Reconstructing past climate

• Sea-floor sediments

  Fossil shells of tiny sea creatures called foraminifera, which accumulate in sea-floor sediments, can be used to reconstruct past climates.

  Chemical composition for foraminifera shells indicates the ocean temperatures in which they are formed.
• Ice Core

  Ice cores from the polar regions contain tiny bubbles of air i.e. gaseous composition of the atmosphere in the past.

  Scientists can measure relative frequency of hydrogen and oxygen atoms with stable isotopes;

  Colder climates result in less of these isotopes.
• Tree rings (Dendrochronology)

  Tree rings (annule) vary in width each year depending on temperature condition and moisture availability.
• Fossils

  Plants and animals can be used as proxies for climate, due to specific environmental conditions needed for survival.

  e.g. some herbivorous dinosaur species only survived in sub-tropical habitats.
• Lake Sediments

  Pollen grains, spores, diatoms (single-celled algae) and valves (alternating light and dark bands) in lakes can be used to reconstruct past climates.

  Light banded valve; form coarser sediments, indicates high-energy meltwater run-off in spring and summer.

  Dark banded valve; finer sediments, show deposition during winter months.
• Long-term changes.

  About 100 million years ago (mid- Cretaceous) average global temperatures were 6 to 8 degrees higher than today, with carbon dioxide five times higher.

  A further, shorter spike in temperatures occurred 55 million years ago in the Palaeocene-Eocene thermal maximum, when average global temperatures were 23 degrees.

  Current global temperature is at 13.9 degrees; the change to colder conditions occurred 35 million years ago.
• Glaciation of Antarctica

  40 million years ago the continents had sub-tropical conditions. Change occurred because;

  Carbon dioxide levels dropped 35 million ago;

  Continental drift led to isolation of the continent in the South Pole;

  Ocean currents isolated the continent from warm water.
• Quaternary glaciation

  characterised by cyclic change in climate from cold glacials to water interglacials.

  Most recent glacial was around 20000 years ago when about 1/3 of the continental surface was covered with ice.

  The climate warmed the ice remained only in mountains until approximately 13000 BP (before present)
• Present interglacial, the Holocene

  Except for Antarctica and Greenland, glaciers and ice sheets have shrunk.

  Ice fields and valley glacier remain only in the highest mountains (e.g. Himalayas) and high latitudes (e.g. Alaska).
• Scientists believe that the climate change of the last 200 years has been driven by human activity, entering us into the Anthropocene period.