Climate Change

Evidence for past and present climate change

Natural archives (proxy evidence for past climate)

• Ice cores (Antarctica/Greenland): trapped air bubbles record ancient atmospheric CO₂ and methane levels; stable isotopes in ice reveal past temperatures. Vostok core shows 800,000 years of data — CO₂ and temperature rise and fall together.
• Tree rings (dendrochronology): wide rings = warm, wet growing season; narrow = cold/dry. Provides annual records over thousands of years.
• Pollen records (pollen analysis): fossilised pollen in lake sediments reveals past vegetation and, by inference, climate over millennia.
• Historical records: wine harvest dates (medieval Europe), Viking settlement in Greenland (~950–1300 CE — the Medieval Warm Period), diaries and paintings showing frozen Thames.
• Sea-floor sediments: foraminifera shells record past ocean temperatures and chemistry.

Observed instrumental evidence (since 1850)

• Global mean surface temperature has risen ~1.2 °C above pre-industrial levels (IPCC AR6, 2021).
• Arctic sea ice extent has declined ~13% per decade since satellite records began (1979).
• Global mean sea level has risen ~20 cm since 1900; accelerating to ~3.7 mm/year currently.
• Ocean heat content and acidification increasing as CO₂ dissolves.
• Increased frequency of extreme weather events (heatwaves, heavy precipitation).

Natural causes of climate change

1. Milankovitch cycles: long-term changes in Earth's orbit (eccentricity, ~100,000-year cycle), axial tilt (obliquity, ~41,000-year cycle), and wobble (precession, ~26,000-year cycle) alter the amount and distribution of solar radiation received — key drivers of glacial/interglacial cycles.
2. Solar output variation: sunspot cycles (~11-year) and longer-term solar luminosity changes affect energy received. The Maunder Minimum (1645–1715, few sunspots) correlates with the Little Ice Age.
3. Volcanic eruptions: major eruptions (e.g. Mt Pinatubo 1991) inject sulphur dioxide into the stratosphere → sulphate aerosols → reflect sunlight → temporary cooling (0.5 °C for 1–2 years). Cannot explain sustained modern warming trend.

Human causes (anthropogenic drivers)

Enhanced greenhouse effect

Natural greenhouse gases (CO₂, CH₄, N₂O, H₂O vapour) trap longwave radiation from Earth's surface, keeping the planet ~33 °C warmer than it would otherwise be. Human activities are enhancing this:

Human activities increasing GHG emissions

• Fossil fuel combustion (energy, transport, industry): largest single source of CO₂.
• Deforestation: removes carbon sinks (forests absorb CO₂); burning biomass releases stored carbon.
• Agriculture: cattle produce CH₄ (enteric fermentation); wet rice paddies emit CH₄; artificial fertilisers release N₂O.
• Industrial processes: cement production (1 tonne of cement releases ~0.9 t CO₂); steel smelting.
• Urbanisation and transport: increased vehicle use; urban heat islands (albedo reduction).

Consequences of contemporary climate change

Physical consequences

• Rising sea levels (thermal expansion + ice melt): threatens low-lying nations (Bangladesh, Tuvalu, Maldives). By 2100, 1–2 m rise under high-emission scenarios.
• Melting ice: Greenland and West Antarctic Ice Sheets; Arctic sea ice loss accelerates warming (albedo feedback — dark ocean absorbs more heat than reflective ice).
• Extreme weather: more intense heatwaves (European 2003: ~70,000 deaths); more intense rainfall events; longer/more severe droughts in already dry regions.
• Ocean acidification: CO₂ dissolves in seawater → carbonic acid → threatens coral reefs and shell-forming organisms (pteropods, oysters).
• Ecosystem shifts: species ranges moving poleward/upslope; coral bleaching (Great Barrier Reef); earlier spring events (phenological mismatch).

Human consequences

• Food security: some regions benefit (Canada, Siberia — longer growing seasons); many subtropical regions face drought and crop failure (sub-Saharan Africa, India).
• Water stress: glacial retreat threatens freshwater supply for billions (Himalayan rivers: Ganges, Brahmaputra).
• Climate refugees: coastal flooding, desertification and extreme heat are expected to drive 200 million+ internal migrants by 2050 (World Bank).
• Economic costs: Swiss Re estimates unmitigated climate change could reduce global GDP by 10–23% by 2100.
• Health: increased range of vector-borne diseases (malaria, dengue) as mosquito habitats expand.

Global atmospheric circulation and its role (link to T1.1)

Climate change is disrupting the Hadley, Ferrel and Polar cells that drive global weather patterns:

• Weakening of the polar vortex and jet stream → more frequent blocking events → prolonged heatwaves and cold snaps.
• Expansion of the Hadley Cell poleward → subtropical deserts expanding.
• Changes in ITCZ position affecting monsoon reliability.

Edexcel B exam tip

For climate change questions, evidence, causes and consequences form three distinct command areas. A common L3 strategy: name the evidence type → explain the mechanism → link to modern observation → evaluate relative importance (e.g. "natural causes explain past cycles but cannot account for the acceleration since 1950").