Quick questions on Evidence for climate change: HSC Earth and Environmental Science Module 7

Isotopes are atoms of an element with different masses. The ratio of oxygen-18 to oxygen-16 in ice and in marine shells depends on temperature, because lighter water molecules evaporate more readily and the balance shifts with the climate. Measuring this ratio in ice cores and in the calcium-carbonate shells of tiny marine organisms preserved in sediment gives a temperature proxy stretching back millions of years. Carbon isotopes help identify whether extra atmospheric carbon comes from burning fossil fuels.

Trees in seasonal climates add one growth ring per year. Wide rings indicate good growing conditions (warm, wet); narrow rings indicate stress (cold, dry). Counting and measuring rings, a method called dendrochronology, gives an annually resolved climate record going back hundreds to thousands of years, useful for reconstructing temperature and rainfall over recent centuries.

Layers of sediment on lake and ocean floors accumulate steadily and trap climate indicators: pollen grains reveal past vegetation and therefore climate, the chemistry and isotopes of fossil shells record ocean temperature, and grain size reflects wind and current strength. Cores from the sea floor and from Australian lakes extend the record over very long timescales.

Direct evidence comes from instruments. The global surface temperature record, compiled from land stations and ships since the 1800s, shows clear warming, especially since the mid-twentieth century. The longest continuous record of atmospheric carbon dioxide comes from Mauna Loa in Hawaii, where measurements since 1958 trace the steady rise known as the Keeling Curve. In the Southern Hemisphere, the Cape Grim Baseline Air Pollution Station in north-west Tasmania, run by the CSIRO and the Bureau of Meteorology, has measured background carbon dioxide and other greenhouse gases in clean Southern Ocean air since 1976, providing a globally important southern record.

Explain how an ice core can provide evidence of both past temperature and past atmospheric composition. [4 marks]

Distinguish between proxy and direct evidence for climate change, giving one example of each. [3 marks]