*Pollen in peat bogs are used to show how temperature changed over thousands of years unlike temperature records which are only used for short term global temperature change
I've revised this from my CGP guide:
1) Pollen is preserved in peat bogs [acidic wetland areas]
2) Peat bogs accumulate in layers so the age of the preserved pollen increases with depth
3) Scientists can take cores from the peat bog and extract pollen grains from different aged layers. They can identify the plant species the pollen came from.
4) Only fully grown [mature] plant species produce pollen, so the samples only show the species that were successful at the time.
5) Scientists know the climates that different plant species live in now. When they find preserved pollen from similar plants, it indicates that the climate was similar when that pollen was produced.
6) Because plant species vary with climate the preserved pollen will vary as climate changes over time
7) so a gradual increase in pollen from a plant species that's more successful in a warmer climates would show a rise in temperature [a decrease in pollen from a plant that needs cold conditions would show the same thing]