Methane

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Methane

Introduction

At normal atmospheric pressure and temperature, methane is a colorless, odorless gas composed of carbon and hydrogen (chemical formula CH₄). It can burn with oxygen to form carbon dioxide (CO₂) and water (H₂O). Methane is the primary ingredient of natural gas, which is found underground in large deposits and is used as a fuel. It is also produced by some anaerobic bacteria, that is, bacteria that thrive in the absence of oxygen. Large deposits of methane exist in the Arctic permafrost and the ocean floors as methane hydrate ice (also called methane clathrate); there may be more carbon in these ices than in all coal, natural gas, and petroleum deposits on Earth.

Although there is far less methane in the atmosphere than CO₂, it is about 20 times as powerful a greenhouse gasasCO₂, molecule for molecule, and so is a significant cause of greenhouse warming. Both human-caused and natural releases of methane are important to climate change.

Historical Background and Scientific Foundations

Methane is created by microorganisms called methanogens, meaning methane-makers, and possibly by green plants as well (though this is disputed). Most of the methane found in natural gas deposits and in methane hydrate ices on the sea floors was formed by ancient methanogens; methanogens in wetlands, insects, animals, and oceans are important producers of methane today. Human activities that release methane, usually by encouraging the growth of methanogens, are waste disposal in landfills, energy production from natural gas and coal, raising cattle and sheep, growing rice, and burning fossil fuels or wood and other biomass.

Once in the atmosphere, methane (CH₄) molecules gradually combine with hydroxyl radicals (OH molecules). This reaction produces water and CH₃, which is not a greenhouse gas. About 90% of all methane removal from the atmosphere is by this reaction. Another 7% is by uptake by microbes living in soil, and a little less than 2% is due to reaction of methane with chlorine atoms in the ocean.

The average CH₄ molecule remains in the atmosphere for 8.4 years before it is removed by one means or another. While it is there, it causes about 20 times more greenhouse warming than a molecule of carbon dioxide. Although there is much less CH₄ than CO₂ in the atmosphere—less than one two-hundredth the number of molecules—methane produces over one-fourth the radiative forcing that carbon dioxide does. The radiative forcing of a greenhouse gas is the amount of heat that Earth retains per square meter of its surface as a result of that gas being in the atmosphere. In 2005, the radiative forcing of carbon dioxide was 1.66 watts per square

meter and the radiative forcing of methane was .48 watts per square meter.

Atmospheric concentrations of both methane and carbon dioxide today are much greater than they have been at any time for at least 650,000 years, as shown by tiny air bubbles trapped in ancient ice in Antarctica and Greenland. In 2005, the concentration of CH₄ was about 1,774 parts per billion, more than double the pre-industrial (pre-1760, approximately) concentration. Over the last 10,000 years, CH₄ concentrations varied slowly between 580 and 730 parts per billion, but in the last 200 years they have risen by about 1,000 parts per billion. Not only is the concentration high, but the rise has been the fastest for at least 80,000 years. Several lines of evidence show that the recent rise in the concentrations of CH₄ and CO₂ is anthropogenic (human-caused), not natural.

Unlike carbon dioxide, the rate of increase of atmospheric CH₄ has recently been slowing. Methane increased most rapidly in the late 1970s and early 1980s, more than 1% per year, but in the early 1990s slowed its increase and was practically unchanging from 1999 to 2005. The reason is methane's relatively short lifetime in the atmosphere, about a tenth of carbon dioxide's. Methane builds up in the atmosphere when annual emissions are greater than annual removals. The fact that emissions and removals have evened out implies that emissions have become nearly constant. This is in contrast to carbon dioxide emissions, which continue to increase.

Levels would be stable if emissions and removals increased together, but removals of methane are known to be approximately constant because the main removal mechanism is reaction with the hydroxyl ion, and the amount of hydroxyl ions in the atmosphere has remained steady. In short, anthropogenic methane emissions and natural removal of methane from the atmosphere are, for the moment, in balance. If emissions were to increase again, the methane concentration would increase also until a new balance was struck—one that contributed more to greenhouse warming. Conversely, if methane emissions were to decrease while removals remained steady, methane would contribute less to greenhouse warming.

Impacts and Issues

Methane Hydrate Ice

Although petroleum engineers discovered the existence of methane hydrate ices in the 1930s, when it was found that they were clogging gas pipelines, scientists did not discover until 1970 that vast quantities of methane hydrate are embedded in the ocean floor. This is a white compound made of methane molecules trapped inside porous water ice. The methane is produced by anaerobic, methanogenic bacteria digesting the bodies of other tiny organisms that rain down out of the upper layers of the ocean.

