Methane emissions in the pre-industrial era
Since the industrial revolution began in the mid-18th century, the human impact on atmospheric methane concentrations has been evident, through our consumption of fossil fuels. But according to a new study in the journal Nature, it seems human activities altered the amount of methane in the atmosphere long before that.
A team of scientists from Europe and the US analysed ice cores from North and Central Greenland, which contain tiny air bubbles stored over thousands of years. By looking at methane trapped in these bubbles, they were able to work out how emissions of the gas have changed over the last 2000 years.
Their records showed the rise and fall of human civilisations can be seen on at least three occasions in the pre-industrial era – as the amount of emissions from burning biomass like wood and coal increased and decreased.
Separating sources
Before the industrial revolution, there were three main sources of methane emissions. Some came from geological sources, such as volcanic eruptions, and some came from biological (plant) sources like wetlands and rice paddies.
The other big source was burning biomass, also called pyrogenic emissions. The authors suggest 20-30 per cent of pyrogenic methane emissions were due to humans, e.g. by clearing land for cultivation or burning wood and charcoal for fuel. They also point to a large contribution from natural sources, like wildfires.
By looking closely at the chemical makeup of methane molecules trapped in the ice core, scientists were able to work out the emissions from each of these sources, relative to one another.
It’s possible to tell the sources apart because the methane molecules they emit are slightly different. The carbon in methane comes in a number of forms, or isotopes, some of which are heavier than others. Methane released through burning and from geological resources contains more of this heavy carbon, and biological sources contain less.
Human civilisations
The methane record in the ice cores showed three blips in time when atmospheric methane contained more of this heavy carbon. When they compared this to other records of population growth, deforestation, temperature and rainfall, they were able to link the blips to the rise and fall of human civilisations.
During the first blip, at around AD 200, temperatures were high and the Roman empire was thriving. During this time, people burned through large amounts of charcoal, emitting lots of heavy carbon. The rise of Asian populations may have also contributed methane as rice paddies expanded to feed more people. But after AD 200, temperatures dropped, the human civilisations in Europe and Asia declined, and the rate of deforestation slowed.
The second blip occurred around the time of the Medieval Climate Anomaly, when many parts of the world experienced warmer than normal conditions. From AD 800 – AD 1200, human activity caused rates of deforestation to rise, in line with increased emissions from biomass burning. It’s thought the warm conditions might have also enhanced wildfires.
The third blip occurred around the time of the Little Ice Age – A period of colder temperatures between the 16th and mid-19th centuries. Colder and drier conditions meant the proportion of methane from biological sources (which are short on heavy carbon) like wetlands reduced. At the same time, the authors suggest emissions from burning rose as rapid land clearance took place – meaning carbon-heavy methane rose.
The bigger picture
Looking at carbon isotopes over this period reveals changes not immediately obvious in the longer-term trend, which shows the amount of methane in the atmosphere (relative to other things) increased fairly steadily over the pre-industrial period.
Having assumed that geological methane emissions didn’t vary much over those hundred-year blips, the scientists worked out that the changes had to be down to either an increase in emissions from burning (rich in heavy carbon), or a decrease in emissions from biological sources (depleted in heavy carbon).
The authors conclude both an increase in pyrogenic emissions and decrease in biological emissions must have happened, since the total amount of methane in the atmosphere didn’t vary wildly over the last 2000 years. Instead it rose in a fairly steady way until the turn of the industrial revolution.
We know current human activities that produce greenhouses gases affect the chemistry of our atmosphere. So it isn’t surprising that this research finds evidence of human impact in the pre-industrial era. There have been a number of periods before when civilisations have thrived, producing lots of greenhouse gases in the process. While this doesn’t account for all those methane emissions, humans certainly make up a considerable chunk.
Natural and anthropogenic variations in methane sources during the past two millennia
C.J. Sapart et. al
doi:10.1038/nature11461