Worldwide climate changes
The Earth’s climate is not stable and fluctuates between colder periods called Ice Ages, and warmer periods known as Interglacials. It is possible to determine when these different climatic conditions occurred in the past by studying oxygen found in ice cores, speleothems (stalactites and stalagmites) and the remains of certain ocean creatures such as the tiny shelled Foraminifera. Oxygen is a useful indicator of past climates because it is plentiful and occurs in different forms. Some of those forms are more common in the atmosphere during cold periods than during warm periods. The forms of oxygen that were present at different times in the past reveal the pattern of past cold and warm periods.
Climate records using data from Foraminifera and ice cores
During cold periods, such as Ice Ages, the oceans and the shells of ocean dwelling Foraminifera contain a higher proportion of oxygen-18 (a heavy isotope of oxygen) than oxygen-16. This occurs because oxygen-16 is relatively light and quicker to evaporate. This oxygen-16 falls to the ground as rain or snow and, during an Ice Age, becomes locked-up in glaciers and ice sheets. During warmer periods the ice melts and oxygen-16 is returned to the oceans where it is incorporated into Foraminifera shells.
Foraminifera are single-celled ocean creatures that often have a shell made of calcium carbonate. Some species of Foraminifera are better adapted to living in cold water, others are adapted to life in warm water. The shells of these different species can be identified from sediments cored out of the sea bed and can indicate the temperature of the ocean in past times. In addition, the proportions of oxygen-18 and oxygen-16 within their shells can reveal past sea temperatures and climates.
Ice cores and cave formations
Ice cores are long cylinders of ancient ice collected by driving a hollow tube into the thick ice sheets of Antarctica, Greenland and some glaciers. Each layer of ice corresponds to a single year (or sometimes a single season) and almost everything that fell in the snow during that period, including gases, dust and volcanic ash, will be trapped in the ice sheets. Ice cores currently provide details on climate changes up to 200,000 years ago. The results from ice cores are complemented by studies of limestone cave formations, such as stalactites and stalagmites. These have annual growth rings that preserve the different forms of oxygen found in the calcium carbonate that formed them.
Reconstructing local environments
Scientists can reconstruct a general picture of an ancient environment by collecting information about the soil and the plant and animal remains that are found at a site. Comparisons of living plants and animals with these ancient remains can then indicate the types of environments that existed in the past.
Many animal species are adapted to living in particular types of environments. Animals fossils found at a site can therefore reveal the types of environments at that site and the environmental changes over time. For example, certain species of antelope (such as gazelles and springboks) are common in grasslands but not in forests. If a number of gazelle or springbok fossils are found at a site then it can be assumed that this area was once grassland. Additional information can be obtained by analysing the chemicals found in fossil teeth. This can reveal whether the animal ate grasses or the leaves of forest trees.
Plant remains occur in many forms including fossils, charcoal, pollens or as chemical traces in the soil. These remains help scientists identify the types of plants that once grew in a particular area so that past environments can be reconstructed and climatic changes identified. For example, if a site has mainly tree pollens in the lower layers, but the upper layers contain mainly grass pollens, this can be used to indicate that the climate became drier and forest was replaced by grassland.
Plant pollens are microscopic but preserve well as they have an almost indestructible outer shell. Pollens have distinctive shape and surface patterns so they can be used to identify the plants they came from.
Diatoms are types of single-celled algae with cell walls made of silica. Their microscopic silica ‘skeletons’ accumulate on the bottom of lakes and oceans and can be retrieved from sediment cores to provide information about the water conditions in previous times.
Prehistoric soil can be analysed for its texture, which influences the type of plant that can grow in the soil, and also for its chemical composition. Trees and woody plants contain different types of carbon compared with grasses. These carbon isotopes remain in the soil after the plant decomposes and can be detected by chemical analysis.
The link between human evolution and climate change
Changes to past climates and environments have been linked with certain major events in human evolution. For example, three million years ago an Ice Age began which produced a worldwide trend toward cooler, drier climates. In east Africa, this climate change brought about changes to the local environments in which broad expanses of woodland were replaced by grassland. This environmental change probably resulted in physical and behavioural changes by some species as they adapted to the new conditions. Soon after this environmental change, the first human fossils (genus Homo) and manufactured stone tools appeared in east Africa.