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Understanding how atmospheric methane varied before the onset of industrialisation

21 February 2017

Bristol-led team publishes a new study on the glacial methane cycle in Nature Communications.

Atmospheric methane (CH4) is the third most important greenhouse gas after water vapour and carbon dioxide (CO2). After CO2, CH4 is also the second most important cause of man-made global warming since the start of the industrial revolution, contributing around 20% of the increase in the man-made greenhouse effect since AD 1750.

The increase in CH4 since the start of the Industrial Revolution from around 680 p.p.b.v. (parts per billion by volume) to over 1800 p.p.b.v. today was almost entirely due to human activities. However, prior to this, natural variations in atmospheric CH4 occurred. The largest and most recent natural change was the increase in CH4 since the end of the last ice age.

This increase is very likely due to climatic warming and associated changes in rainfall patterns, as well as the exposure of land under receding ice-sheets. These climatic changes impacted the sources and sinks of atmospheric CH4, and the increase in land area not covered by ice allowed formation of peatland soils which emit CH4. However, the precise reasons for the observed CH4 increase remain uncertain.

In this new paper, a team of researcher, led by Dr Peter Hopcroft and Prof. Paul Valdes, both of the University of Bristol's School of Geographical Sciences and Cabot Insitute, used the comprehensive Met Office Hadley Centre Earth System model to understand how these environmental changes led to the observed increase in atmospheric CH4 since the ice-age.

The results of this study showed that although there are large contributing factors for the CH4 sinks (such as emissions of biogenic volatile organic compounds (BVOCs) from forests, and climate), the net effect of these is that the CH4 lifetime during the ice-age is longer than during the pre-industrial era by 2-8%. This means that the observed increase in CH4 since the ice-age must have been driven by increased CH4 emissions, particularly from wetlands.

This model study was unable to fully reconcile the required wetland CH4 source increase, and so highlights the need for further research to better understand how natural CH4 sources (particularly wetlands) respond to environmental changes, including climatic warming and changes in atmospheric CO2. Because the natural CH4 cycle could also represent a positive feedback on future climate change, a better understanding these factors will help to inform estimates of the greenhouse gases we can emit whilst still maintaining a likely temperature rise below specified levels.

Further information

Paper available here:
http://www.nature.com/articles/ncomms14383

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