Natural Climate Change

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.

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Evidence for Past Climate Change

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

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:

• Air bubbles trapped in the ice contain samples of the atmosphere at the time the snow fell. Scientists analyse the concentration of greenhouse gases such as CO₂ and methane in these bubbles. Results show a clear correlation between CO₂ levels and temperature over the past 800,000 years.
• Oxygen isotope ratios (the ratio of ¹⁸O to ¹⁶O) in the ice indicate the temperature at the time the snow formed. A higher proportion of the lighter isotope ¹⁶O indicates warmer conditions; a higher proportion of heavier ¹⁸O indicates colder conditions.
• Dust layers indicate periods of increased aridity or volcanic eruptions.
• Annual layers (similar to tree rings) allow precise dating.

Tree Rings (Dendrochronology)

Tree rings provide evidence of climate over the past few thousand years:

• Each ring represents one year of growth. Wide rings indicate warm, wet growing seasons; narrow rings indicate cold or dry conditions.
• By overlapping ring patterns from living trees, dead timber and archaeological wood, scientists have constructed continuous records stretching back over 10,000 years in some regions.
• Tree ring data is particularly useful for reconstructing temperature and rainfall patterns over the past millennium.

Historical Records

Historical records include written accounts, paintings, diaries, harvest records and photographs:

• Records of the Thames freezing in London (Frost Fairs were held on the frozen Thames between 1608 and 1814) provide evidence for the Little Ice Age (approximately 1300–1850 CE).
• Harvest dates recorded by monks and farmers indicate growing season temperatures.
• Paintings and photographs of glaciers show their extent at different times, demonstrating retreat since the mid-19th century.
• Viking sagas describe settling Greenland around 985 CE during the Medieval Warm Period (approximately 900–1300 CE), when temperatures were warm enough to support farming in southern Greenland.

Sea-Floor Sediments

Sea-floor sediments are obtained by drilling into the ocean bed and can provide climate records stretching back millions of years:

• The shells of tiny marine organisms called foraminifera accumulate on the sea floor over time. The oxygen isotope ratio in their calcium carbonate shells indicates the temperature of the water when they were alive.
• Pollen grains preserved in sediments indicate which plants were growing nearby, which in turn indicates the climate.
• Sediment layers can also reveal changes in ocean currents and ice sheet extent.

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.

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The Quaternary Period

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).

• During glacial periods, global temperatures dropped by 5–10°C compared to today. Ice sheets expanded from the poles, covering much of northern Europe, North America and Asia. Sea levels dropped by up to 120 metres as water was locked up in ice.
• During interglacial periods, temperatures were similar to or slightly warmer than today. Ice sheets retreated, and sea levels rose.
• We are currently living in an interglacial period called the Holocene, which began approximately 11,700 years ago.

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).

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Natural Causes of Climate Change

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:

1. Eccentricity (approximately 100,000-year cycle)