Humans Have Changed North America More Than an Ice Age

Humans Have Changed North America More Than an Ice Age
An aerial shot north of Miami showing islands filled with houses, apartments, and other buildings. (Photo: Daniel Slim/AFP, Getty Images)

It’s clear that human activities such as land use and fossil fuel extraction have caused ecological changes like the climate crisis and biodiversity loss. New research shows just how massive the scale of those changes are.

The Pleistocene epoch, which began 2.5 million years ago and ended 11,700 years ago, saw Earth’s most recent Ice Age, when glaciers covered huge parts of the planet. When it ended, ecosystems across North America changed rapidly. Forests and grasslands sprouted up across the region. A new era was ushered in.

It’s only been some 12,000 years since the end of the Pleistocene, which in terms of our 4.57-billion-year-old planet is nothing. Most geological epochs last several million years. But for about a decade, scientists have warned that in just the past 250 years, human activity has ushered in a new epoch called the anthropocene.

New research, which was presented this month at the virtual annual meeting of the Ecological Society of America, solidifies that theory. It shows that the past 250 years of human land use changed North American landscapes even more than the recession of mile-thick glaciers 12,000 years ago did.

“This puts our modern world in context, and it shows that these changes are really unprecedented,” said Stanford University paleoecologist M. Allison Stegner, who led the new study.

To quantify these ecological changes, the researchers examined hundreds of years of fossil records from the global Neotoma Paleoecology Database. By looking at shifts in fossilised pollen records gleaned from records of sediment cores, they were able to determine what types of vegetation lived in different locations across North America at different times. In particular, they looked for signs of abrupt, system-wide transitions, such as when a grassland becomes a forest, or when an oak forest sprouts up where a spruce forest once was.

The researchers looked at how the pollen records changed over 250 year-long periods. Throughout the Pleistocene, they observed an average of 10 abrupt shifts across 100 sites in each 250 year stretch. That’s a massive amount of fast change, but even more change followed once humans showed up in force. Between 1700 and 1950, the researchers observed 20 abrupt changes per 100 sites.

The scientists can’t yet pinpoint what exact human activity drove each of those rapid shifts, but much previous research indicated that agriculture, pollution, logging, fishing, and extracting fossil fuels have all played a role. The climate crisis — driven by some of those very activities — is also a likely culprit. Regardless of which activity is mostly to blame for specific shifts, the new findings as a whole suggest human actions changed North American ecosystems more than the last Ice Age.

In the 250-year period, some areas saw more abrupt changes than others. The U.S. Midwest, Southwest, and Southeast saw the most massive shifts. Other areas, including northern Canada, Alaska, and the Pacific Northwest, remained comparatively stable, likely because they’re colder and less fertile. While the researchers don’t know exactly why the regions that saw abrupt changes were so impacted, they think it’s likely because those areas saw so much change due to logging and farming.

“You had a lot of logging early in that 250 year period that took out a ton of the native trees and then mass agriculture following that,” said Stegner.

This authors findings are, to say the least, troubling, especially because we’re not done making changes yet. We — or usually, those of us with power — are continuing to exploit the natural world, burning down trees, extracting fossil fuels, and clearing more and more land for farming. Studies show that many more of these abrupt shifts could be on the way unless we makes big changes. We could lose rainforests, coral reefs, and wetlands. That’s not just a problem for the wildlife and unique flora that live there.

“We rely on these ecosystems for our lives,” said Trisha Spanbauer, an ecologist at University of Toledo who worked on the new study. “I work in freshwater ecosystems. If we were to have an abrupt change in those systems, since there are people who rely on them for drinking water or agricultural water, that’s a huge issue.”

The same is true for changes in vegetation and marine ecosystems. If either the Amazon rainforest or Great Barrier Reef collapses, for instance, entire societies will lose their sources of food, and the planet will lose an important carbon sink, further putting us in peril. Other ecosystems would replace them, but we don’t know what they’d look like or what exactly it would mean for us.

“We have a lot to lose,” said Spanbauer.