Milankovich Cycles
(Also spelled Milankovitch or, properly, Milanković.)
For a glossary of the astronomical terms used on this page, see here.
Milutin Milanković was a Serbian civil engineer who, during the early 20th Century, developed a theory of Ice Ages based upon variations in Earth's orbit. These are:
- Orbital Eccentricity. As a consequence of the gravitation of other bodies in the solar system, Earth's orbit varies in shape from nearly a circle (e = 0.005) to a distinct ellipse (e = 0.058). There are a number of overlying cycles, the most important having periods of 95 000 years, 136 000 years and 413 000 years; the consequence is a cycle of approximately 100 000 years. Currently e = 0.017. The difference between perihelion and aphelion leads to a difference in insolation of about 4×e, i.e. about 7% more solar energy meets Earth at perihelion than at aphelion. The range is approximately 0.5% to 23%. These differences are partly responsible for climate change.
- Obliquity of the Ecliptic. The obliquity of the ecliptic varies between 22.1º and 24.5º over a period of approximately 41 000 years. It is currently 23.5º. When it is at its greatest, higher latitudes receive increased annual insolation and lower latitudes receive decreased annual insolation. The consequence is generally warmer summers and cooler winters. When the position is reversed (i.e. cooler summers), the conditions favour the development of an Ice Age.
- Precession of the Equinoxes. Earth's axis precesses with a period of 25 770 years. Currently, it is the northern hemisphere that has longer summers, leading to global warming. In about 12 000 years, when the axis is tilted the opposite way, the southern hemisphere will have longer summers and Earth will be generally cooler.
- Precession of the Peihelion. (Also known as Precession of the Line of the Apsides.) This has a period of 21 000 years. Perihelion currently occurs on 3 Jan (+/- 1 day due to leap year cycle), i.e. in northern winter. At perihelion, Earth is moving fastest in its orbit (Kepler's Second Law). This means that, at present, northern hemisphere winters are shorter than northern hemisphere summers (and southern hemisphere winters are longer than southern hemisphere summers). Because there is more land mass in the northern hemisphere, the relative lengths of seasons has an effect on climate.
Based on our current position in the Milankovich cycles, we should expect Earth to be warming, and to continue to do so for several thousand years. It would do this without any human influence. We should also expect this warming to result in an increase in atmospheric carbon dioxide, just as has happened in the past.
Some, perhaps most, of the AGW lobby accepts that the warming and cooling cycles revealed by ice core analysis were initiated by Milankovich cycles. However, it wants us to believe that, after about 800 years of warming, the Milankovich cycles stopped being influential and carbon dioxide took over in a positive feedback system (more CO2 -> higher temperatures -> more CO2 ->higher temperatures....). Until the Milankovich cycles brought about cooling, despite the amount of positive feedback carbon dioxide in the atmosphere. Yeah, whatever...
In short, we have a system that is accepted as having been responsible for global warming. There are some problems with the understanding of it (not surprising, considering how little research effort and funding there is for understanding Milankovich cycles as compared to AGW -- reminiscent of the R&D funding disparity between renewables and civil nuclear in the 1970s!), but the main problems are (a) that Governments can't tax Milankovich cycles and (b) the doom-mongers can't rake in public funding for combatting Milankovich cycles. AGW wins hands down! The point is that climate is a complex system and there are many influences on it; nobody is claiming that it has to be Milankovich Cycles alone: that is a straw man argument invented by the AGW lobby.
© 2007