Causes of long- and medium-term climate change

Overview

Milankovitch cycles, eccentricity, what is its period and obliquity, what is its period.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Demonstrate knowledge and understanding of the four groups of natural cause of long- and medium-term climate change, and of the timescale over which each works.

   Model answer

   Astronomic causes, meaning cyclical changes in the Earth's orbit and axis, operating over tens of thousands of years. Solar causes, meaning variation in the output of the Sun, operating over decades to centuries and also over geological time. Continental drift, meaning the movement of the plates and the resulting changes in the arrangement of land, ocean and mountain, operating over millions to hundreds of millions of years. And volcanic activity, where a single explosive eruption cools the planet for one to three years, while sustained volcanism over millions of years alters carbon dioxide levels. Note that these are causes, not evidence: ice cores, pollen and ocean sediments show that change happened, these explain why.

2. 2

   What are Milankovitch cycles?

   Model answer

   They are three regular, predictable variations in the Earth's orbit and axis, calculated by the Serbian mathematician Milutin Milankovitch, which alter the amount and the seasonal and latitudinal distribution of solar radiation reaching the Earth. They are eccentricity, obliquity and precession, with periods of roughly 100,000, 41,000 and 21,000 years. They do not change the Sun's output; they change how much of it arrives where and when. Together they are regarded as the pacemaker of the glacial and interglacial cycles of the Quaternary. They must be treated as three separate cycles with different periods and different effects, not as one thing.

3. 3

   What is eccentricity, what is its period, and what is its effect?

   Model answer

   Eccentricity is the shape of the Earth's orbit around the Sun, which stretches from almost circular to mildly elliptical and back over a cycle of about 100,000 years, with a longer component of about 400,000 years. At maximum eccentricity the difference in distance between the closest and furthest points of the orbit is large, so the amount of radiation received varies substantially through the year; at minimum eccentricity, which is close to the present state, it barely varies. The direct effect on the total annual radiation received is very small, but the roughly 100,000-year spacing of the major glacial cycles of the last 800,000 years matches the eccentricity period, and eccentricity also controls how strong the effect of precession can be.

4. 4

   What is obliquity, what is its period, and what is its effect?

   Model answer

   Obliquity is the tilt of the Earth's axis relative to the plane of its orbit. It varies between about 21.8 degrees and 24.4 degrees over a cycle of about 41,000 years; it is currently about 23.4 degrees and slowly decreasing. Tilt is what causes the seasons, so greater tilt means hotter summers and colder winters, with the effect strongest at high latitudes. Lower tilt means cooler summers at high latitudes, and cool summers are what allow the previous winter's snow to survive the melt season and accumulate year on year - so low obliquity favours the growth of ice sheets. A clear 41,000-year rhythm is present in the ocean sediment record, and it dominated the earlier part of the Quaternary.

5. 5

   What is precession, what is its period, and what is its effect?

   Model answer

   Precession is the slow wobble of the Earth's axis, which traces a cone like a spinning top over about 26,000 years, combined with a slow rotation of the orbital ellipse itself; the two together give a climatically effective period of roughly 19,000 to 23,000 years. Precession determines which hemisphere has its summer at the point of closest approach to the Sun. At present the northern hemisphere has winter at closest approach, which softens northern seasons; half a cycle later the northern summer coincides with closest approach and northern seasonal contrast is strongest. Crucially, precession does not change the total annual radiation received at all - it only redistributes it between the seasons, and it works in opposite directions in the two hemispheres.

6. 6

   Distinguish between obliquity and precession in the way each affects the seasons.

   Model answer

   Obliquity changes the strength of the seasons in the same direction in both hemispheres at once: a higher tilt gives both hemispheres more extreme seasons, and the effect increases with latitude. Precession changes the timing of the seasons relative to the Earth's position in its orbit, so it intensifies the seasons in one hemisphere while damping them in the other, and it has its greatest effect in the tropics and mid-latitudes. Obliquity therefore alters the annual radiation total received at high latitudes; precession alters only its distribution through the year. Their periods also differ, at about 41,000 and about 21,000 years.

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