October 4, 2012
Tracking Methane – Emissions Traced All The Way Back To Roman Times
April Flowers for redOrbit.com - Your Universe Online
A research team, led by the Niels Bohr Institute, has determined how much methane originates from natural sources and how much is due to human activity using special analytical methods. The resulting data can be dated back to Roman times and up to the present. More than half the emissions today are man-made.
Partly emitted from natural sources and partly from manmade sources today, methane is an important greenhouse gas. Natural sources of methane vary as the climate changes. An example of this would be the release of methane in the wetlands from bacteria. More methane is released during wet times than during dryer times when the wetlands shrink.
Methane emissions are also the result of human activities such as rice fields, biomass burning, or energy production like coal combustion. How can scientists determine where the methane originates, though?
"The different sources of methane have different isotopic compositions. The methane produced by the burning of biomass, like wood, contains more of the heavier isotope (carbon-13) relative to the lighter isotope (carbon-12), than methane which is produced in wetlands," explains Professor Thomas Blunier, Centre for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen.
The team measured the isotopic composition of methane in ice cores drilled from the Greenland ice cap at the NEEM project. Snow falls year after year, gradually being compressed into ice and forms the ice cap. Tiny air bubbles from the atmosphere travel with the snow and are trapped in the ice. By analyzing the composition of the air, the team was able to get a climate curve, which illuminates both the annual temperature and methane content.
The team wanted to understand how far back in history man has been affecting the methane concentrations in the atmosphere.
"We have analyzed the methane composition more than 2,000 years back in time. We can see that already 2,100 years ago during Roman times, some cultures were spreading out and burning large amounts of wood for fuel in furnaces to work with metals that required intense heat to process. But the level was still low. The next significant increase was during the Middle Ages around 1,000 years ago. It was a warm period and it was dry so there were presumably many forest fires that emitted methane while the wetlands dwindled and reduced methane emissions from that source. We also find emissions from natural forest fires and deforestation during the so-called 'Little Ice Age' (between 1350 and 1850), which was a very cold and dry period, Emissions of methane increased dramatically from around 1800, when the industrial revolution took off and where there occurred a large increase in population," explains Thomas Blunier.
Around the year 1800, there were large increases that are manmade, the study shows. Approximately half of this fabricated methane is from the production of food, specifically rice fields and cattle. Much more is emitted from the decomposition of organic materials that are deposited and from burning coal for energy production.
"The extent to which our ancestors were able to influence the emissions of methane with their activities is surprising. The general trend from 100 BCE to the year 1600 shows a correlation between the increase in the appropriation of land for cultivation and the emission of the biogenic methane. Today, half of the methane emissions stem from human activities," says Thomas Blunier.
The findings of this study are published in the journal, Nature.