Ruddiman’s basic argument goes like this: Although the climate has cycled through a series of ice ages and warm interglacial periods for more than a million years, none of those warm spells looks like the one we’re in. In all previous cases, carbon dioxide and methane concentrations peaked just after the preceding ice age ended and then levels of those greenhouse gases dropped until the planet slipped again into a new glacial epoch. The planet seemed to be following the same routine since the last ice age ended about 11,000 years ago. But then something funny happened. After falling for a few thousand years, carbon dioxide levels started to rise about 8,000 years ago and methane values swung upward 5,000 years ago.
As an explanation, Ruddiman suggests that carbon dioxide concentrations started to grow when early farmers cleared vast stretches of forest to plant crops, thus reducing the planet’s ability to sop up carbon from the atmosphere. Later, when people learned how to irrigate rice 5,000 years ago, the paddies created for that purpose led to a jump in methane emissions. Those changes prevented the planet from slipping into an ice age, he suggested.
At Wednesday’s session, Ruddiman took the provocative stance of saying that the case is closed.
“I think we are at the point where it is a dead end to claim that natural [processes] explain the Holocene trends,” said Ruddiman, an emeritus professor at the University of Virgiinia. If you look at the previous interglacial periods, none show the rising pattern of carbon dioxide and methane. Q.E.D.
Jean Jouzel, director of the Pierre Simon Laplace Institute in Paris, said “the change in methane is huge. It’s difficult to think it’s not natural.” There were so few people alive 5,000 years ago that it would be hard for humans to account for the methane changes, he said. Jouzel was part of a group that examined the interglacials and made the point that they are each unique in some way. So arguments about the uniqueness of the current interglacial leave some researchers cool.