Directions: Read the following passages and choose the best answers to the questions.

   Scientists have thought that humans only started emitting significant quantities of greenhouse gases in the 19th century, after the Industrial Revolution took hold. But a study in Nature today suggests that our history as heavy emitters stretches back much farther, to the charcoal fires of the Roman Empire and the intensive agriculture of Han China.
   To examine carbon emissions past, the research team analyzed more than 50 ice cores from Greenland, gauging levels of the greenhouse gas methane in Earth’s atmosphere going back to 100 B.C. They looked at specific carbon signatures in the methane to determine whether it came from burning coal and other material or natural biological process, then used mathematical models to further narrow down manmade emissions from naturally occurring ones, Human emissions, they found, were noticeable, though minuscule compared to post-industrial levels; only perhaps 10% human methane emissions over the past 2,000 years were produced before 1800. For a time when there were far fewer people around, however, that’s still a lot of methane to be sending off into the atmosphere.
   While our current technologies and our sheer numbers mean we’re emitting greenhouse gases at astonishing rates today, this study shows it’s a far older pattern than scientists believed. And climate models, which often use pre-industrial levels as baseline, may eventually have to be updated to reflect our long past as polluters.

Why are the carbon signatures in the methane important in the research?