A new study shows that human activity has been changing the planet way before the Industrial Revolution – particularly 1,000 years ago when human settlement permanently transformed the forests.
Researchers from MIT and University of Massachusetts tied a widespread, permanent forest lost in Madagascar a thousand years ago to human settlers who set the landscape on fire to pave the way for cattle grazing – not to climate change or any natural occurrence, as they earlier expected.
The “slash and burn” method to create pasture for cattle was discovered through analyzing two stalagmites from a Northwestern Madagascar cave.
In the study, the stalagmites’ calcium carbonate suddenly and totally changed in just a century, from carbon isotope ratios characteristic of shrubbery and trees to those that are more typical of grassland. But there was no corresponding change in oxygen isotopes, eliminating the possibility of natural drops in rainfall or a climate event as a reason for forest loss.
“Both the speed at which this shift occurred and the fact that there’s no real climate signal suggest human involvement,” says study author and MIT assistant professor David McGee.
The team found proof of human settlement in Madagascar about 3,000 years ago and their shift to an agrarian lifestyle, bringing cattle to the island prior to a millennium ago.
Prior scientific analyses yielded evidence of abundant charcoal microparticles – one sign of fires – as well as spikes in levels of grass pollen, which indicates more extensive grasslands. This study provided the missing element of time and proved that man-made environmental impacts did not commence with the Industrial era or Europeans, McGee added.
The researchers seek to study and obtain more samples from Madagascar caves to further determine the extent and timing of human influence on the transformed landscape.
While climate change is no longer a suspect in the transformation of that time, there could be other factors at play, says Laurie Godfrey, study author and anthropology professor.
“[W]e don’t yet know whether similar shifts, also unrelated to natural aridification, occurred elsewhere on the island, and if so, when, exactly.”
The findings were published in the journal Quaternary Science Reviews.
