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Orbital forcing is the effect on climate
Climate

Climate encompasses the temperatures, humidity, atmospheric pressure, winds, rainfall, atmospheric particle count and numerous other Meteorology elements in a given region over long periods of time, as opposed to the term weather, which refers to current activity of these same elements....
 of slow changes in the tilt of the Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
's axis and shape of the orbit
ORBit

ORBit is a Common Object Request Broker Architecture 2.4 compliant Object Request Broker . It features mature C , C++ and Python bindings, and less developed bindings for Perl, Lisp , Pascal , Ruby , and Tcl....
 (see Milankovitch cycles
Milankovitch cycles

Milankovitch cycles are the collective effect of changes in the Earth's movements upon its climate, named after Serbian civil engineering and mathematician Milutin Milankovic....
). 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
Ice age

The general term "ice age" or, more precisely, "glacial age" denotes a geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers....
 cycles.






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Orbital forcing is the effect on climate
Climate

Climate encompasses the temperatures, humidity, atmospheric pressure, winds, rainfall, atmospheric particle count and numerous other Meteorology elements in a given region over long periods of time, as opposed to the term weather, which refers to current activity of these same elements....
 of slow changes in the tilt of the Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
's axis and shape of the orbit
ORBit

ORBit is a Common Object Request Broker Architecture 2.4 compliant Object Request Broker . It features mature C , C++ and Python bindings, and less developed bindings for Perl, Lisp , Pascal , Ruby , and Tcl....
 (see Milankovitch cycles
Milankovitch cycles

Milankovitch cycles are the collective effect of changes in the Earth's movements upon its climate, named after Serbian civil engineering and mathematician Milutin Milankovic....
). 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
Ice age

The general term "ice age" or, more precisely, "glacial age" denotes a geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers....
 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.

Vostok Ice Core Petit
It is sometimes asserted that the length of the current interglacial temperature peak will be similar to the length of the preceding interglacial peak (Sangamonian/Eem Stage), and that 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.” Also, Archer and Ganopolski (2005) report that probable future CO2 emissions may be enough to suppress the glacial cycle for the next 500 kyr.

Note in the graphic the strong 100,000 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 that ice ages deepen by progressive steps, but the recovery to interglacial conditions occurs in one big 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 fall and winter in the northern hemisphere occur at closest approach, the earth is moving at its maximum velocity and therefore fall and winter are slightly shorter than spring and summer. Today, northern hemisphere summer is 4.66 days longer than winter and spring is 2.9 days longer than fall. As axial precession
Precession

Precession refers to a change in the direction of the axis of a rotation object. In physics, there are two types of precession, torque-free and torque-induced, the latter being discussed here in more detail....
 changes the place in the Earth's orbit where the solstices and equinoxes occur, winters will get longer and summers will get shorter, eventually creating conditions believed to be favorable for triggering the next ice age.

The arrangements of land masses on the Earth's surface are believed to reinforce the orbital forcing effects. Comparisons of plate tectonic continent reconstructions and paleoclimatic studies show that the Milankovitch cycles have the greatest effect during geologic eras when landmasses have been concentrated in polar regions, as is the case today. Greenland
Greenland

Greenland is a member country of the Kingdom of Denmark located between the Arctic Ocean and Atlantic Oceans, east of the Canadian Arctic Archipelago....
, Antarctica
Antarctica

Antarctica is Earth's southernmost continent, overlying the South Pole. It is situated in the Antarctica of the southern hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean....
, and the northern portions of Europe
Europe

Europe is, conventionally, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural , the Caspian Sea, and by the Caucasus Mountains to the southeast....
, Asia
Asia

Asia is the world's largest and most populous continent. It covers 8.6% of the Earth's total surface area and, with over 4 billion people, it contains more than 60% of the world's current human population....
, and North America
North America

North America is the northern continent of the Americas, situated in the Earth's northern hemisphere and almost totally in the western hemisphere....
 are situated such that a minor change in solar energy will tip the balance between year-round snow/ice preservation and complete summer melting. The presence of snow and ice is a well-understood positive feedback
Positive feedback

Positive feedback, sometimes referred to as "cumulative causation", is a feedback loop system in which the system responds to Perturbation of biological system in the same direction as the perturbation....
 mechanism for climate.

Further reading



External links

  • includes (calculated) data on orbital variations over the last 50 million years and for the coming 20 million years
  • provide another, slightly different series for orbital eccentricity