Orbital Forcing

Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of the orbit (see Milankovitch cycles). These orbital changes change the total amount of sunlight reaching the Earth by up to 25% at mid-latitudes (from 400 to 500 Wm−2 at latitudes of 60 degrees). In this context, the term "forcing" signifies a physical process that affects the Earth's climate.

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 20,000 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.