Man-made global warming started with ancient hunters: study

June 30, 2010

Even before the dawn of agriculture, people 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, accepted for publication in , a journal of the American Geophysical Union (AGU), 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 the lead author of the study, Chris Doughty of the Carnegie Institution for Science in Stanford, California. 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."

In the new study, Doughty, Adam Wolf, and Chris Field—all at Carnegie Institution for Science—propose a scenario to explain how hunters could have triggered global warming.

First, populations began to drop—both because of natural as the planet emerged from the last ice age, and because of human hunting. 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 meters (6 feet) tall. Nonetheless, they would dominate the grasses.

The trees would change the color of the landscape, making it much darker so it would absorb more of the Sun's heat, 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 from Alaska, , 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 colonize open ground following disturbance," the study says. The 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.

To estimate how much additional area the birch might have covered, they started with the way modern-day elephants affect their environment by eating plants and uprooting trees. If mammoths had effects on vegetation similar to those of modern elephants , then the fall of mammoths would have allowed birch trees to spread over several centuries, expanding from very few trees to covering about one-quarter of Siberia and Beringia—the land bridge between Asia and Alaska. 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.

Another study, published last year, shows that "the mammoths went extinct, and that was followed by a drastic change in the vegetation," rather than the other way around, Doughty says. "With the extinction of this keystone species, it would have some impact on the ecology and vegetation—and vegetation has a large impact on climate."

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 because of people's greenhouse gas emissions.)

Only some portion—about one-quarter—of the spread of the birch trees would have been due to the mammoth extinctions, the researchers estimate. Natural climate change would have been responsible for the rest of the expansion of birch trees. Nonetheless, this suggests that when hunters helped finish off the mammoth, they could have caused some .

In Siberia, Doughty says, "about 0.2 degrees C (0.36 degrees F) of regional warming is the part that is likely due to humans."

Earlier research indicated that prehistoric farmers changed the climate by slashing and burning forests starting about 8,000 years ago, and when they introduced rice paddy farming about 5,000 years ago. This would suggest that the start of the so-called "Anthropocene"—a term used by some scientists to refer to the geological age when mankind began shaping the entire planet—should be dated to several thousand years ago.

However, Field and colleagues argue, the evidence of an even earlier man-made global climate impact suggests the Anthropocene could have started much earlier. Their results, they write, "suggest the human influence on climate began even earlier than previously believed, and that the onset of the Anthropocene should be extended back many thousands of years."

Explore further: New discovery suggests mammoths survived in Britain until 14,000 years ago

More information: "Biophysical feedbacks between the Pleistocene megafauna extinction and climate: The first human‐induced global warming?", Geophysical Research Letters

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2 / 5 (8) Jun 30, 2010
This is an outstanding article. I can hear the deniers wailing now. I can even predict some of what they are going to be saying.

1) It is hubris to think that puny humans can change the behavior of the Earth's ecosystem.

2) This is just a bunch of academics looking for more funding.

3) The Earth is not warming, it is cooling, can't you see the trend in the past 4 years of temperature data??? Besides we had a lot of snow in my back yard this past winter.

4) There is no evidence that humans had anything to do with the extinction of the large mammals (it was a meteor).

5) CO2 is the breath of life and is good for the Earth.

6) Warming will be good for people because it will increase the growing season in my county.

I'm sure I have missed some of the reactions but I can hear the groaning and see the rolling of denier's eyes now. Let the comments begin - sounding like those supporting the tobacco industry.

4.2 / 5 (5) Jun 30, 2010
George Bush killed all the elephants.

A. Gore
3 / 5 (2) Jun 30, 2010
Temporal proximity often suggests a causal relationship. It should not, however, be taken for granted.

Also, in birch forests, the temperature is not at all higher than on open ground. And the leaves of a lone birch in sunlight are colder than the ground.

Even if birches actually warmed the climate, I suggest a recalculation, since the total area of birch vegetation is not very big, and they grow where sunlight falls very obliquely, in other words, most birches are largely in the shadow of the previous birch.

