Orbital forcing is the effect onclimate of slow changes in the tilt of theEarth's axis and shape of the Earth'sorbit around the Sun (seeMilankovitch 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).[citation needed] 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 theice 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 duringgreenhouse 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 theEarly Paleogene.[1] Notably, Milankovitch cycles have been theorised to be important modulators of biogeochemical cycles duringoceanic anoxic events, including theToarcian Oceanic Anoxic Event,[2] theMid-Cenomanian Event,[3] and theCenomanian-Turonian Oceanic Anoxic Event.[4][5]
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.”[6] Also, Archer and Ganopolski (2005) report that probable future CO2 emissions may be enough to suppress the glacial cycle for the next 500 kyr.[7]
Note in the graphic, the strong100,000 year periodicity of the cycles, and the striking asymmetry of the curves. This asymmetry is believed to result from complex interactions of feedback mechanisms. It has been observed thatice ages deepen in progressive steps. However, the recovery to interglacial conditions occurs in a single large step.
Orbital mechanics require that the length of the seasons be proportional to the swept areas of the seasonal quadrants, so when the eccentricity is extreme, the seasons on the far side of the orbit can last substantially longer. Today, when autumn and winter in the Northern Hemisphere occur at closest approach, the Earth is moving at its maximum velocity and therefore autumn and winter are slightly shorter than spring and summer.
Today in the Northern Hemisphere, summer is 4.66 days longer than winter and spring is 2.9 days longer than autumn.[8] Asaxial precession changes the place in the Earth's orbit where thesolstices andequinoxes occur, Northern Hemisphere winters will get longer and summers will get shorter, eventually creating conditions believed to be favourable for triggering the next glacial period.
The arrangements of land masses on the Earth's surface are believed to reinforce the orbital forcing effects. Comparisons ofplate tectonic continentreconstructions and paleoclimatic studies show that theMilankovitch cycles have the greatest effect duringgeologic eras when landmasses have been concentrated in polar regions, as is the case today.Greenland,Antarctica, and the northern portions ofEurope,Asia, andNorth America are situated such that a minor change in solar energy will tip the balance in theclimate of the Arctic, betweenyear-round snow/ice preservation and complete summer melting. The presence or absence of snow and ice is a well-understoodpositive feedback mechanism for climate.
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