Observing past ecosystems
We can analyse the fossil record to observe past ecosystems and changes in their biotic and abiotic components. Different species of plants and animals need different conditions to survive. Some plants and animals can be very sensitive to climate and do not adapt easily to change.
Fossilised Corals
Coral reefs live in tropical waters, requiring a particular temperature, depth of water and light intensity. If the depth of the water changes, their their ability to survive is affected.
Therefore fossilised corals provide valuable information about past conditions (with the assumption that they need the same conditions as those that are present today).
Pollen
Plants produce pollen and spores that can be useful in helping to determine past climates. Due to their small size, resistant outer case and large number, they are quickly covered in mud and therefore have a higher likelihood of being preserved.
As each plant has a specific pollen or spore shape, it is possible to analyse samples and match them with species we know today. Different plants are adapted to certain climates, so the types of pollen present in a layer of rock may be indicative of the climate at the time when they were living.
Ice Cores
Oxygen has a number of different isotopes and in water, the relative amount of each of type of oxygen varies with the temperature. Rain and ice contain water that is high in oxygen-16, so when a lot of ice is present, the ocean water contains a lot of oxygen-18.
Shells and sediments also carry an isotopic ‘signature’ of the water they are located in. Therefore, an analysis of the ratio of oxygen-16 to oxygen-18 recorded in rocks, fossils, ice and sediments can provide evidence of the climate at the time they were formed.
Microfossils
These are the tiny remains of very small organisms or very sample fragments of larger organisms, for example small bones from larger animals.
These organisms may require specific living conditions and so can be a useful indicator of how climate has changed over a period of time.
For example, Cytherelloidea, a microscopic genus of ostracod, has a preference for warmer waters, only being found where the temperature does not fall below 10°C. During the early Cretaceous Period, Cytherelloidea was confined to the Tethys Sea but species migrated northwards on several occasions, flourishing in the North Sea during warm phases of the Barremian, Aptian and Albian, but declining during cold phases.