That one-two punch, say Kutzbach and Vavrus, was enough to set human-induced climate change in motion.
"No one disputes the large rate of increase in greenhouse gases with the Industrial Revolution," Kutzbach notes. "The large-scale burning of coal for industry has swamped everything else" in the record.
But looking farther back in time, using climatic archives such as 850,000-year-old ice core records from Antarctica, scientists are teasing out evidence of past greenhouse gases in the form of fossil air trapped in the ice. That ancient air, say Vavrus and Kutzbach, contains the unmistakable signature of increased levels of atmospheric methane and carbon dioxide beginning thousands of years before the industrial age.
"Between 5,000 and 8,000 years ago, both methane and carbon dioxide started an upward trend, unlike during previous interglacial periods," explains Kutzbach. Indeed, Ruddiman has shown that during the latter stages of six previous interglacials, greenhouse gases trended downward, not upward. Thus, the accumulation of greenhouse gases over the past few thousands of years, the Wisconsin-Virginia team argue, is very likely forestalling the onset of a new glacial cycle, such as have occurred at regular 100,000-year intervals during the last million years. Each glacial period has been paced by regular and predictable changes in the orbit of the Earth known as Milankovitch cycles, a mechanism thought to kick start glacial cycles.
"We're at a very favorable state right now for increased glaciation," says Kutzbach. "Nature is favoring it at this time in orbital cycles, and if humans weren't in the picture it would probably be happening today."
Importantly, the new research underscores the key role of greenhouse gases in influencing Earth's climate. Whereas decreasing greenhouse gases in the past helped initiate glaciations, the early agricultural and recent industrial incr
|Contact: Steve Vavrus|
University of Wisconsin-Madison