Did the Amazon rainforest contribute to the ‘Little Ice Age’ of the 1600s?

Scientists have found new evidence as they scrutinize a theory that Amazon re-growth, following European colonization, affected global climate.

In the century after Europeans arrived in the Americas in the late 1400s, more than 50 million Indigenous people are estimated to have died from epidemics, warfare, and slavery. This human-caused tragedy, also known as the Great Dying, may also have left its mark on the landscape and climate.

In a 2019 analysis, U.K. researchers proposed that the regrowth of forests in places where Native people had cleared the land may have absorbed and stored enough carbon to contribute to a 17th-century dip in global atmospheric CO2 levels. This anomaly is thought to be one of the causes of an unusually cold period known as the Little Ice Age.

But a study published last week in the journal Science finds no evidence for such a scenario in the Amazon.

To test the idea that Amazonian forests bounced back during or after the Great Dying, a group of researchers led by paleoecologists Mark Bush, of the Florida Institute of Technology, and Crystal McMichael, of the University of Amsterdam, analyzed sediments from 39 lakes across the Amazon Basin.

“The sediments at the bottom of the lake represent the area’s history,” Bush explains, “with the oldest layers at the bottom, and the youngest at the top.” Using radiocarbon dating to determine the age of each layer, the researchers carefully screened the samples for pollen and charcoal. “This part of the Amazon doesn’t burn naturally,” says Bush, “so if you find charcoal, it’s a pretty sure sign of people.”

Return of the yarumo

From the pollen, the team identified the plants that grew near the lake at different times. When people clear the forest, there will be fewer pollen grains in the lake from trees, and more from grasses, herbs, and crops, says McMichael. “We often find corn and manioc pollen, but also squash and sweet potatoes.”

When sites are abandoned in the Amazon, the first pollen to show up in lake sediment is that of Cecropia trees, which are locally known as yarumo. “These are really weedy trees,” says Bush. “They grow from nothing to five meters (16 feet) in two years, so fast they are actually hollow, often with ants living inside. They’ll stay for a few decades, and then they are crowded out by other trees. But they produce a huge amount of pollen.”

Analysis of the charcoal and all the different kinds of pollen revealed evidence of forest opening, burning, or cultivation predating the arrival of Europeans in four out of five lakes. “That doesn’t mean 80 percent of Amazonia was deforested, of course,” says Bush. “People concentrate around lakes.”

Yet when the researchers reached the layers containing evidence of forest recovery, those often predated the arrival of Europeans by hundreds of years. “There is a lot of variation,” says Bush, “but what we see is that deforestation is strongest between 350 and 750 A.D. It slows down after that, with forests starting to regrow from around 1000 A.D.” During or after the Great Dying, however, evidence of forest regrowth is rare.

Unprecedented deforestation

That suggests that in the Amazon, at least, forest regrowth during and after the Great Dying seems unlikely to have contributed much to the dip in CO2 that caused the Little Ice Age, says Bush. “To get a noticeable amount of change in atmospheric CO2, you’d need to have a big area of the Amazon all changing at the same time. We don’t see that at any time in the past; it’s spread out in space and time.”

That should not make us worry less about the current deforestation in the Amazon, he adds. “The scale of today’s fires and deforestation is much larger, so I think that today, the threat of reaching a tipping point where the Amazon would become a source rather than a sink of CO2 is unfortunately very real.”

Geographer Alexander Koch of Hong Kong University, lead author of the 2019 paper suggesting a link between the Great Dying and the Little Ice Age, says that “pollen data only tell us if forest grew back at a specific location.” He believes the new study “does not refute the main hypothesis” of his article, which referred to the Americas as a whole.

He says the new research makes an important contribution but adds that Amazonia’s influence may have been limited compared to regions in Mexico, central America, and the Andes, where population decreases were larger. “Most of Amazonia was likely harder to reach and less affected by disease and colonizers,” Koch says. In his own analysis, only 4 percent of the increased CO2 uptake was estimated to have happened in the Amazon.

“European arrival in the Amazon was a gradual process,” says McMichael. The most devastating impacts on Amazonian people may have occurred later than the Great Dying in Mexico or the Andes, where there is more evidence of high mortality soon after the arrival of Europeans.

Conflict and disease

Based on their new data showing that forest recovery often predated the arrival of Europeans by hundreds of years, Bush and McMichael believe that population numbers in Amazonia probably peaked long before the arrival of Europeans on the continent. They think the number of people in the region may have declined and then stabilized at a lower level much earlier, allowing forests time to recover from the most intense phases of human activity.

Manuel Arroyo-Kalin, an archaeologist at University College London who was not involved in the study but has used archaeological evidence to reconstruct population trends, agrees. He points out that “ethnohistorical evidence clearly indicates population collapse as a result of European colonization,” but adds that his own research also suggests that the population peak for the Indigenous Amazonian population “may have come centuries earlier.”

But what might explain a decrease in Indigenous Amazonian populations if there were no foreign invaders? In their paper, Bush and McMichael point to evidence of increased hostilities in the adjoining Andes starting between A.D. 1000 and 1200, including “cracked skulls” and “defensive stockades.” Other researchers have reported increasing evidence of fortified settlements in Amazonia after 1200, says Bush. “This suggests people coalescing into certain areas, reorganizing to become less spread out and more defensively oriented,” he says, while avoiding border areas, allowing the forest to regrow there.

There’s also evidence of tuberculosis in the Andes between 1000 and 1300, which may have spread to Amazonia through trade. “It’s valid to ask whether Amazonian populations may have confronted similar challenges as their neighbors in the high Andes, who were experiencing tumultuous times,” says anthropologist Tiffiny Tung of Vanderbilt University. Tung is researching the upheaval that befell the Andean people but was not involved in the Amazon study.

It’s challenging to integrate pollen data from lowland lakes with evidence on disease and violence in the highlands, Tung says. “So I hope we’ll get better environmental data from the areas where we have rich archaeological data, and vice versa.”

This is also what Bush and McMichael are after. “We’re working with archaeologists now,” says McMichael, “and the next thing we want to do is go to lakes near some of their sites and see what we can get from those.”



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