Methane: A Scientific Journey from Obscurity to Climate Super-Stardom
The first survey in 1971 on the possibility of inadvertent human modification of climate stated that "Methane has no direct effects on the climate or the biosphere [and] it is considered to be of no importance". The gas did not even appear in the index of the major climatology book of the time (Lamb's Climate Past, Present and Future). Yet in the 2001 IPCC report, large parts of multiple chapters are dedicated to examining the sources, sinks, chemistry, history and potential future of this humble molecule. New papers are published every month relating paleo-climate changes to methane variability and discussing the possibility of significantly reducing future anthropogenic climate change by aggressively managing methane emissions. New hypotheses such as the "clathrate gun hypothesis" (more below) place methane variability at the centre of the debate on rapid climate change.
What has fueled the rapid rise of methane from an obscure trace gas to a major factor in past, present and future climate change? As is usual in science, it is the conflation of multiple lines of evidence, that only when taken together do the connections and possible feedbacks seem obvious.
Methane as a Greenhouse Gas
First some basics: methane (CH4) is a very simple molecule (one carbon surrounded by four hydrogen atoms) and is created predominantly by bacteria that feed on organic material. In dry conditions, there is plenty of atmospheric oxygen, and so aerobic bacteria which produce carbon dioxide (CO2) are preferred. But in wet areas such as swamps, wetlands and in the ocean, there is not enough oxygen, and so complex hydrocarbons get broken down to methane by anaerobic bacteria. Some of this methane can get trapped (as a gas, as a solid, dissolved or eaten) and some makes its way to the atmosphere where it is gradually broken down to CO2 and water (H2O) vapor in a series of chemical reactions.
Although methane was detected in the atmosphere in 1948, its importance to climate was only recently revealed by three key discoveries. The first, by Wei-Chyung Wang and colleagues at NASA GISS in 1976, was that methane in the atmosphere was actually a significant greenhouse gas — it absorbs some frequencies of infrared radiation (emitted from the Earth's surface) that would otherwise go straight out to space. In combination with other greenhouse gases (water vapor, CO2 and N2O), this leads to a surface temperature that is about 30°C warmer than if there were no atmosphere.
The second key result was due to the recovery and analyses of Greenland and Antarctic ice cores. These multi-kilometer cores, drilled through the ice sheets by both European and American research teams, have shown in unprecedented detail climate changes over centuries, millennia, and hundreds of thousands of years. Indeed, annual layers can be discerned for much of the length of the cores, which allowed researchers to construct an extremely accurate timescale for the climate-related changes they found.
Along with isotopic analyses of the ice itself (which is mainly related to temperature), the researchers (such as Jérôme Chappellaz in Grenoble, France) were able to isolate the gases trapped inside tiny bubbles in the ice. The greenhouses gases, CO2 and CH4, within those bubbles showed that since the industrial period began (around the mid-1800s) concentrations of both CO2 and CH4 have been increasing rapidly. In fact, CH4 concentrations have more than doubled over the last 150 years, and the contribution to the enhanced greenhouse effect is almost half of that due to CO2 increases over the same period.
The changes over the last century seem to be mostly related to increased emissions due to human activity: leaks from mining and natural gas pipelines, landfills, increased irrigation (particularly rice paddies, which are essentially artificial wetlands) and increased livestock producing more intestinal CH4 (!) among other factors. However, over the last ice age, and particularly in the turbulent world just prior to the modern Holocene period (roughly the last 11,500 years), methane was observed to oscillate almost hand-in-hand in response to rapid climate changes such as the Younger Dryas cold interval (a return to almost full ice age conditions 12,500 years ago).
Methane Sensitivity to Climate
The third key piece of evidence was an exceptional investigation by Jeff Severinghaus of Scripps Institution of Oceanography and Ed Brook of Washington State University. They convincingly showed (using some novel geochemistry involving the isotopes of nitrogen that react to rapid changes in surface temperatures) that methane rapidly increases in a warming climate with a small lag behind temperature. Therefore, not only does methane affect climate through greenhouse effects, but it in turn can evidently be affected by climate itself.
