What you need to know
- 5.16Evidence for climate change and its causes: carbon dioxide records, temperature records, pollen in peat bogA waterlogged habitat where dead plants decompose very slowly, so their carbon is stored as peat. and dendrochronologyDating and reconstructing past climate from the pattern of annual growth rings in trees..
Correlation versus causation is developed in 5C 6. Ice cores appear here as a CO₂-record method (enrichment detail of “records of carbon dioxide levels”), not as an extra named proxy you must invent.
By the end, you should be able to…
- Outline four specified types of evidence.
- Explain what each actually measures.
- State a limitation of each method.
Build your understanding
Direct air and thermometers
Records of carbon dioxide include the Mauna Loa / Keeling series from 1958: a measured rise in atmospheric CO₂ with a seasonal wiggle as northern photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen. draws down CO₂ in summer. Ice cores extend the gas record over much longer times by trapping ancient air bubbles. Temperature records include thermometer networks such as NASA GISTEMP. Direct records are strong for the industrial period; they do not by themselves reach the last ice age.
A rise in CO₂ together with a rise in global mean temperature is evidence that both have changed. Linking them still needs the greenhouse mechanism from 5C 4 and a careful look at timing — that evaluation is the next lesson.
Pollen in peat bogs
Peat can preserve pollen in anaerobic, acidic conditions. Identifying pollen types shows which plants grew nearby. Those plants indicate past climate: more sphagnum and cool-wet speciesA group of organisms that can interbreed to produce fertile offspring. in one layer, more warmth-demanding trees in another. Depth can be dated, so you get a local vegetation — and therefore climate — history.
Limitations: pollen blows in from a region, not a single quadratA square frame of known area used to sample organisms.; identification can be to genusThe classification rank above species. It is the first word of a binomial name. not species; peat accumulation is not a perfect yearly clock; humans also change vegetation. Pollen is a proxy for plant communityAll the populations of different species living in one habitat at the same time., not a thermometer.
Dendrochronology
dendrochronologyDating and reconstructing past climate from the pattern of annual growth rings in trees. uses tree rings. In seasonal climates many trees add a ring each year. Ring width and density respond to growing conditions, including temperature and rainfall, so matched series can reconstruct past climate and date timber. NOAA’s International Tree-Ring Data Bank is the professional archive of such measurements.
Limitations: not every species or site is sensitive in the same way; a wide ring might mean warmth or moisture; young trees, pests and competition also change growth; tropical trees may lack clear annual rings. Rings are a growth record, not a CO₂ meter.
NOT: tree rings do not measure carbon dioxide. Pollen is not a direct temperature instrument.
- The Mauna Loa series shows a rise and a seasonal dip
- Ice cores extend the gas record because they trap old air
- Pollen can blow in from a region, and peat is not a perfect yearly clock
- A wide ring can mean warmth or moisture, or less competition
- A rise in both CO₂ and temperature still needs the mechanism in the next lesson
Two traps this diagram blocks
- Do not say tree rings are a carbon dioxide record.
- Do not treat one pollen grain as the temperature of one year.
From the diagram alone
Question
State what pollen in peat and tree rings each provide as evidence of past climate, and one limitation of rings. [4]
Show a model answer
Pollen shows which plants grew nearby, and those plants indicate the climate. Tree rings record growth, and width or density can be linked to temperature or rainfall. A limitation is that a wide ring can be caused by moisture, fewer pests or less competition, not only by temperature.
| Evidence | What is measured | A limitation |
|---|---|---|
| CO₂ records | Atmospheric concentration (flasks, analysers, ice-core air) | Direct series are short; ice cores need careful dating |
| Temperature records | Instrumental temperature, then averages | Coverage and urban sites must be handled; one station is not the globe |
| Pollen in peat | Plant types through a dated bog profile | Pollen source area and human land use confuse a pure climate signal |
| Dendrochronology | Ring width/density as a growth response | Multiple limits on growth; not all trees ring annually |
Make your reasoning visible
Say what the proxy stands in for
“Pollen shows it was warmer” needs “because those taxa indicate warmer conditions”.
