Scientist Links Man to Climate Over the Ages
By KENNETH CHANG
SAN FRANCISCO, Dec. 9 — Humans have altered the world's climate by generating heat-trapping gases since almost the beginning of civilization and even prevented the start of an ice age several thousand years ago, a scientist said on Tuesday.
Most scientists attribute a rise in global temperatures over the past century in part to emissions of carbon dioxide by human activities like driving cars and operating factories.
Dr. William Ruddiman, an emeritus professor at the University of Virginia, said at a meeting of the American Geophysical Union here that humans' effect on climate went back nearly 10,000 years to when people gave up hunting and gathering and began farming.
Dr. Ruddiman is also reporting his findings in the journal Climatic Change.
In a commentary accompanying the article, Dr. Thomas J. Crowley of Duke University, said he was first taken aback by Dr. Ruddiman's premise. "But when I started reading," Dr. Crowley wrote, "I could not help but wonder whether he just might be on to something."
The climate of the last 10,000 years has been unusually stable, allowing civilization to flourish. But that is only because people chopped down swathes of forest in Europe, China and India for croplands and pastures, Dr. Ruddiman said. Carbon dioxide released by the destruction of the forests, plus methane, another heat-trapping gas, produced by irrigated rice fields in Southeast Asia, trapped enough heat to offset an expected natural cooling, he said.
"The stability is an accident," Dr. Ruddiman said.
Levels of carbon dioxide and methane rise and fall in natural cycles lasting thousands of years, and both reached a peak at the end of the last ice age 11,000 years ago. Both then declined as expected.
Both should have continued declining through the present day, leading to lower temperatures, and a new ice age should have begun 4,000 to 5,000 years ago, Dr. Ruddiman said. Instead, levels of carbon dioxide reversed 8,000 years ago and starting rising again. The decline in methane levels reversed 5,000 years ago, coinciding with the advent of irrigation rice farming.
This is fascinating. I've never bought the "global warming caused by industrial CO2 emissions" theory because it clashes so badly with the Medieval Climate Optimum (Vikings in Greenland) and Little Ice Age. This is something with a long timeline fitted to the data. I'm not convinced--human agriculture is not that large a fraction of the biosphere--but this definitly calls for more research. Particularly some attempts to quantify how much forest clearing and cropping went on and compare that to global C02 changes.
My reading of the research is that climate change is still driven by changes in solar output and other long-term cycles in the Earth's chemistry rather than human factors. There's a lot of feedback loops that make it very non-linear--the classic one is that warming the Earth produces more water vapor, which makes clouds, which reflect away sunlight, which makes the Earth colder. Not an easy to simulate problem.
When we do get a grasp on it we'll be facing the possibility of climate engineering, deliberately modifying the climate to optimize it for humans and favored wildlife. Probably not cost-effective in this millenium, but I have to say running the trade study to select the proper global average temperature would be a technically joyous project.