Humans may have affected the earth's climate thousands of years ago.
Primitive climate change. I'm Bob Hirshon and this is Science Update.
Humans may have affected earth's climate thousands of years ago—by burning forests for farming. This according to University of Virginia climate scientist William Ruddiman. He notes that about five or six thousand years ago, greenhouse gases mysteriously increased in the atmosphere. He also says that although there were far fewer people on the planet back then, they consumed up to ten times as much land per person as we do today.
And that larger use of land means that tens of millions of people actually have the effect of hundreds of millions of people.
... Enough, he argues, to explain the spike in greenhouse gases—if you account for all the consequences of that slash-and-burn farming. If he's right, then today's more drastic climate change may have piggybacked on a smaller warming effect from long ago. I'm Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
When scientists talk about climate change, they're usually talking about climate change resulting from human activities of the past 150 years—in other words, since the start of the Industrial Revolution. To be sure, humans have put far more greenhouse gas into the atmosphere over the past century and a half than we had at any previous time. However, human activities like farming have contributed some amount of greenhouse gas to the atmosphere for ages, and Ruddiman suggests that at one point in our ancient past, it was enough to make an impact on the earth's climate.
The point he's talking about is about 5,000 to 6,000 years ago. According to geological evidence, greenhouse gases in the atmosphere had been slowly declining for about 5,000 years. This is consistent with natural climate cycles that had been going on long before humans ever appeared on the planet. If things went the way they usually went, greenhouse gases would have continued to slide down, and average global temperatures would have continued to drop, all the way through today.
But they didn't. Instead, the concentration of greenhouse gases in the atmosphere ticked up instead of down. And although global temperatures varied in the millennia that followed, the expected deep freeze never happened. Ruddiman argues that the only thing about the planet that was truly different 5,000 to 6,000 years ago, compared with similar points in prior climate cycles, was the human population—specifically, the presence of human farmers.
Now, farming today generates quite a bit of greenhouse gas. But most scientists had assumed that pre-industrial farming just wasn't widespread enough to make a measurable impact on the climate. And if farmers in the time of George Washington didn't make a difference, it would seem obvious that farmers 5,000 to 6,000 years ago wouldn't either. After all, there are more people in the state of Florida today than there were in the entire world back then.
However, Ruddiman argues that although there were far fewer people, they used much more land per person than farmers do today—about ten times as much. He says that historical evidence suggests that early farmers burned down acres and acres of forest to plant their crops, plant seeds in the cleared soil, and then do it all again in a few years, when the soil's fertility declined. As farming grew more sophisticated, later civilizations learned to do a lot more with less.
It's important to note that Ruddiman's view remains controversial. Furthermore, his theory shouldn't be misinterpreted to mean that climate change today is no different from that of the ancient past. We now emit greenhouse gases from many sources, like factories and cars, that didn't exist thousands or even hundreds of years ago, and today's global population of over six billion generates much more total greenhouse gas from farming than our early ancestors did. In fact, his work suggests that temperatures at the start of the Industrial Revolution may have already been artificially high, due to human activities—and that modern emissions simply pile much more on top of that. In other words, our responsibility for the planet's climate may extend much further back in time than we thought.
Now try and answer these questions:
- What is the basic point of Ruddiman's theory?
- How might the small global population of 5,000 years ago have an impact on climate?
- In what ways might this study be misinterpreted by climate change skeptics?
- What other evidence might support Ruddiman's theory, if it were found? What evidence might contradict it?
In the PBS lesson Affluenza, students learn about the world's finite resources, and consider the impact of the American lifestyle on the environment.
In the National Geographic News article Early Australians to Blame for Mass Extinctions, Study Finds, read about a study that indicates that the arrival of humans in Australia 60,000 years ago caused the extinction of roughly 60 species of the continent's animals. The extinctions are believed to be the direct result of a change in the ecosystem brought on by massive fires set by the early settlers.