Before the Industrial Revolution, our planet’s atmosphere was still untainted by human-made pollutants. At least, that’s what scientists thought until recently, when bubbles trapped in Greenland’s ice revealed that we began emitting greenhouse gases at least 2,000 years ago.
Célia Sapart of Utrecht University in the Netherlands led 15 scientists from Europe and the United States in a study that charted the chemical signature of methane in ice samples spanning 2,100 years. The gas methane naturally occurs in the atmosphere in low concentrations. But it’s now considered a greenhouse gas implicated in climate change because of emissions from landfills, large-scale cattle ranching, natural gas pipeline leaks and land-clearing fires.
Scientists often gauge past climate and atmosphere conditions from pristine ancient ice samples. The new research was based on 1,600-foot-long ice cores extracted from Greenland’s 1.5-mile-thick ice sheet, which is made up of layers of snow that have accumulated over the past 115,000 years.
Sapart and her colleagues chemically analyzed the methane in microscopic air bubbles trapped in each ice layer. They wanted to know if warmer periods over the past two millennia increased gas levels, possibly by spur- ring bacteria to break down organics in wetlands. The goal was to learn more about how future warm spells might boost atmospheric methane and accelerate climate change.
The researchers did find that methane concentrations went up—but not in step with warm periods. “The changes we observed must have been coming from something else,” Sapart says.