Biology is the only mechanism that can realistically account for the CO2 and CH4 record. The short term record shows immediate CO2 concentration changes in opposition to temperature changes, while the long term record shows delayed CO2 and CH4 changes in the same direction as temperature changes.
The dependence of the growth and evolution of plant and animal life on the relative abundance of CO2 and energy, provides a strong, temperature driven, delayed influence on CO2 and CH4 levels. This occurs as ecosystems adapt to the continuously changing distribution of local climate zones. For example, as the temperature warms, there's less ice and more of the Earth participates in biomass production.
Short term CO2 concentration variability is a consequence of rapid seasonal temperature changes acting on plant growth and decomposition rates.