With these observations — that methane is a greenhouse gas, that changes to emissions can affect the atmospheric concentration, and that climate can cause methane emissions to vary — there is a potential for some very interesting positive feedbacks. The question then turns to what controls this variability and how large these effects can be. Researchers have only started to delve into the details necessary to understand the links more thoroughly.
I have already discussed the principle methane sources, but the other side of the coin is the sink of methane. Once emitted into the atmosphere, what happens to it? The answer is intimately bound up in atmospheric chemistry.
In the lower part of the atmosphere, below about 10-12 km (the troposphere), the key cycles are mediated above all by the presence of what are called OH radicals — colloquially known as the atmospheric detergent. All hydrocarbon chemical species that are emitted can be eventually broken down (or oxidized) by these radicals to CO2 and H2O, and methane is no exception. An average molecule of CH4 lasts around eight to nine years before it gets oxidized. This is a long time compared to most atmospheric chemicals but is fast enough so that there can be significant year-to-year variability. Around 10% of the CH4 makes it into the upper atmosphere (the stratosphere, between 15 and 50 km above sea level)) where it also gets oxidized, though through a slightly different set of reactions. A key point is that in the very dry stratosphere, the water produced from methane oxidation is a big part of the water budget and stratospheric water vapor is a greenhouse gas in its own right! This indirect process enhances the climate impact of methane changes by about 15%.
Climate Impacts on Methane
There are many possible climate influences on the methane sink. For instance, as the climate warms or cools, the amount of water in the air changes, which in turn affects levels of OH. As it gets warmer, the sink becomes more efficient and CH4 levels would fall in the absence of other changes. Changing emissions of other chemicals (e.g. carbon monoxide in biomass burning, complex hydrocarbons from vegetation) can compete for OH and again cause CH4 to change proportionately. In fact, the CH4 level itself has a positive feedback on its own lifetime: i.e., the more methane there is, the more OH is "used up" and the longer the methane can stick around.
What about variations in emissions that might be climatically controlled? I mentioned above that natural methane emissions depend on the extent of organic decomposition in very wet conditions. It turns out that for an individual wetland, an increase in the water table (for instance as rain increases) and/or an increase in temperature will lead to greater emissions on a very short time scale. Over longer periods, wetlands and river deltas come and go as a function of sea level or changes in large-scale rainfall patterns. In the high latitudes, the freezing and thawing of permafrost regions can significantly change the extent of peat bogs and hence emissions. A recent study in Sweden demonstrated that emissions increased by between 20 and 60% over a 30-year period due to permafrost thawing.
Over recent decades the growth rate of methane has oscillated significantly and, indeed, has been basically zero (i.e., no increases) for the last three years. The combination of changes in wetland emissions and climate-related cooling during the Mt. Pinatubo eruption (1991-1993) combined with changes in economic activity, particularly in the former Soviet Union, seem to explain most of this variability although there are still large error bars in these estimates. There is, however, one additional reservoir of methane about which very little is known: the methane clathrate reservoir in the oceans — the 600-pound gorilla of methane variability!
Methane Clathrates and Climate
Clathrates are a class of compound that consist of a cage of molecules that can trap gases, such as methane, in a solid form. For methane, the most important "cage" is one that is made of water molecules, and so is described sometimes as a hydrate. Some key facts about clathrates make them particularly interesting to climatologists. First, they may make up a significant portion of total fossil carbon reserves, including coal and oil. Current best guesses suggest that maybe 500 to 2000 gigatonnes of carbon may be stored as methane clathrates (5-20% of total estimated reserves). Some estimates are as high as 10,000 gigatonnes. They occur mainly on the continental shelf where the water is relatively cold, there is sufficient pressure and enough organic material to keep the methane-producing bacteria happy. Most importantly, clathrates can be explosively unstable if the temperature increases or the pressure decreases — which can happen as a function of climate change, tectonic uplift or undersea landslides.