In the high latitudes, birches carry leaves only for a few months of the year.
3 / 5 (6) Jun 30, 2010
Bad humans were planning destroying mother earth even before automobile era! Seriously, they attribute puny 0.1C global temperature increase to cause a change in vegetation distribution?
2.6 / 5 (5) Jun 30, 2010
How did humans create the warm climate for the dinosaurs?
"Doughty and colleagues then used a climate simulation..."
Mammoth were replaced by tens of millions of bison from Asia. Why didn't all that methane production stop subsequent cooling cycles?
2.3 / 5 (3) Jun 30, 2010
Concerning the hypothesis of man made global warming, it's proven statistically, people are making weather warmer and drier on per week basis - we can just extrapolate these weekend fluctuations to decades of years.

During last warming periods the rise of carbon dioxide followed warming with delay of many decades with compare to present situation - so we can see this argument of many skeptics rather as another evidence for man-made origin of global warming.

Nevertheless the idea, forty thousands of human hunters could affect the global climate sounds too bravely for me. IMO the causality was exactly reversed there.

4.7 / 5 (3) Jun 30, 2010
I think the study is quite useful and that AGW needs to be subdivided into multiple categories.

We have greenhouse gases which we can argue about until the cows come home. We have direct human influence on the ecosystem which is immediate and quite possibly profound.

As the years pass and populations increase we cover more and more ground with concrete and bitumen. Let us add up the effects of this. Then add in aerosols and particulate matter. Then add in pollution and industial accidents. Let us not forget pasture and changes of land use not covered in concrete.

Add up all these effects and their contribution to climate change and see what figure we get.

After doing all that then we can add in the changes to atmospheric gas ratios and see where that leads us.
1 / 5 (1) Jun 30, 2010
"Sophisticated hunters not to blame for driving mammoths to extinction"
""Some people thought humans arrived and decimated the populations of these animals in a few hundred years, but what we've found is not consistent with that rapid 'blitzkrieg' overkill of large animals," said Jacquelyn Gill, a PhD student at the University of Wisconsin, Madison, who led the research team."
"Writing the US journal Science, the researchers describe how the amount of mammal dung started to fall around 14,800 years ago, long before advanced spearheads became commonplace. The animals had been almost completely wiped out a thousand years later."
2.5 / 5 (2) Jun 30, 2010
"Dale Guthrie of the University of Alaska has added 600 radiocarbon-dated fossils to the established collection, and his examination reveals that mammoths and wild horses were in serious decline before humans arrived on the scene in Alaska and the Yukon Territory."
Trust radiocarbon dating or a computer model?
3.4 / 5 (5) Jul 01, 2010

The liberal tactic of bashing your dissenters does not work. The facts are not in your favor and statistical possibilities do not create realities.

Stop wasting our time and become a real scientist. Judge the theory on the facts and the facts on the methods and biases.

BTW- your bias is showing....
5 / 5 (1) Jul 03, 2010
Unbelievably naive 'science'.
Posit a premise, search ONLY for evidence that agrees with your premise, ignore everything else.
It's enough to get you upset.
Thank heavens for the 'Thermodynamics' of this world, otherwise I would have nothing to laugh about.
1 / 5 (1) Jul 03, 2010
Posit a premise, search ONLY for evidence that agrees with your premise, ignore everything else.

That's how the 'hockey stick' was created.
1 / 5 (1) Jul 04, 2010
I propose that we argue about this until it is too late, so as to put off action and not take responsibility.
5 / 5 (1) Jul 04, 2010
I propose that we argue about this until it is too late, so as to put off action and not take responsibility.

There are many actions that can be taken that don't require a government take over of the world's economy. Unfortunately, too many government regulations inhibit such actions by entrepreneurs and individuals.
5 / 5 (1) Jul 05, 2010

There are many actions that can be taken that don't require a government take over of the world's economy. Unfortunately, too many government regulations inhibit such actions by entrepreneurs and individuals.

The actions of the individual are often motivated by a self interest that may tend to benefit a single lifetime at the expense of subsequent generations. Government and regulations evolved to mitigate and distribute this risk.
1 / 5 (2) Jul 05, 2010
Government and regulations evolved to mitigate and distribute this risk.

Since when?
History has shown this only increases the power of the state over the individual.
Individuals are motivated, for the selfish reason of benefiting their children and grandchildren, to pass on a better world to their offspriing.
Most who immigrate to the USA work hard and sacrifice to make a better life for their children. What self interest!
5 / 5 (1) Jul 05, 2010
History is written by the survivors, as hindsight through corrective spectacles. European colonists immigrated to the USA and all but wiped out an indigenous population. They deluded themselves into thinking that they were creating a better world as well.

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