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Ice cores reveal extent of human contributions to methane
9 October 2012, by Adele Rackley
A new study suggests that human activity began adding to atmospheric methane levels long before the industrial revolution.
The results challenge the benchmark for measuring global warming, because what have formerly been assumed to be 'natural' levels of this powerful greenhouse gas, before the widespread use of fossil fuels, have in fact been inflated by human activities since Roman times.
An international team of researchers looked at carbon isotopes in methane trapped in Arctic ice cores, to reveal with unprecedented accuracy the changing balance of methane from different sources. By mapping their results onto independent data on climate variability and land-use change, they have shown that human activity has influenced atmospheric methane levels for much longer than we thought.
'With our research, we can now show that the emissions of methane have indeed been influenced by human activities for at least two thousand years,' says lead researcher Dr Célia Sapart from Utrecht University, The Netherlands.
The work is published in Nature.
Methane has a variety of man-made and natural sources. It can be released from wet areas like swamps and rice fields, produced by fires, or released from geological sources.
The key to these new findings is that these different sources produce methane with different amounts of the isotope Carbon-13. So by studying these isotopes Sapart and colleagues could tease out the different sources of the methane trapped in the ice, and see how the relative contributions from those different sources had changed over the centuries.
Over the two millennia covered by the cores, the total amount of methane in the atmosphere rose gradually, until a sharp increase from the industrial revolution; 90 per cent of the total methane increase seen over the whole period has happened since 1850AD.
But the precision of the 13C data meant the researchers could see the changing contribution of the different sources of this methane over time. In particular they saw several spikes in levels of methane from fire.
The next step was to understand whether these changes were related to natural or human events. The team combined their isotope data with information about past climate variation, population growth and changes in land use.
'It was clear that climate alone could not explain the changes we were seeing in the isotope data,' explains Sapart.
Instead, the pattern matched human events, including the changing fortunes of the Roman Empire and the Han Dynasty in China. At their peaks these saw rapid population expansion, with fire being used for metal-working and heating, and swathes of vegetation being burnt to make way for agriculture.
The researchers associated other peaks in pyrogenic methane with a warm period known as the Medieval Climate Anomaly (MCA) and a cold period commonly known as the Little Ice Age (LIA).
'This shows that the peaks are not driven by climate variability,' says Sapart.
Though the warmer, dryer conditions of the MCA could have been conducive to forest fires, the colder wetter conditions in the LIA clearly were not. Instead, the population peak around the MCA led to another phase of land clearance as demands on agriculture increased. Conversely, in the LIA people would have had to burn extra fuel to keep warm.
The biggest impact of these new findings will be on our understanding of how natural methane emissions are likely to respond to changing climate conditions.
'If we hope to predict how methane levels are likely to vary in the future we must understand how natural methane sources have behaved in the past. But our findings show that man has influenced atmospheric methane for much longer than we thought, so we have to look further back in time to be sure we are looking at the behaviour of natural methane emissions.'