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 published on Wednesday 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, it said.

Celia Sapart at Utrecht University in the Netherlands and colleagues analysed 56 ice core samples drilled in north and central Greenland for levels of carbon 13, a telltale isotope of methane.

They overlaid this data against other tables, including deforestation and charcoal found in sediment, an indicator of human activity and wildfires.

Between 100 BC and 1600 AD roughly 28 extra billion 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 on a short timescale.

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 six percent 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 (206 BC-220 AD) and the Roman empire (27 BC to the last western emperor in 476 AD), which along with an advanced Indian civilisation at the time chopped down millions of trees to heat homes and power their metal-working industries, often to provide weapons.

"Based on archaeological metal production estimates, we calculate that the charcoal used for metal production at the peak of the Roman empire alone could have produced 0.65 teragrams (650 million tonnes) per year of methane," says the study.

During the early medieval period, land was cleared for fields to help population expansion in Europe and Asia. In the Little Ice Age -- a cold snap in the northern hemisphere in the 1300s and 1400s and again from 1600s to the 1800s that has been linked by some to changes in solar activity -- the evidence points to another pickup in fire activity.

From around 1800, there was another surge in methane emissions, and most of the gas emitted today is man-made, says the study.

"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, nor probe whether any warming affected weather patterns, but it has clear implications for work on climate change, said Sapart.

"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 email exchange with AFP.

The UN Framework Convention on Climate Change (UNFCCC) has set a goal of limiting warming to two degrees Celsius (3.6 degrees Fahrenheit). This figure is rooted in a comparison with carbon emissions in 1750, before the Industrial Revolution began voracious burning of coal, oil and gas -- fossil fuels whose carbon dioxide is released to the atmosphere and traps solar radiation.

Many scientists are cautious about the 2C (3.6 F) figure. They say Earth's climate system has many unknowns and it is unwise to set a "safe" target.

The study looked at carbon 13, an indicator of biomass burning, meaning wood and vegetation that is deliberately burned by humans but also by natural wildfires.

According to NASA's Goddard Institute for Space Studies, landfills emit around 25 billion tonnes of methane per year; farm animals around 80 billion tonnes; and biomass burning between 10 and 50 billion tonnes (http://icp.giss.nasa.gov/education/methane/intro/cycle.html).