Climate Change Evidence and Projections

Understanding the evidence for past and present climate change — and the methods used to project future change — is a core requirement of the AQA A-Level Geography specification. This lesson examines proxy records, instrumental data, natural causes of climate variability, and the IPCC's latest scenarios and projections.

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Proxy Records of Past Climate

Direct instrumental records of temperature extend back only to the mid-19th century. To reconstruct earlier climates, scientists use proxy records — natural archives that preserve information about past environmental conditions.

Ice Cores

• Source: Extracted from ice sheets in Antarctica (e.g. Vostok, EPICA Dome C) and Greenland (e.g. GRIP, NEEM).
• What they record: Trapped air bubbles contain samples of past atmospheres, allowing direct measurement of CO₂, CH₄, and other gases. Oxygen isotope ratios (δ¹⁸O) in the ice indicate past temperatures.
• Temporal range: The EPICA Dome C core extends back approximately 800,000 years, revealing eight glacial-interglacial cycles.
• Key finding: Atmospheric CO₂ varied between approximately 180 ppm (glacial periods) and 280 ppm (interglacials) over the past 800,000 years. Current levels (~420 ppm) are unprecedented in this entire record.

Tree Rings (Dendrochronology)

• Principle: Trees in temperate and boreal regions produce annual growth rings. Ring width, density, and chemical composition reflect growing conditions — temperature, rainfall, and soil moisture.
• Temporal range: Living trees provide records of several hundred years; overlapping sequences from fossil and subfossil wood can extend records back thousands of years (e.g. the European oak chronology extends back ~12,000 years).
• Limitations: Trees record growing season conditions only; interpretation requires careful calibration; not all tree species produce clear annual rings.

Ocean Sediment Cores

• Source: Drilled from the ocean floor using specialised vessels (e.g. JOIDES Resolution).
• What they record: The shells of foraminifera (tiny marine organisms) preserved in ocean sediments contain oxygen isotope ratios (δ¹⁸O) that reflect both ocean temperature and global ice volume.
• Temporal range: Ocean sediment records extend back hundreds of millions of years, though resolution decreases with age.
• Key finding: Combined with ice core data, ocean sediments confirm the cyclical nature of glacial-interglacial cycles driven by orbital variations.

Other Proxy Records

Proxy	What It Records	Temporal Range
Pollen analysis (palynology)	Vegetation assemblages, indicating climate zones	Up to ~1 million years
Coral growth bands	Sea surface temperature, ocean chemistry	Up to ~500 years per colony; fossil corals extend range
Speleothems (cave formations)	Temperature, rainfall via δ¹⁸O and growth rates	Up to ~600,000 years
Historical records	Harvest dates, frost fairs, diary entries	Centuries (varies by region)
Lake sediments (varves)	Seasonal deposition patterns, pollen, diatoms	Up to ~50,000 years

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Instrumental Records

Temperature Records

• Reliable global instrumental records begin in the 1850s, when networks of weather stations were established.
• Global mean surface temperature has risen by approximately 1.1°C above the 1850–1900 average (as of 2023, IPCC AR6 and subsequent updates).
• The 20 warmest years on record have all occurred since 1998.
• 2023 was the warmest year in the instrumental record, approximately 1.45°C above pre-industrial.

Atmospheric CO₂ Measurements

• Continuous atmospheric CO₂ measurements began at Mauna Loa Observatory, Hawaii in 1958, initiated by Charles David Keeling.
• The Keeling Curve shows a relentless upward trend from ~315 ppm in 1958 to ~424 ppm in 2024, superimposed with an annual seasonal cycle (reflecting Northern Hemisphere vegetation growth and dormancy).
• Pre-industrial CO₂ was approximately 280 ppm (from ice core data).

Sea-Level Records

• Tide gauge records (from the mid-19th century) and satellite altimetry (from 1993) show:
  • Average sea-level rise of approximately 1.3 mm/yr from 1901 to 1971.
  • Accelerating to approximately 3.7 mm/yr from 2006 to 2018 (IPCC AR6).
• The acceleration is driven by increasing contributions from ice sheet mass loss (Greenland and Antarctica) in addition to thermal expansion.

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Natural Causes of Climate Variability

Milankovitch Cycles

Milutin Milankovitch (1920s-1940s) identified three orbital variations that affect the amount and distribution of solar radiation reaching Earth: