Human-caused methane emissions can be traced back thousands of years through the use of Greenland ice sheet cores. Special analytical methods allow researchers to determine how much of the methane originated from natural sources and how much originated from human activities. This data extends all the way from early Roman times up to the present day, currently more than half of all the methane emissions are human-made.
Methane is a very potent greenhouse gas, currently it’s being emitted in significant quantities both from natural sources and from human activities. The emissions from natural sources wax and wane based on variations in the climate. As an example, wetlands release methane through the actions of bacteria, but considerably less is emitted during as the wetlands dry and lose area.
“Emissions of methane into the atmosphere also come from human actions. For example, 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. Energy production through coal combustion also produces methane gases. But how can you determine where the methane gas comes from?”
“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.
For the research, the isotopic composition of the methane in ice cores was measured, these were drilled up from the Greenland ice cap at the NEEM project in northwestern Greenland. “The ice cap is formed from snow that falls year after year and remains, gradually getting compressed into ice. The ice contains tiny air bubbles from the atmosphere in the snow that fell, and by analysing the composition of the air you can get a climate curve, which tells you about both the annual temperature and methane content.”
So the question that the researchers had was how far back in history has man had a significant impact on the methane concentration in the atmosphere?
“We have analysed 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.
Their analysis showed that from “around the year 1800 there are large increases that are human-made. Approximately half originates from the production of food — especially rice fields and cattle. Then a lot is emitted from the decomposition of organic materials that are deposited and methane is emitted from burning coal for energy.”
“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 new research was just published in the journal Nature.
Source: University of Copenhagen
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