3 October 2012

Air bubbles in ancient ice cores provide proof

Man already contributed to greenhouse gases in Roman times

The influence of humans on the emissions of the greenhouse gas methane dates back to Roman times, which is much earlier than previously assumed. These conclusions of Utrecht University researchers, the SRON Netherlands Institute for Space Research and international fellow-researchers were published in the scientific magazine Nature on 4 October.

It has long been assumed that man only started to make a major contribution to the emission of methane from the Industrial Revolution onwards. However, ten years ago, the famous American climatologist William Ruddiman suggested that humans influenced the climate much sooner, but his hypothesis received much criticism.

"Research conducted over the last ten years has provided clues that the burning of biomass by people in the Middle Ages possible resulted in increased methane emissions. 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 Dr. Célia J. Sapart from Utrecht University.

Air bubbles trapped in Greenland ice

Little amounts of old atmosphere is trapped in air bubbles that are located in Greenland ice. Sapart and her colleagues developed a technique with which they could determine the origin of the methane with unprecedented accuracy.

Methane can end up in the atmosphere in different ways. It originates in wet areas, such as swamps and rice fields, and is formed during the combustion of biomass and fossil fuels, and it may be released from so-called mud volcanoes. The technique used by Sapart and her colleagues has made it possible to make a distinction between the different sources.

"Each source reveals itself by its unique fingerprint", explains Sapart. "The ratio between regular methane and its stable carbon isotope, which is methane in which the carbon atom has one neutron more, and thus is slightly heavier, is unique. With our technique, we can determine the isotope ratio much more accurately than before."

The decline of the Roman Empire

The researchers compared the emission of methane per source to other data about the period concerned, such as the climate variability, the population and land use data. This has clearly revealed several periods of higher methane emissions originating from the burning of biomass
For instance, the starting point of the measurements coincided with the peak of the Roman Empire. The huge population burnt a considerable amount of wood. Researchers discovered that the burning of biomass decreased in the subsequent period in concert with the decline of the Roman Empire. The population grew considerably later on in the Middle Ages, and researchers observed another increase in the amount of methane from this source during this period.

The researchers also compared their data to information about the use of land by humans. There again, they discovered a connection between the emissions of methane due to the growing of crops and the area used by humans for agriculture, for example. "These results support Ruddiman's hypothesis", says Sapart.

"This research is a good example of the added value you can achieve when you combine experimental research with computer models", concludes SRON researcher Dr. Sander Houweling.

Publication

'Natural and anthropogenic variations in methane sources during the past two millennia',
Célia J. Sapart, G. Monteil, M. Prokopiou, R.S.W. van de Wal, J.O. Kaplan, P. Sperlich, K.M. Krumhardt, C. van der Veen, S. Houweling, M.C. Krol, T. Blunier, T. Sowers, P. Martinerie, E. Witrant, D. Dahl-Jensen en T. Röckmann,
Nature 2012, DOI: 10.1038/nature11461

This research was sponsored by the Netherlands Organisation for Scientific Research (NWO) and the Swiss National Science Foundation. The project is a part of the deep drilling project in northwest Greenland.

More information

Monica van der Garde, press officer Faculty of Science, + 31 6 13 66 14 38, m.vandergarde@uu.nl.