Pre-industrial man was already changing climate, study shows
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Humans were big emitters of greenhouse gases long before the Industrial Revolution, a finding that raises worrying questions about the benchmark for measuring global warming, a study said.
For 1,800 years before industrialisation took off in the 19th century, emissions of methane rose in line with expanding populations, human conquest and agricultural techniques, a report published last week said.
Dr Celia Sapart at Utrecht University in the Netherlands and colleagues analysed 56 ice-core samples drilled in northern and central Greenland for levels of carbon-13, a telltale isotope of methane.
They overlaid this data against other tables, including for deforestation and charcoal found in sediment, indicators of human activity and wildfires.
Between 100BC and AD1,600, roughly 28 billion extra tonnes of methane per year were added to the atmosphere, according to the analysis.
Methane is a potent greenhouse gas, more than 20 times more efficient than CO2 in trapping solar heat in the short term.
Twenty-eight billion tonnes is roughly equivalent to the annual emissions of this powerful greenhouse gas from all the landfills in the world today, which in turn account for about 6 per cent of global methane emissions.
The major contributors are likely to have been deforestation, biomass burning and rice paddies, rather than geological sources such as mud volcanoes, according to the study published in the journal Nature.
Big early increases coincided with the Chinese Han dynasty (206BC-AD220) and the Roman empire (27BC to the last Western emperor in AD476), which along with an advanced Indian civilisation at the time, chopped down millions of trees to heat homes and power metal-working industries, often to provide weapons.
From around 1800, there was another surge in methane emissions, and most of the gas emitted today is man-made.
"The footprint is visible on a global level. That's what surprised us," said Sapart.
The study was not designed to calculate the additional warming from the methane emissions, but it had clear implications for work on climate change, Sapart said.
"This study shows the urgency of controlling greenhouse-gas emissions as soon as possible, because it shows that the disequilibrium in the climate system caused by humans existed for much longer than we expected," she said in an e-mail.