Planet Earth Online

Coal deposits provide a record of ancient methane emissions

8 April 2009, by Tom Marshall

Ancient wetlands were the major source of methane emissions.

This is the conclusion of a new study, published in American Journal of Science, that uses present day coal deposits as a window into past concentrations of methane in the atmosphere.

Global atmospheric methane, or CH4, is a significant contributor to the greenhouse effect - it is around 20 times more effective as a greenhouse gas than CO₂. Present day atmospheric methane concentrations are more than double those of the pre-industrial era, and future increases will significantly worsen global warming resulting from higher levels of CO₂.

Large changes in atmospheric CH4 have also been observed in Earth's recent geologic past. Studies of air bubbles in glacial ice show CH4 concentrations at least doubled between the last glacial maximum, 21,000 years ago, and the pre-industrial era.

This study aimed to extend the record of CH4 concentration in the atmosphere far beyond that contained in ice cores, in order to investigate methane's effects on more ancient global climates. A better understanding of the past climate will improve our ability to predict the course of future climate change.

'The key result is that although CO₂ was still the dominant greenhouse gas over the Phanerozoic [the last 550 million years of Earth's history], methane is important over periods when wetland extent and activity is high' says Professor David Beerling of Sheffield University, lead author of the study. 'During the Permo-Carboniferous period around 300 million years ago, when there was extensive coal swamp formation, the forcing by methane was probably greater than forcing by CO₂'. The study's authors also include scientists from Yale, Hawaii and Cambridge Universities.

The team were able to estimate the input of CH4 to Earth's atmosphere in the ancient past by using CH4 emissions calculated for wetland areas, such as swamps, marshlands, bogs and lakes, combined with the past extent of wetland areas known from coal deposits, This estimate is known as a 'proxy' for past methane emissions.

Beerling describes the development of this proxy. 'We lacked a methane proxy during Earth's ancient geologic past, but recognised that the main terrestrial methane flux in the pre-human world was wetlands, and that coal deposits in turn represent a proxy for wetland abundance.'

The creation of this proxy was not straightforward, though. To estimate the past extent of wetlands based on present day abundances of coal and non-commercial organic matter deposits, the researchers had to take deposit loss due to erosion and past tectonic activity into account.

Additionally, while previous proxies simply used a pre-industrial value for CH4 emitted by a given area of wetland, this study calculated a value for wetlands 3-4 million years ago - a time when Earth's climate was warmer, wetter and more representative of ancient periods when wetlands were abundant. To get this value, a complex computer model of Earth's climate system was combined with computer models of vegetation and wetland gas emissions.

Using coal abundance as a proxy for fossil wetlands, the scientists calculated how much methane has been emitted into the atmosphere over the last 400 million years of Earth's history - a time when the existence of large land plants made substantial wetland coal deposits possible. CH4 emissions over the last 400 million years from other sources, such as oceans, wildfires, termites, volcanoes and hydrates [underground frozen methane deposits] were ignored as they are relatively small compared to wetland emissions.

The final step was to feed the calculations of wetland CH4 emissions into a computer model of atmospheric chemistry. This model captures the complex chemical reactions that take place in Earth's atmosphere, and used the estimated wetland emissions to give concentrations of atmospheric CH4. Using these concentrations the greenhouse effect due to CH4 could then be calculated over the last 400 million years.

The use of this complex chemistry model is a substantial improvement over previous studies, which used much less sophisticated atmospheric chemistry. This produces not only improved estimates of past CH4 concentrations, but also estimates of related changes in ozone and water vapour in the high atmosphere that in turn produce an additional greenhouse effect.

The study also included the effects of another greenhouse gas, nitrous oxide, commonly known as laughing gas. Atmospheric concentrations of CH4 and nitrous oxide changed hand in hand throughout the glacial-interglacial cycles of Earth's recent geologic history, and the scientists used this relationship to extend records of atmospheric nitrous oxide concentrations over the last 400 million years.

The result of all this effort is an estimate of the greenhouse effect caused by methane, ozone, stratospheric water vapour and nitrous oxide over the last 400 million years of Earth's history. Importantly, this shows that the changes in past climate driven by these non-CO₂ greenhouse gases are not unimportant relative to those caused by variations in both atmospheric CO₂ and radiation from the sun.

'When we're modelling ancient climate we need to think about these non-CO₂ greenhouse gases,' says Beerling. 'Most past climate studies tend to assume pre-industrial levels of those gases, whereas our results indicate this is invalid. By obsessing with CO₂ you're underestimating the contribution to warming by these non-CO₂ greenhouse gases.'

Understanding Earth's past climate will also help us better predict the effects that increasing greenhouse gas concentrations may have on our climate in the future.

Beerling explains 'Some IPCC [Intergovernmental Panel on Climate Change] future greenhouse gas emissions scenarios show by 2100 we could have an atmosphere very similar to what we had 50 to 100 million years ago in terms of methane, ozone, nitrous oxide and CO₂. In the past, the equilibrium response to these conditions was an ice free Earth.'

Beerling then goes on to consider the role of non-CO₂ greenhouse gases in helping us mediate future climate change.

'CO₂ and water vapour in the atmosphere are the primary greenhouse gases, but if you put a lot of effort into controlling our non-CO₂ greenhouse gases then we could perhaps buy ourselves a little more time for getting CO₂ levels down,' he says.

This is a view also promoted by James Hansen, a well-known climate scientist and head of NASA's Goddard Institute for Space Studies, whose work is cited in this study.

Cutting emissions of non-CO₂ greenhouse gases would also have other benefits. Reducing atmospheric concentrations of ozone and nitrous oxide would benefit people's health, while reducing ozone would also improve crop productivity.

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