Climate change: causes and evidenceEdexcel A-Level Biology A: Revision notes
Section 1
Records of carbon dioxide and temperature
Evidence for climate change comes from direct records and from indirect (proxy) evidence.
Ice cores are drilled from polar ice. Each year's snowfall forms a layer, so depth and layer counting give the age. Air bubbles trapped in the ice are samples of the past atmosphere and are analysed to find the carbon dioxide concentration. Ice cores show it stayed between about 180 and 280 ppm for 800 000 years.
Direct measurements of atmospheric carbon dioxide began in 1958 at Mauna Loa, Hawaii, and show a rise from 316 ppm in 1959 to over 420 ppm today. Temperature records from thermometers cover about 170 years and show a rise of about 1.1 °C since 1850. Earlier temperatures are inferred from proxies such as the ratio of oxygen isotopes in ice cores.
Direct measurements are the most reliable, but they only cover recent decades, so older evidence is needed for long-term trends.
Section 2
Pollen in peat bogs
Peat forms in waterlogged, acidic, low-oxygen bogs where decomposers are inhibited, so plant remains and pollen are preserved in layers. The outer wall of pollen is very resistant and each species has a distinctive shape.
A core is taken, and the pollen grains in each layer are identified and counted under a microscope. Layers are dated by radiocarbon dating or by depth. The proportions of species show how the vegetation changed, for example from cold-tolerant birch and pine to warmth-loving oak and hazel, so the climate is inferred.
Limitations: vegetation depends on rainfall, soil and human activity as well as temperature; species produce different amounts of pollen; and pollen may be blown in from afar.
Pollen does not measure temperature directly. Say that the climate is inferred from the species that were present.
Section 3
Dendrochronology
Dendrochronology is the study of tree rings. In temperate climates a tree adds one ring each year as the cambium lays down new xylem. In a favourable year (warm, with enough water) the ring is wide; in a poor year it is narrow.
Patterns of wide and narrow rings from living trees are matched with those from older timbers, such as building beams and bog oaks, in cross-dating. This builds a continuous record covering thousands of years and allows wood to be dated exactly. Ring widths are compared with known climate records to infer past conditions.
Limitations: ring width is affected by rainfall, light, nutrients, pests and competition as well as temperature.
Section 4
Correlation and causation
A correlation is a relationship between two variables, for example carbon dioxide concentration and global temperature. A causal relationship means that a change in one variable directly produces the change in the other.
A correlation alone does not prove causation, because a third factor may affect both, or the link may be chance. Evidence for causation comes from a plausible mechanism (here, the greenhouse effect), repeated findings from independent lines of evidence, and a link in the right time order.
In an exam, state the correlation, state that this does not prove cause, and then give the extra evidence that makes the causal link convincing.
Writing that a correlation proves a cause. It only shows association.
Section 5
The greenhouse effect
Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface. The surface warms and re-emits energy as longer-wavelength infrared radiation.
Greenhouse gases, mainly water vapour, carbon dioxide and methane, absorb infrared radiation and re-radiate some of it back to the surface. Less heat escapes to space, so the lower atmosphere is warmer than it would otherwise be.
The natural greenhouse effect keeps the Earth habitable. The enhanced greenhouse effect occurs when human activity increases the concentrations of these gases, so more infrared radiation is trapped and the Earth warms.
Section 6
Anthropogenic causes: carbon dioxide and methane
Anthropogenic means caused by humans. The main causes are:
- Carbon dioxide: combustion of fossil fuels (coal, oil and gas) releases carbon stored for millions of years; deforestation reduces photosynthesis so less is removed, and burning or decay of felled trees releases it.
- Methane: produced by anaerobic microbes in the guts of cattle and sheep, in flooded rice paddies, in landfill sites, and as leaks from natural gas extraction. Warming can release more from thawing permafrost.
Methane is present at a far lower concentration than carbon dioxide and stays in the atmosphere for a shorter time, but each molecule absorbs infrared radiation more strongly. Atmospheric carbon dioxide has risen from about 280 ppm before 1850 to over 420 ppm.
Must know
- Evidence: ice cores, direct carbon dioxide and temperature records, pollen in peat bogs and tree rings
- Pollen and tree rings are indirect (proxy) evidence with limitations
- Correlation does not prove causation; a mechanism and independent evidence do
- Greenhouse gases absorb infrared radiation and re-radiate some back to Earth
- Carbon dioxide and methane have increased because of human activity
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Climate change: causes and evidence
- 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.Suggest two reasons why pollen from a single peat core may not give a reliable record of past temperature.2 marks
- 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 newspaper claims that this correlation proves that higher summer temperatures cause wider rings. Explain why the correlation alone does not prove this.2 marks
- 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.Describe how ice cores can be used to find the carbon dioxide concentration of the atmosphere in the past.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).