Evidence for climate change and its limits

Direct records

• Temperature records: thermometer readings from weather stations and ships, kept since the mid-1800s, show global average temperature rising, especially since about 1980. Early records are fewer and less precise.
• Atmospheric CO2: continuous measurements began at Mauna Loa, Hawaii, in 1958, when the first reading was about 313 ppm. In May 2026 the monthly peak was about 432 ppm. The rising, zigzag line (the Keeling curve) shows a yearly cycle from seasonal photosynthesis plus a steady upward trend.
• Ice cores: bubbles of air trapped in Antarctic ice give CO2 levels and, from isotopes, temperatures going back hundreds of thousands of years.

Indirect evidence

• Pollen in peat bogs: peat is acidic, waterlogged and low in oxygen, so pollen does not decay. Pollen grains are distinctive for each plant type, and peat builds up in layers that can be dated. The plants present in each layer show what the climate was like then.
• Dendrochronology: trees add one growth ring per year. Wide rings form in warm, wet years, narrow rings in cold or dry years. Counting rings dates them, and their widths show past conditions. Overlapping patterns from old timber extend the record back thousands of years.

Models, extrapolation and controversy

Extrapolation extends a trend beyond the measured data to predict the future. Climate models are limited because:

• future human emissions are unknown
• feedbacks (melting ice reflecting less light, release of methane) are hard to predict
• past data may be incomplete, and the climate system is complex

So models give a range of predictions.

Conclusions about climate change can be controversial. How people judge a conclusion can depend on who reached it: funding, political or commercial interests can introduce bias. Peer review, independent replication and agreement among many scientists increase confidence.

Worked example

Atmospheric CO2 was 313 ppm in 1958 and 432 ppm in 2026. Calculate the percentage increase.

1. Increase = 432 - 313 = 119 ppm
2. Percentage = 119 / 313 x 100 = 38.0%

Answer: About 38%.

Check yourself

1. CO2 rose from 280 ppm to 420 ppm. Calculate the percentage increase.
   Show answer

   50 %. 140 / 280 x 100 = 50%.

2. CO2 is 432 ppm and rising by 2.5 ppm per year. Assuming the trend continues, what will it be in 10 years, in ppm?
   Show answer

   457 ppm. 432 + 25 = 457 ppm.

3. Why is pollen preserved in peat bogs?
   • Peat is very warm and dry
   • Pollen is made of plastic
   • Decomposers feed only on pollen
   • The acidic, waterlogged, low-oxygen conditions slow decay
   Show answer

   The acidic, waterlogged, low-oxygen conditions slow decay. Few decomposers can work in acidic, anaerobic peat.

4. What does a wide tree ring usually indicate?
   • A warm, wet growing season
   • A cold, dry year
   • That the tree was diseased
   • That the tree was cut down that year
   Show answer

   A warm, wet growing season. Good growing conditions give more growth in that year.

5. Why does the Mauna Loa CO2 record show a small rise and fall every year?
   • Measurement errors each winter
   • Volcanic eruptions every summer
   • Seasonal changes in photosynthesis by plants in the northern hemisphere
   • Changes in ocean depth
   Show answer

   Seasonal changes in photosynthesis by plants in the northern hemisphere. In summer more photosynthesis lowers CO2; in winter it rises.

6. Why do climate models give a range of predictions rather than one exact value?
   • Future emissions and feedbacks are uncertain
   • Scientists do not record temperatures
   • CO2 has no effect on climate
   • Computers cannot do calculations
   Show answer

   Future emissions and feedbacks are uncertain. Uncertainty in inputs gives a spread of outcomes.

7. A report doubting climate change was funded by an oil company. What should a scientist do?
   • Accept it because it is a report
   • Reject it without reading because of the funder
   • Check whether the work was peer reviewed and agrees with independent studies
   • Assume all climate research is biased
   Show answer

   Check whether the work was peer reviewed and agrees with independent studies. Funding may introduce bias, so look for peer review and replication.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• UC San Diego Today: Annual carbon dioxide peak reaches 432 parts per million (June 2026)

Last updated 7 November 2026. Spotted a mistake? Tell us.