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.
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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.
See how examiners mark answers
Access past paper questions with model answers. Learn exactly what earns marks and what doesn't.
IB-style question — evidence that the climate is warming [3]
Scientists studying the planet's recent climate point to several long-term records. Outline three pieces of evidence that show the Earth's climate is warming. [3]
How to answer it, step by step
- Pick records that change over time
• Rising global average temperatures over the last century
• Shrinking glaciers and reduced Arctic sea-ice extent - Don't repeat the same idea twice
• Rising sea levels (melt + thermal expansion)
• Or: ice cores show CO₂ and temperature rising together
Final answer
Give three separate records — 'it's hotter' and 'temperatures are up' count as one point, not two.
IB-style question — reading a temperature graph [2]
A graph shows global mean temperature rising from 13.7°C in 1920 to 14.9°C in 2020. Calculate the total temperature change, and state what it suggests about the climate. [2]
How to answer it, step by step
- Later value − earlier value
• Change = 14.9 − 13.7
• = +1.2°C - Link the number to a trend
• A positive value means the climate is warming
• The rise over 100 years shows a long-term warming trend
Final answer
Always keep the sign and the °C unit — a bare '1.2' with no direction or unit can lose the mark.