Evidence for past climate change

Long-term change: the Quaternary

During the Quaternary (the last 2.6 million years or so) climate has swung repeatedly between cold glacials and warmer interglacials.

• Ice cores: layers of snow compress to ice. Isotopes in the ice record temperature, and trapped air bubbles record past CO2. The EPICA core from Antarctica (3,270 m long) gives about 800,000 years of data: eight glacial and eight interglacial periods, with temperature closely matching CO2. CO2 fell below 180 ppm in a glacial between 650,000 and 750,000 years ago (CORDIS, 2008).
• Ocean sediment cores: the shells of tiny marine organisms record ocean temperature and ice volume; a 1976 study linked these to orbital cycles over 450,000 years (NASA).
• Tree rings: wide rings show good growing years; useful for recent millennia.

Medium-term change

• Medieval Warm Period: about the 9th to 13th centuries, warmer over the Northern Hemisphere; Norse seafaring and colonisation across the North Atlantic suggest warmth (NOAA).
• Little Ice Age: about the 15th to 19th centuries, cooler.
• Evidence: pollen in lake and peat sediments (which plants grew), historical sources (harvest records, diaries, ship logs) and art (paintings of frozen rivers and advancing glaciers).

NOAA notes the data become sparse before the last four centuries, so the regional pattern is still being researched.

Strengths and limits of the evidence

• Proxies are indirect: they must be calibrated against modern measurements.
• Coverage is uneven: ice cores come from polar regions, tree rings from land and mid to high latitudes.
• Historical and art sources are local and may be subjective.
• Combining several proxies gives more reliable reconstructions.

Worked example

EPICA showed glacial CO2 below 180 ppm, and levels in 2008 just over 380 ppm. Calculate the percentage by which 380 ppm exceeds 180 ppm.

1. Difference = 380 - 180 = 200
2. 200 / 180 x 100

Answer: About 111% higher.

Check yourself

1. Glacial CO2 fell below 180 ppm; in 2008 it was just over 380 ppm. Calculate the percentage increase from 180 to 380 ppm, to the nearest whole number.
   Show answer

   111 %. 200 / 180 x 100 = 111%.

2. Which evidence gives the longest continuous record of both temperature and atmospheric CO2?
   • Antarctic ice cores
   • Tree rings
   • Paintings
   • Harvest records
   Show answer

   Antarctic ice cores. EPICA covers about 800,000 years and traps air bubbles.

3. Pollen grains in lake sediments show past climate because
   • pollen changes colour with temperature
   • pollen traps carbon dioxide
   • the types of plants growing reflect the climate at the time
   • pollen only forms in glacials
   Show answer

   the types of plants growing reflect the climate at the time. Different species need different climates.

4. The Little Ice Age is generally dated to about
   • 800,000 years ago
   • the 15th to 19th centuries
   • the 9th to 13th centuries
   • the period since 1960
   Show answer

   the 15th to 19th centuries. NOAA describes the cooler period of the 15th to 19th centuries.

5. How many glacial periods does the EPICA ice core record over 800,000 years?
   • Two
   • Twenty
   • One
   • Eight
   Show answer

   Eight. Eight long glacials and eight shorter interglacials (CORDIS).

6. Explain one limitation of using historical sources and art as evidence of past climate.
   Show answer

   They are local and subjective, so they may not represent the global climate and are hard to compare with measured data.. Local coverage and subjectivity limit their use.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Geography specification (Issue 3, September 2025)
• European Commission CORDIS: Ice cores give up 800,000 years of climate data (15 May 2008)
• NOAA Paleoclimatology (archived): The Medieval Warm Period
• NASA Science: Milankovitch (orbital) cycles and their role in Earth's climate (updated 22 October 2024)

Last updated 8 November 2026. Spotted a mistake? Tell us.