Methane emissions in the pre-industrial era
- 08 Oct 2012, 14:40
- Freya Roberts
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
new study in the journal Nature, it seems human activities
altered the amount of methane in the atmosphere long before
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.
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
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.
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
Updated 9/10/12: Corrected the date of the industrial
revolution to mid-18th century.