Long before electricity or automobiles or the Industrial Revolution, the world had air pollution. One cause was the same as today: human beings. A new study suggests people have been boosting greenhouse gas levels for at least 2,000 years.
By studying carbon isotopes in centuries of air bubbles trapped in Antarctic ice, Dominic Ferretti, an atmospheric chemist at the National Institute of Water and Atmospheric Research in New Zealand Ltd., determined that humans burned huge expanses of vegetation, releasing methane into the skies. "What a lot of early indigenous people did was to practice burning for hunting and agriculture," he says. "Animals run out into the open after the fires are lit, and subsequent to that is new growth, a new generation of seeds."
At first it wasn't clear that people were to blame for the glut of greenhouse gas. Naturally occurring forest fires, especially in warm and dry years, also release methane into the air. But about 500 years ago, the methane record shows a dramatic reduction in wildfires that cannot be tied to climate. Ferretti and his team believe the drop was caused by European exploration in the Americas. "About 1500, explorers brought all sorts of diseases, like influenza and chicken pox," Ferretti says. Thriving cultures of between 50 million and 100 million people were decimated, leaving only 5 million survivors—enough upheaval to be reflected in the methane record, revealing how much early Americans had been polluting before then. "The really amazing, surprising thing that has come out of [this research]," says Ferretti, "was the impact of humans in a period in which everyone thought there were very few humans in the world."