In a way, human history is etched in the soil.
Recently, an international team of researchers found evidence that we humans have been leaving our mark on this planet since long before the Industrial Revolution. Around 4,000 years ago, human activities had already significantly accelerated soil erosion around lake beds on a global scale.
“We have been imprinting our presence [on] the landscape and in the natural world further back than we thought,” said Nuno Carvalhais, a research group leader at the Max Planck Institute for Biogeochemistry and the senior researcher of the study published in Proceedings of the National Academy of Sciences of the United States of America on 28 October.
The findings required an interdisciplinary approach, with different types of analyses allowing a more comprehensive picture of how human activities could be behind the accelerating erosion, Carvalhais said.
Jean-Philippe Jenny, a French geoscientist affiliated with the Max Planck Institute for Biogeochemistry and the Alpine Center for Research on Trophic Networks and Liminic Ecosystems and the lead author of the study, analyzed core samples of sediments from 632 lake beds collected from around the world. Because sediments accumulate in lakes at continuous rates, lake sediment cores can be used as a natural archive of fluctuations in soil erosion over time.
Combining sediment rates with radioactive carbon dating data from each site, Jenny and his collaborators inferred the changes in lake sedimentation accumulation rates and found that 35% of the sampled lakes had accelerated erosion over the past 10,000 years.
Crucially, the acceleration in erosion began around 4,000 years ago, and the researchers sought out the mechanisms that could explain this trend. “We built up our hypotheses, and based on these hypotheses, we [collected] the data that would either destroy or support the different hypotheses that were behind the trends,” Carvalhais explained.
In the end, humans were the most likely culprit.
Changes in erosion were less related to fluctuations in precipitation and temperature, researchers found, whereas trends in deforestation coincided with the rise in erosion. Jenny and his collaborators analyzed pollen samples at each lake bed site to produce a proxy for tree coverage of the surrounding land; they found that decreases in tree cover were tightly coupled with accelerated erosion. “Deforestation at the time was caused by the human beings, because at that time they were starting to develop agriculture,” said Jenny.
Humanity’s Past and Future Written in the Dirt
Although soil erosion accelerated 4,000 years ago in Europe, similar trends occurred only recently in North America, probably following European immigration and importation of agricultural practices.
The research team also found that 23% of lake sites had a decrease in erosion rates, which may be the result of human-driven river management, such as the construction of dams.
“It means that we as human beings are now living in a time period where we have a huge effect on everything on the Earth, and all our activities will be recorded in the natural archives,” said Jenny.
“These guys have done a really remarkably ambitious job putting the story together,” said David Montgomery, a professor of Earth and space sciences at the University of Washington and author of Dirt: The Erosion of Civilizations. The results of the paper “put into perspective just how powerful a force people are on the planet today,” he said.
Montgomery, who was not involved in the study, suggests that it was not merely deforestation that accelerated soil erosion, but subsequent agricultural activities as well. Though deforestation is a necessary first step for widespread farming, increased soil erosion is mainly driven by “the plow that followed,” he said. “It wasn’t simply cutting down the trees that caused the erosion; it was keeping them off the landscape through farming practices.”
The erosion rates produced by conventional agricultural practices are not sustainable, and they sap crucial nutrients from the soil. “What you come away with is the lesson that societies that don’t take care of their soil don’t last,” Montgomery said.
And there are broader environmental implications too. As with many types of large-scale human activities, increased soil erosion “can impact the climate in the long term,” said Jenny.
The results of this study provide more data about “the sensitivities of the Earth system to climate and environmental factors, including humans,” said Carvalhais. “And this can help us improve our ability to understand and also to predict or forecast future scenarios.”
“To go into the future, we also need to understand our history,” he added.
—Richard J. Sima (@richardsima), Science Writer