Evidence for climate change: HSC Earth and Environmental Science Module 7
“Analyse the proxy and direct evidence used to reconstruct past climates and to identify current climate change, including but not limited to ice cores, isotopes, tree rings, sediment cores and the instrumental temperature record”
Scientists reconstruct past climate using proxies (ice cores with trapped ancient air, oxygen and carbon isotopes, tree rings, sediment cores) and confirm present change with direct instruments (the global temperature record, the Mauna Loa Keeling Curve and Australia's Cape Grim air record). All these independent lines converge to show rapid recent warming and unusually high carbon dioxide.
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What this dot point is asking
NESA wants you to explain how scientists know what past climates were like and how they know the climate is changing now. You need to describe proxy evidence (indirect records preserved in nature) and direct evidence (instrumental measurements), and explain what each tells us. Australian datasets such as the Cape Grim air record strengthen the answer.
The answer
Reliable thermometer records only began in the mid-1800s, so to understand climate over thousands and millions of years scientists rely on proxies, natural records that respond to climate and preserve a signal of past conditions.
Ice cores
Ice sheets in Antarctica and Greenland build up year by year as snow compacts into layers of ice. Cores drilled through them are climate archives reaching back hundreds of thousands of years. Bubbles trapped in the ice hold samples of the ancient atmosphere, letting scientists measure past carbon dioxide and methane directly. The ratio of oxygen isotopes in the ice records past temperature. Antarctic cores show that carbon dioxide and temperature have risen and fallen together through ice ages, and that today's carbon dioxide level is far above anything in at least the last 800,000 years.
A proxy is an indirect, natural record that stands in for a direct measurement. Ice cores, tree rings, sediment cores and isotope ratios are proxies because the organisms or materials that formed them responded to temperature, precipitation or atmospheric chemistry, preserving a measurable climate signal long before instruments existed.
Isotopes
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.
Tree rings
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.
Sediment cores
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 instrumental evidence
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. Satellites add measurements of sea level, ice extent and sea-surface temperature. Together these show rising temperature, rising carbon dioxide, retreating glaciers, shrinking sea ice and rising seas.
Why multiple lines matter
No single record is decisive on its own, but proxies and instruments agree. Ice cores, isotopes, tree rings and sediments all show that recent warming is rapid and that carbon dioxide is unusually high, and the instrumental record confirms continuing change. This convergence of independent evidence is the strongest argument that the climate is changing.
- Confusing proxy and direct evidence
- Thermometers and the Cape Grim or Mauna Loa records are direct measurements. Ice cores, tree rings and sediment cores are proxies. Classify each correctly.
- Claiming ice cores measure ancient air indirectly
- The trapped air bubbles are actual samples of the past atmosphere, a direct measure of past carbon dioxide, even though the ice itself is a proxy archive.
- Relying on one line of evidence
- Markers reward showing that independent proxies and instruments agree, which is what makes the conclusion robust.
Try this
Q1. Explain how an ice core can provide evidence of both past temperature and past atmospheric composition. [4 marks]
- Cue. Oxygen-isotope ratios in the ice record temperature; air bubbles trapped in the ice are direct samples of past carbon dioxide and methane.
Q2. Distinguish between proxy and direct evidence for climate change, giving one example of each. [3 marks]
- Cue. Proxy is an indirect natural record (e.g. tree rings); direct is an instrumental measurement (e.g. the Cape Grim carbon dioxide record).
Exam-style questions
Questions in the style of NESA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
2023 HSC-style6 marksDiscuss the use of dendrochronology AND changes in rock types as evidence for past variation in global temperatures.Show worked answer →
Discuss means present advantages and limitations of each method. For 6 marks, cover BOTH lines of evidence and weigh how good each is.
- Dendrochronology (tree rings)
- The width of each annual growth ring reflects the conditions in which the tree grew, so a narrower-than-normal ring can indicate a cooler year. Advantage: it gives an annual, well-dated record. Limitations: ring width is also affected by other factors such as rainfall, so it is not a pure temperature signal; it is limited to the lifetime of the tree and to local rather than global conditions.
- Changes in rock types
- Different rocks form in different climates, so the rock record provides evidence over a much longer time frame. For example, coal seams indicate a warm, swampy environment, while glacial tillite indicates cold, glacial conditions. Advantage: covers millions of years. Limitation: it again reflects the local environment, and (depending on the position of the continent) may not directly equal a global temperature.
- Discussion
- Tree rings give high-resolution but short, local records, whereas rock types give long but coarse records; used together they build a fuller picture of past temperature variation.
2021 HSC-style5 marksHow does scientific evidence contribute to our understanding of ancient variations in global temperature? Support your answer using TWO specific examples.Show worked answer →
For 5 marks, explain (not just name) how TWO distinct proxies reveal past temperatures.
- Example 1: pollen and spores
- Fossil pollen and spores preserved in sediments record which plant species were living at a given time. Because different plants thrive in different climates, the types and abundance of pollen indicate the climate of that period, with records extending back hundreds of millions of years.
- Example 2: sedimentary rocks (and isotopes)
- Sediments and the rocks they form preserve climate clues. For example, dropstones in Permian-aged sediments near Kiama on the NSW south coast show that glaciers were once present, indicating a cold climate at that time and place. Oxygen isotope ratios in sediment also vary with temperature.
- Bringing it together
- Each proxy is indirect, but by combining biological evidence (pollen) with physical evidence (sediments and isotopes) scientists build a reliable, cross-checked picture of how global temperatures varied in the ancient past.
2022 HSC-style1 markDirect measurements of greenhouse gases present in the atmosphere 400 000 years ago can be obtained from A. tree rings. B. fossilised pollen grains. C. gas bubbles trapped in ice cores. D. oxygen isotopes in ocean sediments.Show worked answer →
The correct answer is C: gas bubbles trapped in ice cores.
As snow accumulates and compresses into ice on ice sheets, it traps tiny bubbles of the atmosphere from the time the ice formed. Drilling an ice core and analysing these bubbles gives a direct sample of ancient air, allowing scientists to measure past concentrations of greenhouse gases such as carbon dioxide and methane going back hundreds of thousands of years.
Tree rings (A), pollen (B) and oxygen isotopes (D) are all useful proxies, but they provide indirect evidence inferred from biological or chemical signals, not a direct measurement of the ancient atmosphere itself.