Stalagmites reveal huge shift in ancient Madagascar’s plant life, unrelated to climate change.
There’s no question that our species has had a dramatic impact on the planet’s physical environment, particularly over the last few centuries, with the rise of modern industry, transportation, and infrastructure. But as new research shows, humans have been transforming the landscape, with lasting impacts, since long before the start of the Industrial Era.
Scientists from MIT and the University of Massachusetts at Amherst have found that a widespread and permanent loss of forests in Madagascar that occurred 1,000 years ago was due not to climate change or any natural disaster, but to human settlers who set fire to the forests to make way for grazing cattle.
The researchers came to this conclusion after determining the composition of two stalagmites from a cave in northwestern Madagascar. Stalagmites form from water that percolates from the surface, through the soil, and into a cave. These finely layered pillars can be preserved for thousands of years, and their composition serves as a historical record of the environment above ground.
From their analysis, the team found that around 1,000 years ago, both stalagmites’ calcium carbonate composition shifted suddenly and completely, from carbon isotope ratios typical of trees and shrubs, to those more consistent with grassland, within just 100 years.
Was this landscape transformation triggered by climate change? The team’s results suggest otherwise. Around the same period, they found that oxygen isotope levels remained unchanged in both stalagmites, indicating that rainfall rates — and climate in general — remained relatively stable.
“We went in expecting to just tell a climate change story, and were surprised to see a huge carbon isotope change in both stalagmites,” says David McGee, the Kerr-McGee Career Development Assistant Professor in the Department of Earth, Atmospheric and Planetary Sciences at MIT. “Both the speed at which this shift occurred and the fact that there’s no real climate signal suggest human involvement.”
The team’s results are published this week in the journal Quaternary Science Reviews.
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