Evidence for past climate change: ice cores, pollen and ocean-floor deposits

Overview

Why climates before 1850 have to be read from proxies, what an ice core, a pollen sequence and an ocean-floor deposit each measure, how far each reaches back, and where each falls short.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why do we have to use proxy evidence to reconstruct climates before about 1850, and what does a proxy actually give you?

   Model answer

   Direct instrumental measurements of temperature, rainfall and atmospheric composition only exist for roughly the last 150-170 years, and for much of the world for far less than that. A proxy is a natural recorder that responds to climate and preserves that response in a datable material - the isotopic composition of glacier ice, the pollen in a peat bog, the shells in deep-sea mud. It gives an indirect, calibrated estimate of a climate variable rather than a reading of it, so every proxy carries an error range, needs an independent dating method, and is only as good as the assumption that the modern relationship between climate and the recorder also held in the past.

2. 2

   Describe what is actually measured in an ice core, and explain what each measurement tells you.

   Model answer

   Four main things. First, the oxygen and hydrogen isotope ratios of the ice itself, which give the air temperature at the time the snow fell. Second, the air trapped in bubbles, which gives the actual past concentration of carbon dioxide and methane. Third, the dust and sea-salt content, which indicates how arid, windy and stormy conditions were. Fourth, layers of volcanic ash and sulphate, which record eruptions and provide dated tie-points between cores. Annual layers can also be counted like tree rings where snowfall is high, and the thickness of each layer records the accumulation rate.

3. 3

   How does the oxygen isotope ratio in glacier ice record past air temperature?

   Model answer

   Water containing the lighter isotope oxygen-16 evaporates more readily and condenses less readily than water containing the heavier oxygen-18, so as an air mass travels polewards and cools it loses the heavier isotope preferentially. The colder the air, the more of the heavy isotope has already been rained out before the snow falls, so the snow is more depleted in oxygen-18. A more negative delta oxygen-18 value in the ice therefore means a colder air temperature at the time of the snowfall. The same reasoning applies to deuterium, and modern snow is used to calibrate the relationship.

4. 4

   What do the air bubbles in an ice core give you that no other proxy can?

   Model answer

   A direct, physical sample of the ancient atmosphere. As snow compacts to ice the pore spaces are sealed off, trapping air of that period. Measuring the carbon dioxide and methane in those bubbles gives the actual past concentration in parts per million rather than an inference from a proxy. This is what allows greenhouse gas concentration to be plotted alongside temperature over hundreds of thousands of years, and it is why the ice core record is so central to the argument that present concentrations are without precedent.

5. 5

   How far back do ice cores reach, and why do Antarctic cores reach further than Greenland ones?

   Model answer

   Greenland cores reach reliably to about 120,000 years, covering the last glacial cycle. Antarctic cores reach much further: Vostok to about 420,000 years and the EPICA Dome C core to about 800,000 years, and in 2025 the Beyond EPICA core reached ice at least 1.2 million years old. Antarctica wins because it is colder, larger and drier, so annual accumulation is very low and a given thickness of ice spans far more time; Greenland has higher snowfall and warmer basal conditions, so its record is thicker per year but shorter overall, and its oldest ice is more likely to have been disturbed by flow.

6. 6

   Discuss the main limitations of ice cores as evidence of past climate.

   Model answer

   They can only be drilled where thick ice exists, so the temperature signal is that of high latitudes or high mountains and is not directly a global average - polar regions amplify change. Resolution falls with depth because deeper ice is thinned and deformed by flow, and the lowest ice may be folded or mixed so the oldest part of the record is the least trustworthy. The gas record is smoothed because air mixes in the porous upper snow before sealing. Continuous cores reach only about 800,000 years, and even the Beyond EPICA core, which reached ice at least 1.2 million years old in 2025, covers a small fraction of the Quaternary. Finally, meltwater percolation at warmer sites can smear the isotope signal.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.