In addition to carbon dioxide, the increasing concentrations of methane are of great concern. The relative rate of increase of methane has greatly exceeded that of carbon dioxide in the last several decades. Molecule for molecule, this gas is much more powerful as a greenhouse gas than carbon dioxide, although much shorter lived in the atmosphere. Methane now provides for about 20 percent of the greenhouse effect, so the future development of its abundance in the atmosphere is of great interest. The present rate of increase (1 to 1.5 percent per year) is equivalent to adding another 30 to 40 percent of CO2 to the present input, in terms of radiative effect.
Methane is partially a result of growing food and ranching (rice, cattle), of producing waste dumps (decay of organic matter) and of deforestation (burning and decay of wood), as well as a by-product of the hydrocarbon economy. None of these sources are anticipated to decrease in the future. On the contrary, with 100 million more people needing food and fuel every year, the expectation must be for increased methane release from human activities.
Another source of methane is natural release from a variety of processes, such as decay of wet peat and decay of organic matter in stagnant ponds and warming of the sea floor. For these sources, changes in the environment produced by global warming are important. For example, thawing the permafrost in Alaska and Siberia may considerably influence the methane budget, since previously unavailable organic matter can decay, after thawing, in a wet environment with a lack of free oxygen. This is a prescription for producing methane. We do not know the course global warming will take (it depends on future release of greenhouse gases and all the climate physics associated with that) and we do not know the effect of such warming on the potential methane sources. Hence, all estimates on the future emissions of methane from natural sources are pure guesswork.
Methane in Ice Cores
Vostok Ice Core record of variations in air temperature (relative to the current average temperature of –55.5°C) and CH4 concentrations from gas bubbles in the ice. (Data from Petit et al., 1999)
Some idea about how the methane changes in the atmosphere in response to global warming and cooling can be derived from studies of climate history. Long ice cores drilled at the Vostok station in Antarctica provide a record of atmospheric gas concentrations and climate over the last 400,000 years. Ancient concentration of methane and carbon dioxide are preserved in air bubbles trapped in the ice. In addition, estimates of past temperatures can be derived from isotope ratios of hydrogen and oxygen of the ice molecules. Concentrations of methane have varied considerably over the Earth’s recent history. The correlation between CH4 records and Antarctic temperature records is remarkable. This supports the idea that methane has amplified climate change (i..e. it provided a positive feedback). Major transitions from the lowest to highest methane values are associated with each temperature transition from cold climate (ice-ages, or glacial conditions) to warm climates (inter-glacials). At these times, called “terminations,” the atmospheric concentrations of CH4 rose from 320-350 p.p.b.v to 650-770 p.p.b.v (parts per billion by volume).
Why did CH4 rise so rapidly during these terminations? This is one of the great unknowns in climate research. Melting of permafrost, growth of swamps in low-latitudes as sea level rose, and a general increase in wetlands on the continents are likely candidates. Also, it is possible that a warming ocean could have triggered release of methane from the sea floor due to the melting of methane hydrates. (Learn more about the polar ice research at American Chemical Society Publications)
Methane Hydrates-The Biggest Unknown
The structure of a methane hydrate molecule. It consists of methane gas (CH4) trapped within an accommodating structure of frozen water (H2O). Source USGS.
The biggest unknown — and the one with the greatest potential impact — is the release of methane from a warming sea floor. The amount of methane in the sea floor is thought to be greater than that of all hydrocarbon gases stored in reservoirs on land. In fact, it has been claimed that “methane-laced ice crystals in the seafloor store more energy than all the world's fossil fuel reserves combined” (Erwin Suess, Scientific American, Nov 1999). The icy storage is within a compound called “methane clathrate”, also called “methane hydrate” (See glossary for more on gas hydrates).
Methane hydrates form within smelly mud at depths of several hundred meters within water near freezing temperature below high-productivity regions in the oceans. They are unstable when warmed or depressurized and quickly begin to disintegrate when brought up from the seafloor.
The importance of methane hydrates only emerged in the last twenty years or so. Their abundance suggests they may be a new untapped source of natural gas. Natural gas is relatively benign in terms of producing pollutants since methane has only consists one carbon atom and four hydrogen atoms, and burning it produces more water than carbon dioxide.
(A) Methane hydrates look like white ice (B) The enclosed methane molecules are flammable and when ignited give rise to “burning ice” (Images from the USGS.)
Methane hydrates also represent a potential source of climate instability. As warming proceeds downward in the ocean, reaching deeper water layers, some of the methane ice will reach the limits of stability and will decay, releasing the methane gas. Some of this gas will escape to the air, increasing the greenhouse effect. Release of methane from warming of methane ice can apparently be sudden and possibly catastrophic. For example, an enormous submarine landslide off the coast of Norway was dated at 8000 years ago. It is thought that it was caused by warming of the deep sea and associated release of methane from methane ice. We believe that the associated flood waves on the shores of Norway and Greenland would have been very large. Such waves, presumably, can help remove shelf ice, which could trigger additional responses in ice streams buttressed by such shelf ice. Read more about gas hydrates on the USGS fact sheet:
USGS Coastal and Marine Geology Program