Climate Change: Evidence and Projections

This lesson examines the evidence for contemporary climate change, IPCC assessment findings, future projections, tipping points and uncertainty. It addresses the Edexcel A-Level Geography (9GE0) specification, Topic 6, Enquiry Question: "How are the carbon and water cycles linked to climate change?"

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Observed Temperature Changes

Global mean surface temperature has increased by approximately 1.2°C since the pre-industrial period (1850–1900). This warming has accelerated in recent decades.

Period	Temperature Change (relative to 1850–1900 average)	Rate of Warming
1850–1900	Baseline (0°C)	—
1900–1950	+0.2°C	~0.04°C/decade
1950–2000	+0.6°C	~0.08°C/decade
2000–2023	+1.2°C	~0.25°C/decade
2023	+1.48°C (warmest year on record)	—

Key observations:

• Each of the last four decades has been successively the warmest on record — the 2010s were warmer than the 2000s, which were warmer than the 1990s.
• The Arctic is warming approximately 3–4 times faster than the global average (Arctic amplification).
• Land areas are warming faster than oceans (~1.6°C vs ~0.9°C).
• Night-time temperatures are rising faster than daytime temperatures.
• The 20 warmest years on record have all occurred since 2000 (except 1998).

Temperature Data Sources

Source	Method	Coverage	Reliability
Surface thermometers	Weather stations, ship observations, buoys	Global but uneven distribution	Excellent from ~1880; limited before
Satellite measurements	Microwave Sounding Units (since 1979)	Global, including oceans and remote areas	Good; but shorter record
Reanalysis datasets	Combine observations with climate models	Global, continuous	Best overall picture
Proxy data	Tree rings, ice cores, corals, sediments	Global; varying timescales	Essential for pre-instrumental period

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Ice Core and Paleoclimate Evidence

Ice cores from EPICA Dome C (Antarctica) provide a continuous record spanning 800,000 years:

Parameter	Glacial Minimum	Interglacial Maximum	Current (2024)
CO₂ (ppm)	~180	~280	~424
CH₄ (ppb)	~350	~700	~1,920
Temperature anomaly (relative to present)	−8 to −10°C	−1 to +2°C	+1.2°C (and rising)

Key findings:

• CO₂ and temperature have varied in lockstep over 800,000 years through eight glacial-interglacial cycles.
• Current CO₂ (424 ppm) is 51% higher than the highest natural level in 800,000 years (280 ppm).
• The rate of CO₂ increase (~2.5 ppm/year) is approximately 100 times faster than the fastest natural increases in the ice core record.

Exam Tip: When presenting ice core evidence, emphasise both the magnitude (CO₂ is far outside the natural range) and the rate (the increase is ~100x faster than natural). This combination — unprecedented level at an unprecedented rate — is the strongest argument that current change is anthropogenic, not natural.

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Sea Level Rise

Global mean sea level has risen by approximately 20 cm since 1900 and the rate is accelerating.

Period	Rate of Sea Level Rise (mm/year)	Main Drivers
1901–1990	~1.3	Thermal expansion; glacier melt
1993–2023	~3.4	Accelerating glacier/ice sheet melt
2006–2023	~4.3	Ice sheet contribution increasing
Projected by 2100	0.3–1.1 m (depending on emissions)	All drivers accelerating

Causes of Sea Level Rise

Cause	Contribution (2006–2023)	Mechanism
Thermal expansion	~40%	Warming water expands; the top 2,000 m of ocean has warmed by ~0.09°C per decade
Glacier melt	~25%	Mountain glaciers worldwide are losing mass (Alps, Andes, Himalayas, Alaska)
Greenland Ice Sheet	~20%	Losing ~280 Gt of ice per year (equivalent to ~0.8 mm/year sea level rise)
Antarctic Ice Sheet	~10%	Losing ~150 Gt of ice per year; West Antarctic Ice Sheet particularly vulnerable
Land water storage	~5%	Groundwater extraction (pumped to surface, eventually reaches ocean)

Consequences of Sea Level Rise

Impact	Detail
Coastal flooding	Low-lying areas (Bangladesh, Netherlands, Pacific islands) increasingly vulnerable
Coastal erosion	Accelerated erosion of cliffs, beaches and dunes
Saltwater intrusion	Rising sea level pushes salt water into freshwater aquifers, threatening water supplies
Loss of land	Some Pacific island nations (Tuvalu, Kiribati, Marshall Islands) face existential threat
Infrastructure damage	Ports, roads, railways, buildings in coastal zones at risk
Displacement	Estimated 200+ million coastal climate refugees by 2100 under high-emission scenarios

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Glacier Retreat

Global glacier retreat is one of the most visible indicators of climate change:

Glacier/Region	Evidence of Change
Alps	Lost ~60% of ice volume since 1850; 2022 saw record-breaking melt in Switzerland (~6% of remaining ice lost in one year)
Himalayas	Losing ice at ~0.5 m/year; threatens water supply for ~2 billion people (Indus, Ganges, Brahmaputra)
Kilimanjaro	Lost ~85% of ice cover since 1912; may be ice-free by 2040
Arctic sea ice	September minimum extent declined by ~40% since 1979; some projections suggest ice-free Arctic summers by 2050
Greenland Ice Sheet	Losing ~280 Gt/year; surface melt area expanding; moulin formation accelerating basal sliding

Exam Tip: Glacier retreat provides evidence of warming AND has consequences (sea level rise, water supply disruption, natural hazard risk from glacial lake outburst floods). In an exam, you can use glaciers as evidence AND as an impact — a double function that maximises marks.

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Ecosystem Changes