| Lecture 19 Notes November 4, 2004 |
Milankovitch Cycles
We are motivated to learn about the Milankovitch cycles owing to their apparent match with climate cycles during the ice ages. I began my lecture by showing a map of the distribution of ice during the last ice age. The ice mass was greatest over North America. The ice sheets in Euroasia were tiny in comparison. The distribution depends on what drives ice advance. Milankovitch claimed that variations in the incoming solar radiation at 65N during summer was key. Climatologists debate this still today. In the week of Nov 15, I will return to this debate.
We have already discussed
how the solar luminosity varies on very long timescales because the sun is
growing stronger on very long timescales (noticeable perhaps over >10
million years or so). I also described variations in sun spot frequency (revelevant
on all timescales), which we will return to when we talk about modern climate
change. Sunspot frequency has probably had a substantial influence on climate
in the past - such as the little ice age. Unfortunately sun spot frequency
is not recorded reliably in any proxy data and it not very predictable - especially
in advance of a few weeks.
The sunlight
incident on Earth varies due to variations in Earth's orbit around the sun.
The distribution of sunlight can change (relative amplitude at a particular
point at a partiuclar time), and the overall global average (i.e, the "solar
constant" S) can change as the annual average Earth-Sun distance varies.
The influence of these orbital changes on Earth's sunlight are known collectively
as Milankovitch cycles. The cycles are well described in the text on pages
276-277, but here the book is surprising shy on pictures. I've included the
pictures I showed in class below.
 Figure 1 Earth orbits the sun on an ellipse with the sun sitting at one of the foci. The point of closest approach is perihelion and furthest approach is aphelion. Perihelion occurs in early January at present, which is when the NH is tipped away from the sun. NH dwellers receive little additional sunlight at this time, while SH summers are enhanced considerably more. |
 Figure 2 Earth's axis of rotation is tipped away from the line pependicular to the orbital plane (plane of the ecliptic), which is why we have seasons. But the axis precesses slowly (25,700 years), so NH dwellers do not always experience winter at perihelion. The orbit in Fig 1 precesses too. The combination reduces the period and makes it slightly irregular with peaks every 19,000 to 23,000 years or so. |
 Figure 3 The tilt of Earth's spin axis wobbles too, but only by about 1 deg every 40,000 years or so. This makes seasons more and less extreme. |
 Figure 4 The cycles described in Figs 2-3 alter the annual average earth-sun distance, so the solar constant in unchange. In contrast variations in eccentricity alter the total flux reaching earth. Lower eccentricity increases the average distance and decreases S. Eccentricity also influences the extremes possible associated with the time of perihelion and aphelion. |
All the above mentioned components of the orbit vary because of the gravitational attraction between the Earth and the other planets. The orbital changes are known as Milankovitch orbital changes. He did put forward the theory that the periodic changes of climate between glacial and interglacial are related to the orbital changes of the Earth. They are only important for climate changes on very long timescales. Hence, when modelling the enhanced greenhouse effect, the solar constant can probably be handled as a true constant.
Contact the instructor at: atms211@atmos.washington.edu
Last Updated: 10/6/2004