The paper

• David Archer & Andrey Ganopolski (2005), "A movable trigger: Fossil fuel CO2 and the onset of the next glaciation", Geochemistry Geophysics Geosystems 6:5.

provides evidence that periods of glaciation begin when northern hemisphere insolation (which varies due to changes in the precession, obliquity and eccentricity of Earth's orbit) falls below a "trigger" level that depends on the amount of CO2 in the atmosphere. Archer & Ganopolski suggest that we're currently approaching a solar minimum, but we've already released enough CO2 into the atmosphere to avoid a glaciation in the next few thousand years. (If we burn all available fossil fuels, "The model predicts the end of the glacial cycles, with stability of the interglacial for at least the next half million years".)