Recent bad news: The Wilkins Ice Shelf is comparable in size to the US state of Texas. Although it had been cracking for years, a Connecticut-sized portion began calving into icebergs in April 2009 (although the news media missed it because they were fixated upon the relatively smaller H1N1 Mexican influenza outbreak as well as minutia from Hollywood)
1) Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of Earth's orbit (see Milankovitch cycles in the next section). These orbital changes modulate the total amount of sunlight reaching the Earth by up to 25% at mid-latitudes (from 400 to 500 W/m-2 at latitudes of 60 degrees). In this context, the term "forcing" signifies a physical process that affects the Earth's climate.
2) This mechanism is believed to be responsible for the timing of the ice age cycles. A strict application of the Milankovitch theory does not allow the prediction of a "sudden" ice age (rapid being anything under a century or two), since the fastest orbital period is about 15,000 years. The timing of past glacial periods coincides very well with the predictions of the Milankovitch theory, and these effects can be calculated into the future.
3) Scientists think Milankovitch Cycles enable/disable ice ages but climatic feedback loops are responsible for the actual flip. For example, Milankovitch warming causes the oceans to heat up which triggers the release of dissolved CO2, water vapour, and methane hydrates. These added greenhouse gasses raise atmospheric temperatures even higher which melt polar ice (changing albedo from light to dark) as well as melting of permafrost which releases even more methane.
1) Precession is the change in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of roughly 26,000 years. This gyroscopic motion is due to the tidal forces exerted by the sun and the moon on the solid Earth, associated with the fact that the Earth is an oblate spheroid shape and not a perfect sphere. The sun and moon contribute roughly equally to this effect.
2) The angle of the Earth's axial tilt (obliquity) varies with respect to the plane of the Earth's orbit. These variations are roughly periodic, taking approximately 41,000 years to shift between a tilt of 22.1° and 24.5° and back again. When obliquity increases, the temperature difference between winter and summer increases.
3) The shape of Earth's orbit around the Sun is an ellipse and eccentricity being is a measure of the departure from circularity. The shape of the Earth's orbit varies from being nearly circular (low eccentricity of 0.005) to being mildly elliptical (eccentricity of 0.058) and has a mean eccentricity of 0.028. The major component of these variations occurs on a period of 413,000 years. A number of other terms vary between components 95,000 and 125,000 years, and loosely combine into a 100,000-year cycle.
4) The result of these waves combine to enable glaciation cycles with an average period of 100,000 years. (feedbacks from greenhouse gases actually throw the final lever; Volcanoes introduce a randomness which can go either way). Within this cycle you will find an average interglacial period of 15,000 years.