Ruddiman sees the signals of human interference in the pattern of methane and CO2 in the atmosphere from old antarctic ice cores. About 8000 years ago the CO2 pattern diverged from previous inter-glacial patterns, starting to rise after 2500 years of falling; this seems to coincide with humans starting to burn down forests for agriculture. Then about 5000 years ago the methane pattern also abruptly shifted, which Ruddiman attributes mainly to the flooding of Asian rice fields, still a major source of atmospheric methane.
Not every climate scientist agrees with Ruddiman – strangely enough, some global warming deniers have come out in support of Ruddiman – even though he’s talking about global warming effects due to human-caused greenhouse gas increases that match very closely climate models, the very thing these guys usually like to deny!
RealClimate has more on the ironies here. The main point, of course, is that the human-caused changes in greenhouse gases of the past 8000 years, if that’s really what they were, have already been doubled by the fossil fuel burning of recent decades. Plotting CO2 concentrations, for example, on the same graph as Ruddiman’s for the past 8000 years, you get a huge vertical spike for the last 100 years. It’s that near-vertical spike in CO2 that we need to worry about, not the gradual millenial changes. I’m sure we’ll figure out how to engineer the world over the next thousand years or so to keep it at whatever glaciation level we like – as long as we survive the next hundred, that is!