1. Orbital Changes
Changes in how the Earth moves around the Sun are believed to cause ice ages.
According to Milankovitch (a Serbian physicist), every 100,000 years or so the Earth’s orbit changes from a circular to elliptical (egg-shaped) pattern. It is when the orbit is in the circular pattern that the ice ages are most likely to occur.
This changes how much sunlight we receive.
He also identified that the Earth’s axis moves and wobbles about, changing over 41,000 and 21,000 year cycles.
This also affects how much sunlight is received.
Put all of this together and the history of ice ages can be explained!

3. Solar Output The Sun’s output is not constant.
Cycles have been detected that reduce or increase the amount of solar energy.
The most well-known phenomenon is sunspot activity, when uneven temperatures develop on the Sun’s surface.
These can be seen as tiny black spots on photographs of the sun taken by experts (never try this yourself).
Sunspots seem to come and go following an irregular cycle that lasts about 11 years.
Interestingly, temperatures are greatest when there are plenty of spots – because it means other areas of the Sun are working even harder!

4. The Occurrence of Sunspots since 1600

5. Volcanic Activity
Major volcanic eruptions lead to a brief period of global cooling, due to ash and dust being ejected high into the atmosphere, blanketing the earth.
The 1883 explosion of Krakatoa is believed to have reduced world temperatures by 1.2 °C for at least one year afterwards.
The most recent explosion to have a similar effect was Pinatubo (1991) – link to Restless Earth work completed recently.
Sunlight reaching earth was reduced by 10%.
World temperatures fell by nearly half a degree in the following year.

6. Your Tasks
Write down the names of the three orbital changes and the time each change takes.
a) When was the Little Ice Age?
b) How many years did it last?
c) How many sunspots were there during this time?
d) Approximately how many sunspots have there been
since 1800?
e) What effect has this had on global temperatures?
Explain how variations in the Sun’s output can affect global temperatures.
Explain how big volcanic eruptions might change our climate.