Climate change: evidence, causes, effects and management

The Earth's climate has fluctuated naturally over geological time — between ice ages and warmer interglacials. Today's climate change is different: it is unusually rapid (over decades, not millennia) and tightly coupled to human emissions of greenhouse gases. The IPCC's strongest finding is that humans are unequivocally the dominant cause of the warming observed since 1850.

Evidence for climate change

Examiners look for at least three types of evidence. Use the mnemonic TIPS: Temperature, Ice, Polar/sea, Species.

1. Temperature records. Direct thermometer measurements show ~1.2 °C of warming above the 1850–1900 baseline by 2024. The 10 warmest years globally on record have all occurred since 2014.
2. Ice loss. Arctic sea-ice extent in September has fallen ~13 % per decade since 1979. The Greenland Ice Sheet has lost ~280 Gt of ice per year. Glaciers worldwide are retreating; tropical ice (Kilimanjaro) is nearly gone.
3. Sea-level rise. ~21 cm rise since 1900, accelerating to ~4 mm/year now. Two contributors: melting land ice + thermal expansion of warming ocean water.
4. Ocean acidification. ~30 % more acidic since 1750 — from absorbing CO₂.
5. Species shifts. Birds, butterflies and trees moving polewards/upslope. Earlier UK spring (frogspawn, blossom).
6. Proxy data for the deeper past — ice cores (air bubbles record CO₂), tree rings, pollen layers, coral growth bands.

Natural causes of past climate change

• Milankovitch cycles — slow changes in Earth's orbit, axial tilt and precession (eccentricity ~100 000 yrs; obliquity ~41 000 yrs; precession ~26 000 yrs). Drove the ice ages.
• Volcanic eruptions — large eruptions (Tambora 1815, Pinatubo 1991) inject sulfate aerosols into the stratosphere, cooling the surface for 1–3 years.
• Solar output (sunspot cycles). ~11-year cycle and longer-term variation. Recent solar output has been flat to slightly declining — so it cannot explain modern warming.

Human causes (anthropogenic)

The chemistry is well understood. Greenhouse gases (CO₂, CH₄, N₂O, water vapour) are transparent to short-wavelength solar radiation but absorb the long-wavelength infrared radiation re-emitted by Earth's surface, trapping heat in the atmosphere — the enhanced greenhouse effect.

Sources:

• CO₂ — burning fossil fuels (coal, oil, gas) for electricity, transport, industry; deforestation. CO₂ has risen from ~280 ppm (pre-industrial) to ~425 ppm (2024).
• Methane (CH₄) — livestock (cattle), rice paddies, leaks from natural-gas extraction, landfills. ~30× more powerful per molecule than CO₂ over 100 years.
• Nitrous oxide (N₂O) — fertilisers, vehicle exhausts. ~270× more powerful.
• F-gases — refrigerants, industrial uses. Tiny amounts but very powerful.

Effects of climate change

Environmental: melting ice, sea-level rise (low-lying nations like the Maldives and Tuvalu existentially threatened), ocean acidification (coral bleaching — Great Barrier Reef bleached 7 times since 1998), shifts in biome boundaries, increased wildfire risk (California, Australia 2019–20).

Economic: insurance costs rising; agricultural yields shifting (UK could grow more wine; tropical agriculture suffers heat stress); climate-related migration; cost of building flood/heat infrastructure.

Social: 'climate refugees' (Pacific islands, Sahel); inequality (LICs hit hardest, lowest historical emissions); health effects (heatwaves, malaria expanding range).

Management — mitigation and adaptation

Mitigation (reducing emissions):

• Switching to renewables — solar, wind, hydro, geothermal, tidal. Renewables now ~30 % of global electricity (2024).
• Nuclear — low-carbon baseload (debated for cost and waste).
• Carbon capture and storage (CCS) — early-stage technology; expensive.
• International agreements — Paris Agreement (2015) commits 195 countries to limit warming to 1.5–2 °C.
• Carbon taxes / emissions trading (UK ETS).
• Reforestation / land-use change.
• Energy efficiency — better insulation, electric vehicles.

Adaptation (living with the change that's already locked in):

• Coastal defences — sea walls (Thames Barrier), managed retreat.
• Drought-tolerant crops — GM and traditional breeding.
• Water management — desalination plants (Israel, UAE), rainwater harvesting.
• Heat-resilient cities — green roofs, urban tree planting.

Examiner tips

Always distinguish natural and human causes when asked. Always say enhanced greenhouse effect when discussing modern climate change — the natural greenhouse effect is what makes Earth habitable. Use specific data: ppm values, °C, dates, named places. Most candidates lose marks for vague answers.