Climate change: causes and evidenceEdexcel A-Level Biology A: Subtopic test

1. 1
   A palaeoecologist extracts a vertical core from a peat bog in northern England. Peat is waterlogged, acidic and contains very little oxygen. She identifies the pollen grains in each layer of the core under a microscope and dates the layers using radiocarbon dating. Moving from the deepest layers to the surface, the proportion of birch and pine pollen decreases while the proportion of oak and hazel pollen increases. Birch and pine tolerate colder conditions than oak and hazel.

   (a)

   Why are pollen grains so well preserved in the layers of a peat bog?

   [1 mark]

   • AThe low pH and high oxygen concentration harden the pollen wall.
   • BWaterlogged, acidic, low-oxygen conditions inhibit the decomposers that would otherwise break the pollen down.
   • CThe low temperature of peat permanently kills all saprotrophic organisms.
   • DPeat contains enzymes that digest the cells that surround each pollen grain.

   (b)

   Which conclusion about the past climate is best supported by the pollen data?

   [1 mark]

   • AThe atmospheric carbon dioxide concentration rose steadily while the peat formed.
   • BRainfall in northern England increased throughout the period.
   • CHumans planted oak and hazel in place of birch and pine.
   • DThe climate became warmer as the peat accumulated.

   (c)

   Suggest two reasons why pollen from a single peat core may not give a reliable record of past temperature.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A forestry researcher takes thin cores from the trunks of living oak trees and from oak beams in medieval buildings. Each core shows a series of annual rings, and the width of each ring depends on how much new wood the cambium laid down in that year. Over a 150-year period, she finds that the ring width in the living oaks has a positive correlation with mean summer temperature (r = +0.81).

   (a)

   What can be inferred from a very narrow ring in a core?

   [1 mark]

   • AGrowing conditions in that year were unfavourable, so little new wood was added.
   • BThe tree was growing at its fastest rate in that year.
   • CThe tree grew only during the winter of that year.
   • DThe tree was infected by a fungus for its entire life.

   (b)

   How can the researcher find the felling date of a medieval beam of unknown age?

   [1 mark]

   • ACount the rings and add that number to the current year.
   • BMeasure the thickness of the bark on the beam.
   • CMatch its pattern of wide and narrow rings with overlapping ring sequences from living trees and older timbers.
   • DMeasure the amount of lignin in the outer ring.

   (c)

   A newspaper claims that this correlation proves that higher summer temperatures cause wider rings. Explain why the correlation alone does not prove this.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Scientists drill ice cores in Antarctica. Each layer of ice formed from snow that fell in a particular year and trapped tiny bubbles of air. Analysis of the bubbles shows that over the last 800 000 years the carbon dioxide concentration varied between about 180 and 280 parts per million (ppm). Direct measurements of air at the Mauna Loa Observatory in Hawaii show that the concentration rose from 316 ppm in 1959 to 421 ppm in 2022.

   (a)

   Describe how ice cores can be used to find the carbon dioxide concentration of the atmosphere in the past.

   [3 marks]

   (b)

   Calculate the percentage increase in carbon dioxide concentration between 1959 and 2022. Show your working. Then use the ice core data to explain why the recent rise is unlikely to be part of the natural pattern.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Since 1850 the mean global surface temperature has risen by about 1.1 °C. Over the same period the atmospheric carbon dioxide concentration has risen from about 280 ppm to 421 ppm, and the methane concentration from about 0.72 ppm to 1.9 ppm. A commentator claims that the warming is caused by natural variation and that human activity has no significant effect.

   (a)

   Explain how greenhouse gases cause the greenhouse effect and how human activities have increased the concentrations of carbon dioxide and methane.

   [6 marks]

   (b)

   Evaluate the commentator's claim, using the data and your knowledge of the evidence for climate change.

   [6 marks]

   Total for question 4: 12 marks

End of questions