Direct and indirect evidence
Big idea: Climate change evidence comes from direct measurements (recent, precise) and proxy data (historical, indirect). Together, they show Earth is warming at an unprecedented rate.
Direct evidence
Modern instruments have measured climate variables directly for about 150 years:
- Temperature records: Weather stations show ~1.1°C warming since pre-industrial times
- Sea level measurements: Tide gauges and satellites show ~20 cm rise since 1900
- Atmospheric CO₂: Mauna Loa observatory shows rise from 280 ppm (pre-industrial) to >420 ppm today
- Satellite data: Ice extent, sea surface temperature, vegetation changes
Proxy data (indirect evidence)
Proxy data allows us to reconstruct climate from before instruments existed:
- Ice cores: Air bubbles trap ancient atmosphere — shows CO₂ and temperature correlation over 800,000 years
- Tree rings: Width indicates growing conditions (temperature, rainfall)
- Coral bands: Record ocean temperature and chemistry
- Pollen in sediments: Shows past vegetation (indicates climate)
- Ocean sediments: Fossil shells indicate past ocean temperatures
Exam tip: Be ready to evaluate the reliability of evidence. Direct measurements are more precise; proxy data extends further back but requires interpretation.
Key trends and patterns
Big idea: The data shows clear trends: rising temperatures, rising CO₂, rising sea levels, and declining ice — all accelerating in recent decades.
What the data shows
- Global temperature: ~1.1°C increase since 1850–1900; warmest years all in recent decade
- CO₂ concentration: From ~280 ppm (pre-industrial) to >420 ppm — highest in at least 800,000 years
- Sea level: Rising ~3.7 mm/year (accelerating from ~1.7 mm/year in 20th century)
- Arctic sea ice: Declining ~13% per decade since 1979
- Glaciers: Retreating worldwide; some have lost 50%+ of their mass
Interpreting graphs and data
Exams often show you graphs of climate data. Key skills:
- Identify trends: Overall direction (increasing/decreasing)
- Note the rate of change: Is it accelerating or constant?
- Spot correlations: Do CO₂ and temperature rise together?
- Note anomalies: Short-term variations vs long-term trends
- Check the scale: Time period, units, baseline
Exam tip: When describing trends, use specific data from the graph, describe the overall trend, and note any changes in rate.