In nature, methane hydrate ice is stable only at temperatures near freezing and under the pressures found at depths of 1,640 ft (500 m) or more; it is found mostly near lower edges of the continental slopes, where the continental shelf drops off to the deep part of the ocean. Methane hydrate ice has been found all over the world, and although there is much uncertainty in estimating its total quantity, most assessments agree that there is probably more carbon locked up in these ices than in all fossil-fuel deposits put together, including coal. Many schemes have been suggested for harvesting the methane from methane hydrate ices to burn as a fuel, but none so far have proved affordable compared to other fuel sources.

WORDS TO KNOW

BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.

METHANE CLATHRATE: Form of water ice containing methane in its crystal structure; also called methane ice or methane hydrate. Methane clathrates form in sediments in many areas of the deep ocean floor. Estimates of total methane clathrate deposits have varied widely; such deposits are now thought to contain some hundreds of billion of tons of carbon, more than all natural gas resources put together. Methane clathrates, however, exist as scattered lumps mixed into sediment on the ocean floor and would be extremely difficult to harvest in large quantities.

METHANOGEN: Bacterium that produces methane as a waste product.

PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F(0°C) for several years.

RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing, solar radiation coming to Earth would continue to be approximately equal to the infrared radiation emitted from Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation, reradiating it back toward the surface and creating a warming influence (i.e., positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).

Methane hydrate ice may have played a major role in ancient climate change. There is much geological evidence that a major period of global warming 55 million years ago—the Paleocene-Eocene Thermal Maximum, which caused the extinction of many species and triggered the evolution of modern mammals—was produced by a giant methane release from the ocean floor. Evidence from marine sediments shows that some 15 trillion tons of methane hydrate may have formed beneath the world's oceans, then escaped rapidly into the atmosphere when the oceans warmed (for some asyet unknown reason). A sudden burst of greenhouse warming would have followed the methane burp.

A similar event may be caused by anthropogenic greenhouse warming. About 400 billion tons of ethane hydrate ices are locked in Arctic permafrost. The Arctic is already experiencing some of the most extreme greenhouse warming on Earth, about twice the planetary average, with widespread melting of permafrost. If enough methane is released by this process, and possibly also from marine methane ices, greenhouse warming may be increased by 10-15% over what it would be without the methane. This remains a source of uncertainty in predictions of climate change.

Methane Production by Green Plants

In 2005, researchers claimed to have discovered that green plants produce methane, a basic fact that had escaped the notice of generations of earlier researchers. This source of methane is so large and so recently identified that its role in the global methane budget is not yet well-understood, but as of 2006 green plants were estimated to produce between 70 million and 270 million tons of methane per year, about as much as wetlands, the largest previously known single source of methane. According to the scientists announcing the discovery, green plants produce more methane under warmer conditions, so global warming may cause forests to release more methane, accelerating global warming. However, forests absorb carbon dioxide as they grow, and the researchers' calculations also showed that the greenhouse benefits of planting forests (or refraining from cutting them down), achieved through their absorption of carbon dioxide, would greatly outweigh any possible greenhouse contribution from methane release.

Yet the claim that green plants produce methane has been challenged by other scientists, who in July 2007 announced that their experiments showed that green plants produce almost no methane at all—at most 0.3% of the amount claimed previously. Meanwhile, still other scientists have asserted that satellite measurements showing higher methane concentrations over tropical rain-forests confirm vegetation as a methane source. As of this writing, in early 2008, this important point remains undecided, with competing claims and no scientific consensus as yet.

Concentrations May Rise Again

Although methane concentrations were stable in the atmosphere in the early 2000s, this leveling-off may have occurred only because of an unusual drying of the world's wetlands. If this trend is reversed, methane emissions will increase and atmospheric levels may begin to rise again. Methane may also rise if methane hydrate ice in permafrost melts in large quantities. This is more likely in the near future than increased melting of marine methane ices, which are not as quickly reached by atmospheric warming as Arctic tundra.

See Also Greenhouse Effect; Greenhouse Gases; Tundra.

BIBLIOGRAPHY

Books

Khalil, Mohammad, and Aslam Khan. Atmospheric Methane: Its Role in the Global Environment. New York: Springer, 2000.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.