I don’t think anyone could argue that humans have had a major impact on the planet’s physical environment. However, most of us only look back to the last few centuries, mainly back to the rise of modern industry.
But, scientists have found evidence of our ancestors transforming the landscape as far as 1,000 years ago. Researchers from the Massachusetts Institute of Technology (MIT) and the University of Massachusetts at Amherst have now discovered that human settlers in Madagascar had set fire to the forests to make way for cattle pastures, leaving widespread and permanent loss of forests.
According to MIT:
“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.”
The question is, was this widespread and permanent loss of the forests due to climate change or any natural disaster? The team found that the oxygen isotope levels had remained unchanged in both stalagmites, which indicates rainfall rates and the climate in general had remained relatively stable.
David McGee, the Kerr-McGee Career Development Assistant Professor in the Department of Earth, Atmospheric and Planetary Sciences at MIT, said:
“We went in expecting to just tell a climate change story, and were surprised to see a huge carbon isotope change in both stalagmites.
“Both the speed at which this shift occurred and the fact that there’s no real climate signal suggest human involvement.”
The team consisted of McGee, who studies the composition of stalagmites as an indicator of past climates, Stephen Burns, the lead author and a professor of geosciences from the University of Massachusetts at Amherst, Laurie Godfrey, professor of anthropology also from UMass Amherst, and colleagues from the University of Antananarivo in Madagascar.
Godfrey has been studying the extinctions of giant lemurs that occurred in Madagascar over the past 1,000 years. Populations of other large animals declined dramatically around this time, including pygmy hippos and giant tortoises, MIT said in a statement.
Godfrey believes that the extinction of megafaunal was most likely hastened by habitat loss and the widespread destruction of forests at the time. However, he has found it difficult to pin down exactly why the forests shrank, and when.
Scientists have analyzed sediment deposits from ancient lakes in the region and in other parts of Madagascar. What they observed was an increased abundance of charcoal microparticles (a signal of fire). What was also noticed was a spike in grass pollen levels, which indicates a larger extent of grasslands. However, dates for these sediments are uncertain, McGee says, and that the stalagmites offer a more precise record of environmental change.
“You’d think stalagmites in a cave are insensitive to what’s going on in the landscape above them.
“But, because they’re basically fossilized groundwater deposits, precipitated in very regular layers, they’re a fairly sensitive recorder of climate and ecosystem changes.”
McGee, along with research scientist Benjamin Hardt, had determined the ages of each stalagmite’s layers by measuring the ratio of uranium to thorium (a common geological dating technique). Burns had then measured the carbon and oxygen isotope ratios.
All plants take up carbon dioxide for photosynthesis. While carbon dioxide in the air consists of a fixed isotopic ratio of carbon-12 to carbon-13, all plants preferentially take up carbon-12. Among plants, trees and shrubs more strongly exclude carbon-13 compared with grasses, MIT writes.
After the dating and isotope results were combined, the researchers detected a dramatic shift in the ratio of carbon-12 to carbon-13 around 1,000 years ago from both stalagmites.
McGee said in a statement:
“What we see in the record is that the change from carbon isotopes that look like forest, to isotopes that look like grassland, happens really rapidly, within a century, and it would be unusual for a forest to naturally completely turn into grassland that quickly.”
Burns and McGee, after additional analysis, have determined that there is no corresponding change in oxygen isotopes at the time, which eliminates climate change or any natural drop in rainfall, as a cause for the forest loss.
There is evidence of human’s settlements on Madagascar from around 3,000 years ago, and then when they later adopted a more agrarian lifestyle, where they introduced cattle to the island before 1,000 years ago. McGee suggests that the results show humans had used “slash and burn techniques” to create pastureland for cattle around this time. McGee said:
“I think this is one more piece of evidence that human impacts on the environment don’t just start with Europeans and the Industrial era.”
Now, the team is planning to take more samples from caves across Madagascar to determine the timing and extent to which humans had transformed the landscape, Godfrey said:
“The transition from ephemeral forager to dedicated agro-pastoralist occurred, probably across Madagascar, around 1,000 years ago.
“We know that a dramatic landscape transformation occurred in the northwest. We know that this transformation was not triggered by climate change.
“But we don’t yet know whether similar shifts, also unrelated to natural aridification, occurred elsewhere on the island, and if so, when, exactly. We are currently seeking to answer these questions.”
The researcher’s results titled: “Rapid human-induced landscape transformation in Madagascar at the end of the first millennium of the Common Era” have been published in the journal, Quaternary Science Reviews.