Methane Emission Began in Roman Times; Humans Generate Half of the Greenhouse Gas

Methane is a greenhouse gas, partly emitted from the natural sources and partly from the human activities. Basically, the emission of the greenhouse gas methane into the atmosphere can be traced back to thousands of years in the Greenland ice sheet.

With the help of special analytical methods, the researchers from the Niel's Bohr Institute have determined how much methane originates from natural sources and how much is due to human activity.

They found traces of emission of methane right from the Roman times till the present date where humans are responsible for more than half of the emission.

The details of this study were published in the scientific journal Nature.

According to the study, the emission from natural sources varies due to the climate variations. For example, bacteria in wetlands release methane and less is emitted in dry periods as the wetlands shrink.

Even human actions trigger emissions of methane into the atmosphere such as methane is emitted from rice fields, which are of course wetlands, and methane is emitted from biomass burning, either from burning of forest areas for cultivation or the use of wood in furnaces. Even the energy production though coal combustion produces methane gases.

"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 isotopic composition of the methane in the ice cores have been measured by the researchers. These were drilled drilled up from the Greenland ice cap at the NEEM project in northwestern Greenland. 

The question is how far back in history man has had an impact on the methane concentration 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 Blunier.

"The extent to which our ancestors were able to influence the emissions of methane with their activities is surprising. The general trend from 100 BC 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," said Blunier.