Ancient hunters may have begun global warming

WASHINGTON - Even before the dawn of agriculture, humans hunting the mammoths may have caused the planet to warm up, a new study suggests.

Mammoths used to roam modern-day Russia and North America, but are now extinct - and there’s evidence that around 15,000 years ago, early hunters had a hand in wiping them out.

A new study argues that this die-off had the side-effect of heating up the planet.

“A lot of people still think that people are unable to affect the climate even now, even when there are more than 6 billion people,” says Chris Doughty of the Carnegie Institution for Science in Stanford, California, who led the study.

The new results, however, “show that even when we had populations orders of magnitude smaller than we do now, we still had a big impact”.

Doughty, Adam Wolf, and Chris Field - all at Carnegie Institution - propose a scenario to explain how hunters could have triggered global warming.

First, mammoth populations began to drop - both because of natural climate change as the planet emerged from the last ice age, and because of human hunting.

The researchers postulated that normally, mammoths would have grazed down any birch that grew, so the area stayed a grassland. But if the mammoths vanished, the birch could spread.

In the cold of the far north, these trees would be dwarfs, only about 2 metres tall, but still dominating the grasses. The trees would also make darker the landscape’s colour, and thus absorbing more of the sun’s heat, and in turn, heating up the air.

This process would have added to natural climate change, making it harder for mammoths to cope, and helping the birch spread further.

To test how big of an effect this would have on climate, Field’s team looked at ancient records of pollen, preserved in lake sediments from Alaska, Siberia, and the Yukon Territory, built up over thousands of years.

They looked at pollen from birch trees (the genus Betula), since this is “a pioneer species that can rapidly colonise open ground following disturbance”, the study says.

Researchers found that around 15,000 years ago - the same time that mammoth populations dropped, and that hunters arrived in the area - the amount of birch pollen started to rise quickly, said a Carnegie Institution release.

In those places where there was dense vegetation to start with and where mammoths had lived, the main reason for the spread of birch trees was the demise of mammoths, the model suggests.

Doughty and colleagues then used a climate simulation to estimate that this spread of birch trees would have warmed the whole planet more than 0.1 degrees Celsius (0.18 degrees Fahrenheit) over the course of several centuries.

In comparison, the planet has warmed about six times more during the past 150 years, largely due to greenhouse gas emissions.

These findings are slated for publication in Geophysical Research Letters.