The challenge of modeling interglacial CO2 and CH4 trends

A vigorous debate about the early anthropogenic explanation of greenhouse-gas increases in the late Holocene has been underway for years, but what about the current status of natural explanations for the gas trends? Modelers have called on a wide range of natural factors to explain the Holocene gas increases but have not yet converged on an explanation. No model has yet reproduced both the upward gas trends during the Holocene and the downward gas trends during previous interglaciations.