The importance of these clathrates in climate change has only recently started to be appreciated. The first clue was some puzzling data from a period 55 million years ago. In the early 1990's, Jim Kennett of Scripps Institute of Oceanography and his colleagues noticed that during an extremely short amount of time (geologically speaking) at the transition between the Paleocene and Eocene epochs, carbon isotope ratios everywhere (the deep sea, on land, at the poles and in the tropics) suddenly changed to favour the lighter 12C isotope of carbon at the expense of 13C. The rapidity and size of this change was unprecedented in the period since the demise of the dinosaurs, and this excursion was simultaneous with a short period of extreme global warming (around 3 to 4 degrees globally, more in the high latitudes).
In 1995, Jerry Dickens of Rice University suggested that the only conceivable perturbation to the global carbon cycle to fit these data was a massive input of light carbon that had been stored as methane clathrates, which are observed to be particularly high in 12C. Nothing else could be as fast-acting or have enough of the lighter isotope to have had the observed effects. Given that both CH4 and its oxidization product CO2 are greenhouse gases, this might explain the global warming as well.
Subsequent work, including atmospheric chemistry studies by myself and Drew Shindell of NASA GISS, have confirmed that this hypothesis is still the most likely candidate, although the initial triggering mechanism is unknown. Similar ideas have been proposed to explain short term events in the Jurassic, at the Permian-Triassic boundary and in the Neo-Proterozoic, although the evidence for a unique role of methane in these cases is much weaker than at the Paleocene/Eocene boundary.
With a plausible role for methane clathrates in the Paleocene, it is only natural to examine whether they played a similar role in more recent climate changes, such as rapid climate variability during the last ice age. There are some tantalizing clues. In ocean sediments offshore of California, Kai-Uwe Hinrichs and colleagues at Woods Hole recently found geochemical traces of clathrate releases coincident with warmings in the Greenland ice core records. In some records, there are coincident spikes in the carbon isotope record, reminiscent of the Paleocene/Eocene spike but of lower amplitude. This has led Jim Kennett to propose the so-called "clathrate gun hypothesis", that methane builds up in clathrates during cold periods, and as a warming starts it is explosively released, leading to enhanced further rapid climate warming. This idea is not yet widely accepted, mainly because the records of methane in the ice cores seems to lag the temperature changes, and the magnitudes involved do not appear large enough to significantly perturb the radiative balance of the planet. The more conventional explanation is that as the climate warms there is increased rain in the tropics and thus increased emissions from tropical wetlands which need to have been large enough to counteract a probable increase in the methane sink. There is, however, much that we don't understand about the methane cycle during the ice ages, and maybe hydrates will eventually be considered part of the rapid climate change story.
Methane and the Future
This example does lead to the wider question: What role does methane (including methane clathrates) play in the global carbon cycle? Dickens for one has suggested that all carbon cycling models need to take the clearly significant methane reservoir into account. However, much is still to be learned about the clathrates. How extensive are they? How long is required for the reservoir to fill? What happens when clathrates are exposed to the ocean? What is the role of methane-eating bacteria in the sediment? Until recently these questions were considered extremely esoteric, but such subjects now have moved to the mainstream in carbon-cycle research.
In order to answer these questions, researchers are currently working on improvements in the understanding of many aspects of the methane cycle. They are refining techniques for measuring the carbon isotopes in the ice-core methane to possibly distinguish between different sources. Models of the climate and atmospheric chemistry are being revised and improved in order to simulate the variations in methane seen in the ice core records. Deep in the ocean, researchers are paying close attention to the chemistry and biology that is going around the clathrate deposits. In the high latitudes, scientists are better quantifying the fluxes of methane from peat bogs and thawing permafrost regions. Slowly but surely some of the uncertainties are being reduced.
