[IAN] Stridani dob ledovych

[Martin Gembec]

Na 28. května připadlo 125 let od narození Milutina Milankovice. Tady je 
kratší průřez tímto tématem v AJ:

Serbian astrophysicist Milutin Milankovitch 
<http://www.ngdc.noaa.gov/paleo/slides/images/base/iceage11.gif> 
(1879-1958) was born 125 years ago this week. His claim to fame is that 
he developed a mathematical theory of climate based on changes in the 
Earth's orbit and axial tilt. There are three basic parameters that 
change with time--now known as the *Milankovitch cycles*--that affect 
the amount of solar energy the Earth receives and how it is distributed 
upon the Earth.

First, the orbital eccentricity of the Earth changes with time. The 
eccentricity tells you how elliptical 
<http://zebu.uoregon.edu/%7Esoper/Orbits/eccentricity.gif> an orbit is. 
An eccentricity of 0 means the orbit is perfectly circular. A typical 
comet's orbit, on the other hand, is very elongated, with an 
eccentricity of 0.999 not at all uncommon. Right now, the Earth's 
orbital eccentricity is 0.017, which means that it is 1.7% closer to the 
Sun than average at perihelion (its closest) and 1.7% further from the 
Sun than average at aphelion (its farthest).

Obviously, the greater the eccentricity the greater the variation in the 
amount of solar radiation the Earth receives throughout the year. It is 
thought that over a period of about 100,000 years, the Earth's orbital 
eccentricity (e) changes from close to circular (e = 0.000) to about e = 
0.070 and back to circular again. At present, the Earth's orbital 
eccentricity is close to its minimum value (i.e. most circular). 
(Actually, the Earth's orbital eccentricity appears to change with 
periods of both 100,000 and 400,000 years, making for a slightly more 
complicated 
<http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/Images/orbital_variation.gif> 
variation than a simple sinusoid.)

Second, the tilt of the Earth's polar axis with respect to the plane of 
the Earth's orbit around the Sun--called the /obliquity/ 
<http://www.soes.soton.ac.uk/staff/ejr/DarkMed/9-tilt.jpg>/ to the 
ecliptic/--changes with time. The Earth's current axial tilt is 23.4°, 
but it ranges between about 22.1° and 24.5° over a period of about 
41,000 years. Greater axial tilt means winter and summer become more 
extreme. Presently, the axial tilt is decreasing, albeit very slowly.

Third, the Earth's axis precesses 
<http://www.soes.soton.ac.uk/staff/ejr/DarkMed/F7wobble.jpg> or 
"wobbles" with a period of about 26,000 years. This changes what 
latitude of the Earth is most directly facing the Sun each year when the 
Earth is closest to the Sun. Currently, the southern hemisphere has 
summer when the Earth is the closest to the Sun.

Milankovitch used these three cycles to predict climate change. His 
ideas were largely ignored until 1976, when a paper by James Hays, John 
Imbrie, and Nicholas Shackleton in the journal /Science/ showed that 
Milankovitch's mathematical model of climate change was able to predict 
major changes in climate that have occurred during the last 450,000 years.

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MartinG