The earth's orbit
Milankovitch cycles are natural variations in the shape of the earth’s orbit (which occur with a cycle of approximately 100,000 years); axial tilt (with a 41,000 year cycle); and precession or axial wobble (with a 23,000 year cycle). It is thought that the intersection of these cycles are an important factor in causing the planet's climate to move slowly, over many thousands of years, from ice ages (glacial periods) to warmer periods (interglacial periods) and back again. The 'termination' of glacial periods occurs relatively rapidly (over thousands of years) compared to the cooling process associated with the return of glaciation (which happens over tens of thousands of years), as demonstrated in the figure below.
These orbital variations change the amount of solar radiation heating the earth. The extent and timing of various mechanisms contributing to the transitions between glacial and interglacial periods is uncertain, but it is thought that the changes in orbit at glacial 'termination' trigger an initial warming of the oceans. This in turn releases CARBON DIOXIDE and the extra carbon dioxide in the atmosphere causes further warming, as does the decrease in ALBEDO resulting from melting of ice. These mechanisms amount to a positive FEEDBACK that magnifies the initial tendency and leads to the warm interglacial periods. Concurrent changes in wetlands causing changes in the amount of methane released to the atmosphere may also play a role in the glacial/interglacial transitions.
Bubbles of air trapped in deep layers of ice in the thick ice sheets on Greenland and Antarctica reveal the composition of the atmosphere over the last several hundred thousand years. Furthermore, proxy measures of temperature can be obtained at the same time, using the isotopic composition of the ice itself (the amount of deuterium, or heavy hydrogen, in the ice, which varies with temperature). The figure below shows the changes in atmospheric carbon dioxide concentration and temperature over the last 400,000 years inferred from ice core measurements.
Carbon dioxide and temperature change over the last 400,000 years
Temperature change (blue) and carbon dioxide change (red) observed in ice core records.
Source: US Government, National Oceanic and Atmospheric Administration, National Climatic Data Center, Temperature change and carbon dioxide change.
The 'Last Glacial Maximum' occurred about 20,000 years ago, and for the past 10,000 years the earth has been in an interglacial period known as the Holocene. It is thought that the current trend of the Milankovitch cycle is towards the eventual onset of another ice age but we cannot say with any precision exactly when this will happen. The current orbital configuration has low eccentricity (i.e. it is relatively circular), which minimises the secondary influences of precession, and the configuration of the last glacial maximum that produced extremely cold northern summers could not occur within the next 30,000 years.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 6: Palaeoclimate.
15 November, 2010