It turns out humans have been affecting the Earth's climate for a long, long time. Overhunting the woolly mammoth 15,000 years ago kick-started a chain reaction that changed the surrounding ecosystem and heated up the Earth.

How could humans have affected the prehistoric Earth's climate at a time when their population only numbered in the millionth, less than a thousandth of our current population? A team of scientists at Carnegie Institution for Science have put together a compelling, complicated argument for how human hunting caused a minute but measurable change in the ancient Earth's climate, and it all goes back to the woolly mammoth.

The last ice age ended about 15,000 years ago as the Earth began to naturally get warmer. This obviously placed a pressure on the woolly mammoth population, which was poorly adapted to rising temperatures, and the species began to die out. This process was accelerated by human hunting, which decreased their numbers far more rapidly than would have otherwise occurred.

That's important because mammoths were essential in grazing on birch trees, a plant species particularly adept at spreading itself when given an opportunity. Without sufficient mammoths around to keep the birch in check, the young trees grew to about six feet tall, a seemingly unimpressive figure but more than enough to blot out the sun from the grass below. The trees actually darkened the color of the landscape, increasing the amount of sunlight absorbed and heating up the area.

So began a vicious circle in which the depleted mammoths failed to control the birch, which led to warming climates that further ravaged the mammoth population, allowing the birch to spread even further afield. The researchers found that the amount of birch pollen in northern regions skyrockets in the archaeological record at the same time human hunters arrive and the mammoth population plummets.

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The temperature changes we're talking about are subtle, as such changes on this sort of scale usually are. The spread of the birch trees throughout the northern reaches of Eurasia and the Americas - in Siberia, expanding from just a few trees to covering a quarter of the region - would have raised the entire Earth's temperature by about .1 degrees Celsius. Simulations based on how elephants interact with their environment suggest the disappearance of the mammoths accounted for about a quarter of that temperature change, with the rest due to natural warming processes.

So then, ancient humans were perhaps responsible for warming the Earth by 1/40 degrees Celsius fifteen millennia ago. That may not seem like much, but that's when you spread the effect out over the whole planet; regional warming in places like Siberia would have been as much as .2 degrees due to human intervention. When you consider the Earth's temperature has risen about .5 degrees in the last two centuries, due in part to humans' greenhouse gas emissions, that's hardly a trivial difference.

The finding speaks to just how much control humans both modern and ancient can exert on the Earth's climate. Indeed, these results push back the start of the so-called Anthropocene Era - the period when humans began to shape the nature of the Earth - almost twice as far in the past as previously thought. Scientists had previously placed the start of this time period with the widespread burning of the forests some 8,000 years ago by prehistoric farmers.

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[Geophysical Research Letters]