气候变化领域集成服务门户

A study of ancient pollen reveals that millennia of human activities transformed Earth's ecosystems as quickly as when the Ice Age ended.

Officially, we’re in what’s known as the Holocene, the geological epoch that began at the end of the last ice age. But the influence of human activity on the Earth’s ecosystems has become so extreme that it now seems to be the central driver of environmental change, leading some scientists to argue that we should think of ourselves as living in a new epoch called the Anthropocene. Usage of the term is still being debated, though, and one of the central disputes is when the start of this new epoch would be. The mid-twentieth century? The Industrial Revolution? Or might it be earlier than that—say, when agriculture took hold as a dominant feature of human life?

A new study suggests that the best answer might be the last one. According to a research team led by Ondrej Mottl and Suzette G.A. Flantua of the University of Bergen in Norway, the vegetation of the planet began changing dramatically between 4.6 and 2.9 thousand years ago, and it’s likely that the primary cause was human activity—agriculture, deforestation, and the use of fire to clear landscapes.

“Ours is the first quantitative study showing that humans likely impacted the planet strongly not just in recent decades or centuries, but thousands of years ago,” Mottl says of their research, published today in the journal Science. The landscape changes of the last century or two, as dramatic as they have been, appear to be continuations of trends several thousands of years in the making.

But the second major finding of the research is no less significant. The change in vegetation over the last few thousand years rivals the degree of vegetation change that occurred as the Ice Age yielded to a warming planet between 16,000 and 10,000 years ago. That’s when the ice sheets and glaciers that covered much of the Northern Hemisphere receded, when landscapes of ice yielded to forests and tundra and grasslands, and when a global temperature increase of 6 degrees Celsius (10.8 degrees Fahrenheit) led to changing plant regimes across the globe.

 “We didn’t expect that the change in the last few thousand years would be even larger than what happened as the Ice Age ended,” Flantua says.

The study’s results were derived from 1,181 fossil pollen sequences from sites around the world. Pollen that is blown by wind or washed by rain into a lake or bog can get buried in the sediment at the bottom, preserving a snapshot of the vegetation that existed around the water body at a particular time, which can be determined by radiocarbon dating.

Using a database of well-dated sediment cores from around the world, the researchers were able to identify the extent to which pollen compositions had changed over time. Because they were seeking to extract a global pattern from more than a thousand data sets, they didn’t try to analyze which species of vegetation had been replaced by which others at any particular place. They focused only on the overall rate of change over the past 18,000 years.

That’s how they documented a second period of rapid vegetation change, after the one resulting from the end of the Ice Age. The onset of that second period varied by region, between 4,600 and 2,900 years ago. But the acceleration of vegetation change was seen on every continent except Antarctica.

The study was the first to document that acceleration with quantitative data, but a 2019 study that surveyed 250 archaeologists about past human agricultural activity around the globe came to similar conclusions: By 3,000 years ago, much of the planet’s terrestrial surface had been markedly transformed by human activity. The lead author of that study, Lucas Stephens, an archaeologist and environmental policy expert at Duke University, says the two studies in tandem paint a compelling picture.

“Their database of global pollen records is impressive,” Stephens says. “I think the most novel and important finding is that the rate of vegetation change now is approaching or even exceeding rates at the Pleistocene/Holocene transition”—i.e., the period at the end of the Ice Age. “That rate of change has frightening implications for the future.”

Stephen T. Jackson, an ecologist with the U.S. Geological Survey, agrees that the research is significant. “It is an important and provocative analysis,” he says. But he cautions that other factors besides human activities might be at work, such as natural climate change.

“In some parts of the world, vegetation change is clearly driven by human activities,” Jackson says. “But in other regions we have good documentation of climate change that’s sufficient to drive vegetational changes. And in many of those same areas, there’s very little evidence of widespread human activities.”

Mottl and Flantua are careful to qualify that their research does not demonstrate that human activities caused the vegetation changes they’ve documented. That is a subject for future research, they say. But the correlation is undeniable, says Jonathan T. Overpeck, a University of Michigan climate scientist who authored a commentary article in Science on Mottl and Flantua’s research.

“They don’t make the causal link, but I would agree that the most logical explanation is human land use,” he says. “Because we know that humans are clearing land for agriculture, they’re using fire for managing land surface. It’s up to archaeologists to say exactly what the processes were, but it certainly looks like we’re seeing the fingerprint of human beings as primary causal agents behind these changes that started several thousand years ago.”

And this has important implications for ecosystem management as we try to ameliorate the effects of recent and future global climate change, the researchers say. If what we take to be a “natural” landscape is in actuality one that has developed in tandem with human activities, does it make sense to try to preserve things as they presently are, as if that is a reflection of a natural ideal?

“Maybe what is being considered pristine is not so pristine at all,” Flantua says.

“Instead of trying to maintain species compositions that existed in the past, we have to start managing for change and managing for the future,” Overpeck says. “Many of the forests we have now are dying because those trees established under cooler, moister conditions. As the climate becomes hotter and more extreme, we have to plant species that can handle that.”

What we’re looking at now is what Overpeck calls a “one-two punch.” We’ve got acute and rapidly intensifying climate change on the heels of thousands of years of extreme vegetation change. How is the planet going to handle that? No one knows.

“But that one-two punch is really going to stress out our forests,” he says. “And in order for forests to take up carbon, they’ve got to be healthy.”

In other words: It seems that we’ve long laid a heavy hand on the planet’s ecosystems, and perhaps now it is time to wield that hand more deliberately and creatively. You might call it Anthropocene 2.0.

Copyright © 1996-2015 National Geographic SocietyCopyright © 2015-2021 National Geographic Partners, LLC. All rights reserved