Orbital forcing

Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of the Earth's orbit around the Sun (see Milankovitch cycles). These orbital changes modify the total amount of sunlight reaching the Earth by up to 25% at mid-latitudes (from 400 to 500 W/(m2) 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 (sudden 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. Milankovitch cycles are also associated with environmental change during greenhouse periods of Earth's climatic history. Changes in lacustrine sediments corresponding to the timeframes of periodic orbital cycles have been interpreted as evidence of orbital forcing on climate during greenhouse periods like the Early Palaeogene. Notably, Milankovitch cycles have been theorised to be important modulators of biogeochemical cycles during oceanic anoxic events, including the Toarcian Oceanic Anoxic Event, the Mid-Cenomanian Event, and the Cenomanian-Turonian Oceanic Anoxic Event. It is sometimes asserted that the length of the current interglacial temperature peak will be similar to that of the preceding interglacial peak (Sangamonian/Eem Stage). Therefore, we might be nearing the end of this warm period. However, this conclusion is probably mistaken: the lengths of previous interglacials were not particularly regular (see graphic at right). Berger and Loutre (2002) argue that “with or without human perturbations, the current warm climate may last another 50,000 years. The reason is a minimum in the eccentricity of Earth's orbit around the Sun.