And so what of the future? Over the last few years atmospheric methane concentrations have hardly changed. This is clearly good news for those worried about continued greenhouse warming, but until scientists understand why, there is no assurance that increased emissions won't resume. This stands as a clear reminder that we still do not know everything we need to about methane. Many of the anthropogenic sources of methane such as irrigation, mining, farm practices, etc. are however relatively cheap and straightforward to control. For example, New Zealand's primary contribution to greenhouse gas emissions is from sheep digestive processes, and scientists there are experimenting with changes to feed that have the potential to reduce methane production enormously. Similarly, improved capture of methane released in mining and oil operations is providing a cleaner fuel source and increasing profits for the companies concerned.
The responses of wetlands and clathrate deposits to climate change are hard to foresee, and one wild card is the extent to which the potential exploitation of the clathrate reservoir for energy production might lead to increased releases to the atmosphere. However, the technologies involved in this have yet to be fully developed and so forecasts are extremely uncertain. Research is now being conducted on reducing methane emissions from almost all the sources and that may possibly allow for relatively short term (<50 years) decreases in methane concentrations, and a consequent reduction in the forces driving global warming.
Over the last 30 years, methane has gone from being a gas of no importance, to — in some researchers eyes, at least — possibly the most important greenhouse gas both for understanding climate change and as a cost-effective target for future emission reductions. Whether some of these new ideas stand up to the scrutiny of the wider climate research community remains to be seen, but one thing is certain, the scientific journey of methane is not yet complete.
References and Further Reading
Alley, R.B. 2000. The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. Princeton University Press.
Christensen, T.R., T. Johansson, H.J. Åkerman, M. Mastepanov, N. Malmer, T. Friborg, P. Crill, and B.H. Svenson 2004. Thawing sub-arctic permafrost: Effects on vegetation and methane emissions. Geophys. Res. Lett. 31, L04501, doi:10.1029/2003GL018680.
EPICA Community Members 2004. Eight glacial cycles from an Antarctic ice core. Nature 429, 623-628, doi:10.1038/nature02599.
Hinrichs, K.-U., L.R. Hmelo, and S.P. Sylva 2003. Molecular fossil record of elevated methane levels in late Pleistocene coastal waters. Science 299, 1214-1217, doi:10.1126/science.1079601.
Intergovernmental Panel on Climate Change 2001. Climate Change 2001: The Scientific Basis. Cambridge University Press.
Kennett, J.P., K.G. Cannariato, I.L. Hendy, and R.J. Beh 2003. Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis. American Geophysical Union.
Lamb, H.H. 1972. Climate Past, Present and Future, vol. 1. Methuen.
Ruddiman, W. 2003. The anthropogenic greenhouse era began thousands of years ago. Climatic Change 61, 261-293, doi:10.1023/B:CLIM.0000004577.17928.fa.
Schmidt, G.A., and D.T. Shindell 2003. Atmospheric composition, radiative forcing and climate change as a consequence of a massive methane release from gas hydrates. Paleoceanography 18, doi:10.1029/2002PA000757.
Severinghaus, J.P., and E.J. Brook 1999. Abrupt climate change at the end of the last glacial period inferred from trapped air in polar ice. Science 286, 930-934, doi:10.1126/science.286.5441.930.
Wang, W.-C., Y.L. Yung, A.A. Lacis, T. Mo, and J.E. Hansen 1976. Greenhouse effects due to man-made perturbation of trace gases. Science 194, 685-690.
About the Author
Gavin Schmidt is a research scientist at the NASA Goddard Institute for Space Studies and Center for Climate Systems Research, Columbia University in New York. He works on models of the climate system and their application to problems of past, present and future climate change. This article was commissioned for the September 2004 issue of La Recherche, Paris, and this English-language translation appears here with their permission.