Four named methods
If a question lists the specification’s four, do not substitute only ice cores and skip peat.
Key explanations
- Finding that CO₂ and temperature have both risen is evidence that both changed. The causal argument is a further step.
Avoid this misconception
The claim: Tree rings measure carbon dioxide.
The correction: Rings record growth. CO₂ is measured in air or in ice-core bubbles. Growth can even be affected by several factors at once.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
List the four types of evidence named in 5.16.
Show suggested answer
Records of carbon dioxide levels, temperature records, pollen in peat bogs, and dendrochronology.
Practice question
Retrieval and explanation · suggested answer
How can peat pollen be used as evidence of past climate?
Show suggested answer
Pollen is preserved in peat; identified plant types indicate the climate those species needed, and layers can be dated.
Practice question
Retrieval and explanation · suggested answer
Give one limitation of dendrochronology as a climate record.
Show suggested answer
Ring width responds to more than one factor (temperature, rainfall, pests, competition), or some trees lack clear annual rings.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A student treats a single warm decade at one weather station as proof that peat pollen is unnecessary. What does each record contribute that the other cannot?
Show a hint
Timescale and geography.
Explore the reasoning
Instrumental records give precise recent temperatures but are short. Peat pollen can reach centuries to millennia and a landscape’s vegetation, but it is a proxy. You need both kinds of evidence for different timescales; one warm decade at one station is not a Holocene reconstruction.
Recall this lesson
Lesson recall questions
Question 1 of 5: What is dendrochronology, and what does a wide annual ring usually indicate?
Evidence for climate change
Practice question
Retrieval and explanation · suggested answer
What is dendrochronology, and what does a wide annual ring usually indicate?
Lesson: 5C 5: Looking at the Evidence
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The professional tree-ring bank
5C · Looking at the EvidenceNOAA NCEI hosts the International Tree-Ring Data Bank: raw ring width, density and site chronologies from thousands of sites. That is what “dendrochronology” looks like as a global…Specification 5.16Read & use the scienceClose article
Why this matters
NOAA NCEI hosts the International Tree-Ring Data Bank: raw ring width, density and site chronologies from thousands of sites. That is what “dendrochronology” looks like as a global evidence base. School diagrams of five rings are a teaching sketch, not that archive. Cross-dating matches patterns among trees, and calibration compares growth with measured climate. Ring width can also change with disease, competition or soil water, so a single ring is not a thermometer.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.16 understand the different types of evidence for climate change and its causes, including records of carbon dioxide levels, temperature records, pollen in peat bogs and dendrochronology, recognising correlations and causal relationships
Use the science
Why would five rings from one playground tree fail as a 5.16 climate record?
Explore a possible response
One young tree, one site, no cross-dating and no known sensitivity cannot reconstruct regional climate. The method needs long, matched series.
Explore the source
Air trapped in ancient ice
5C · Looking at the EvidenceNOAA explains that bubbles in ice cores preserve samples of past atmosphere, while properties of the surrounding ice record other environmental information. This gives a different kind of…Specification 5.16Read & use the scienceClose article
Why this matters
NOAA explains that bubbles in ice cores preserve samples of past atmosphere, while properties of the surrounding ice record other environmental information. This gives a different kind of evidence from tree rings or pollen in a peat bog. A paleoclimatologist compares archives because each proxy measures something different and covers different times and places. Past carbon dioxide and temperature can be correlated, but correlation alone does not identify every causal pathway. Physical knowledge of greenhouse gases and independent observations strengthen the interpretation. Ice is therefore an archive to analyse, not a perfect thermometer or a single decisive graph.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.16 understand the different types of evidence for climate change and its causes, including records of carbon dioxide levels, temperature records, pollen in peat bogs and dendrochronology, recognising correlations and causal relationships
Use the science
Why is comparing ice-core gas with temperature proxies stronger than reading only one ice-core variable?
Explore a possible response
They are partly independent measurements. Agreement can support a pattern, while differences reveal uncertainty or other influences.
Explore the source
- How can ice teach us about climate?NOAA NCEI