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The Earth's climate has always changed. Long before humans began burning fossil fuels, the planet experienced dramatic shifts between warm periods and ice ages driven entirely by natural factors. Understanding these natural causes of climate change is essential for the Edexcel B specification, because it helps us distinguish between natural variability and the human-caused warming we observe today. This lesson examines the evidence for past climate change and the natural mechanisms that drive it.
Scientists use a range of proxy indicators — indirect evidence preserved in the natural environment — to reconstruct climates stretching back millions of years. No single source of evidence is sufficient on its own; scientists cross-reference multiple types of evidence to build a reliable picture of past climates.
Ice cores are cylinders of ice drilled from ice sheets in Antarctica and Greenland. The longest core (from Dome C, Antarctica) extends back approximately 800,000 years. Ice cores provide evidence in several ways:
Tree rings provide evidence of climate over the past few thousand years:
Historical records include written accounts, paintings, diaries, harvest records and photographs:
Sea-floor sediments are obtained by drilling into the ocean bed and can provide climate records stretching back millions of years:
| Evidence Type | Timescale | Strengths | Limitations |
|---|---|---|---|
| Ice cores | Up to 800,000 years | Trapped air gives direct measurement of past atmospheres; very precise dating | Limited to polar regions with thick ice sheets |
| Tree rings | Up to ~10,000 years | Annual resolution; precise dating; widely available | Only covers areas where trees grow; limited to a few thousand years |
| Historical records | Up to ~5,000 years (written records) | Direct human observations; qualitative and quantitative | Subjective; geographically limited; only covers literate societies |
| Sea-floor sediments | Millions of years | Very long timescale; global coverage | Lower time resolution; dating can be imprecise for older sediments |
Exam Tip: When discussing evidence for climate change, always mention at least two types of evidence and explain what they actually tell us. Examiners want to see that you understand how the evidence is interpreted, not just that it exists.
The Quaternary period is the most recent geological period, spanning the last 2.6 million years. It has been characterised by repeated cycles of glacial periods (ice ages) and interglacial periods (warmer intervals).
These glacial-interglacial cycles have occurred roughly every 100,000 years over the past million years, and the pattern is closely linked to changes in the Earth's orbit around the Sun (Milankovitch cycles).
In the 1920s, Serbian scientist Milutin Milankovitch proposed that long-term variations in the Earth's orbit around the Sun cause cyclical changes in the amount and distribution of solar energy reaching the Earth. These are known as Milankovitch cycles and consist of three components:
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