Topic 5 Edexcel A2 Biology - evidence for Global Warming (PIDT)

Revision cards for the four main types of evidence for Global Warming and a look at their limitations/criticisms

(Pollen Analysis, Ice Cores, Dendrochronology, Temperature Records)

For Unit 4 (Edexcel A2 Biolohy) - Topic 5

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Dendrochronology (tree ring studies)

In a tree trunk a new layer of xylem vessels grow annually (each year) and are visible as a new 'ring' in the bark of the tree trunk. This enables us to identify the age of the tree and, more importantly to global warming, the year of each tree ring.

The diameter (length across) a tree ring gives an indication of the climatic conditions of that year. This is because the climatic conditions of the environment in which the tree grew determined the success of the growth of xylem vessels.

  • Thicker/Wider - indicates more successful growth of xylem vessels as more are made, thus suggesting there were more favourable conditions for growth (for example, sunshine/light energy and moisture which enable photosynthesis to take place for the tree's growth and survival)
  • Thinner/Narrower - indicates less successful growth of xylem vessels due to unfavourable conditions for growth (for example, overcast conditions limiting light energy and dry conditions hindering photosynthesis
  • Lighter - wide lumen of xylem vessles suggesting more water travelling through hollow tubes (wetter climatic conditions enabling growth) and therefore less dense xylem vessels
  • Darker - narrow lumen of xylem vessels suggesting less water (drier climatic conditions) hindering growth and therefore more dense xylem vessels as more packed in 
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Pollen Data Analysis

Peat bogs - anaerobic conditions (absence of oxygen) that enable plants/insects/pollen to be preserved and not to decay. Layers in peat bogs enable the year to be easily identified with deeper layers signalling the years being older.


  • Pollen/insect/plant remains can be taken from samples, and their species identified
  • The ecological conditions suiting and supporting these species' survival can therefore be used to determine the ecological conditions of that year (identified through the layers of the peat bog going back through time as peat bog deepends)
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Ice Cores

  • Ice Cores extracted and used as samples
  • Air bubbles in Ice Cores can be used to determine the temperature and Carbon Dioxide Concentration of past years

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Temperature Records

Long data sets for temperature records over time allow for temperature and climate conditions over time to be determined and analysed.

The validity of these long data sets can be tested by comparing these datasets to other long data sets to determine whether the two data sets demonstrate a correlation (not necessarily a causation), for example comparing long datasets for fossil fuel consumption and temperature records to see if they have a correlating relationship that may support a theory for their causation.


Considering that equipment used to measure temperature overtime has developed, earlier temperature records taken in long datasets are likely to have used less developed and accurate equipment to take these measurements. This therefore limits the validity of these earlier measurements and consequently questions the realibity of any conclusions made from analysis of these long data sets for temperature records over time.

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