© Eugene S. Takle, 2002
Crowley (2002) gives a brief overview of the occurrences from geological history of glacial and interglacial events and the successes and failures of climate models to simulate these events. He points out that the main forcing factor is change in solar radiation received on Earth (insolation) on time scales of 20,000 years (due to the earth's precession), 40,000 years (due to obliquity or tilt of the earth?s axis), and 400,000 years (due to eccentricity of the earth's orbit around the sun). These cycles of insolation lead to oscillations in climate known as the Milankovich cycles. He points to some geological examples of rather abrupt climate changes and notes that the anthropogenic perturbation (greenhouse gas increase) may take 10,000 years to neutralize by natural processes, and that this perturbation will interact with cycles of natural forcing in ways that are not well understood but that might lead to climate surprises.
Reference
Crowley, T. J., 2002: Cycles, cycles everywhere. Science, 295, 1473-1474.