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Humanity has been affecting the climate for longer than previously imagined, according to scientists studying the history of the planet's atmosphere.

Researchers at New Zealand's National Institute of Water & Atmospheric Research (NIWA) examined tiny air bubbles trapped in ice cores drilled in East Antarctica.

They found that levels of atmospheric methane around 2,000 years ago were much higher than expected. The levels start to drop off after around 1,000AD, however, remaining low until the beginning of the industrial age, when they began to rise rapidly again, the researchers found.

Forest and grassland fires are a significant source of atmospheric methane. The presence of these larger quantities of methane so long ago suggests that there must have been more fires than would have occurred without human intervention.

The results tally well with both climate change, and human land use, according to the paper’s lead author, Dr Dominic Ferretti.

The analysis suggests that between 0 and 1500 AD, the indigenous population of the Americas regularly burned grassland and woodland areas, perhaps for agriculture and hunting. The arrival of Europeans caused a population crash, and with the declining numbers, the burning and methane production was also reduced.

In addition, the researchers say dry periods, like those triggered by El Niño events, correspond to higher levels of methane. They conclude that as global temperatures rise, forest fires will become more common, releasing more methane into the atmosphere.

Methane is a potent greenhouse gas, with an insulating effect 20 times as strong as that of carbon dioxide. The main difference is that it is not very stable, and only persists in the atmosphere for around 20 years. CO2, on the other hand, will happily stick around for a couple of centuries. ®

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