methane is stored in these deposits. By comparison, total human greenhouse gas emissions (including CO2) since 1750 amount to some 350 billion tons.
Significant methane release can occur when on-shore permafrost is thawed by a warmer atmosphere (unlikely to occur in significance on less than a century timescale) and undersea clathrate at relatively shallow depths is melted by warming water. This is now occurring. In both cases, methane gas bubbles to the surface with little or no oxidation, entering the atmosphere as CH4 – a powerful greenhouse gas which increases local, then Arctic atmospheric and ocean temperature, resulting in progressively deeper and larger deposits of clathrate melting.
Methane released from deeper deposits such as those found off Svalbard has to pass through a much higher water column (>300 meters) before reaching the surface. As it does so, it oxidises to CO2, dissolving in seawater or reaching the atmosphere as CO2 which causes far slower warming, but can nevertheless contribute to ocean acidification.
A significant 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 but also in the atmosphere. Resulting hypoxic conditions would 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 km2, is emitting more methane than all other ocean sources combined. She calculates that methane venting from the ECS is now in the order of 8 million tons per annum and increasing. This equates to ~200 million tons/annum of CO2, more than the combined CO2 emissions of Scandinavia and the Benelux countries in 2007. This methane is likely sourced from non-hydrate methane previously kept in place by thin and now melting permafrost at the sea bed, melting clathrates, or some combination of both.
Release of ECS methane is already contributing to Arctic amplification resulting in temperature increase exceeding twice the global average. The rate of release from the tundra alone is predicted to reach 1.5 billion tons of carbon per annum before 2030, contributing to accelerated climate change, perhaps resulting in sustained decadal doubling of ice loss causing collapse of the Greenland Ice Sheet (Hansen et al, 2011). This would result in a possible sea level rise of ~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.
In summary, immense quantities of methane clathrate have been identified in the Arctic. Were a fraction of these to melt, the result would be massive release of carbon, initially as CH4 causing deeper clathrate to melt and oxidise, adding CO2 to the atmosphere. Were this to occur, it would greatly worsen global warming.
While natural global warming during the ice ages was initiated by increased solar radiation caused by cyclic changes to Earth’s orbital parameters, there is no evident mechanism for 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.
The above article refered to the PETM as an event marked by “massive extinction of animals”. Although there was “large extinction (30-50%) of water breathing animals, particularly Benthic species”, this is incorrect. The article has been modified from it's original form accordingly.
Posted by Riduna on Saturday, 23 April, 2011
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