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Climate Change — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

35 min read 40 parts Lesson 20 of 34 Unit quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Lesson summary

Climate Change Edexcel A Level Biology SNAB notes: Explain the greenhouse effect and how human activities enhance it.Q10 = rate of reaction at (T + 10) °C ÷ rate at T °C; no units; typically 2-3 for enzyme reactions between 0 and 40 °C.Percentage change = (new value − original va…

  • Evidence for temperature change
  • Effects on organisms
  • Required practical: seedling growth
  • Temperature and collisions
  • Photorespiration
  • Catalase temperature method

Section 1 of 40Climate Change

The natural greenhouse effect keeps Earth habitable. The enhanced greenhouse effect results from increased greenhouse gas concentrations from human activity since the Industrial Revolution.
Main anthropogenic CO2 sources: combustion of fossil fuels (coal, oil, gas), cement making and deforestation. Main CH4 sources: ruminant livestock, rice paddies, landfill decomposition, natural gas leaks; thawing permafrost releases more CH4 (positive feedback).
Evidence for temperature change
Evidence for temperature change direct temperature records, dendrochronology, pollen in peat bogs (anaerobic, acidic conditions preserve pollen; layers give a timeline) and ice cores (bubbles and isotopes).
Evidence for CO2 change: ice core air bubbles and direct measurements (e.g. Mauna Loa) show CO2 correlates closely with temperature over hundreds of thousands of years.
A correlation between CO2 and temperature alone does not prove causation, but the physical mechanism (absorption of infrared) and controlled experiments support a causal link. Evaluate data by considering sample size, reliability, time span and possible bias.
Scientists use computer models to predict future climate; outputs depend on assumptions about emissions, feedbacks (e.g. melting ice lowers reflectivity/albedo) and are therefore shown as a range. Extrapolation beyond data is uncertain.
Effects on organisms
Effects on organisms enzyme-catalysed reactions speed up with temperature (Q10 ≈ 2) until the optimum, after which the active site changes shape and enzymes denature. Ectotherms' development rate and metabolic rate depend on environmental temperature.
Species may shift distribution to higher latitudes/altitudes; species unable to migrate or adapt may become extinct. Changes in timing of flowering, bud burst, egg laying and migration can cause mismatch between predators and prey (e.g. caterpillar peak and bird hatching).
Changing rainfall patterns affect water availability; rising sea levels flood habitats; ocean warming and acidification (CO2 dissolving) damage coral reefs.
Biofuels are theoretically carbon neutral because CO2 released on combustion was recently absorbed by photosynthesis, but energy for fertiliser, harvesting, processing and transport, and land clearance, add net emissions; they also compete with food crops for land.
Reforestation and managed forests act as carbon sinks: growing trees fix CO2 as biomass; harvested timber used for building locks carbon away, replaced by new growth.
Decisions on climate change involve scientists, governments, businesses and individuals; the peer-reviewed scientific consensus (e.g. IPCC) informs policies, while conflicting interests and economic costs influence action.
Required practical
Required practical seedling growth
Grow seedlings of one variety in identical compost, measure initial heights, then place trays in incubators at different temperatures for the same time (e.g. 7 days). Keep light, water and soil pH constant. Mean growth rate = mean height increase ÷ days; rate rises to an optimum then falls.
Temperature and collisions
As temperature rises, enzyme and substrate molecules gain kinetic energy,
  • so successful collisions and enzyme-substrate complex formation become more frequent.
  • Beyond the optimum, vibration breaks hydrogen and ionic bonds, the tertiary structure changes and the active site is no longer complementary.
Photorespiration
At higher temperatures rubisco catalyses photorespiration more, which wastes fixed carbon and lowers the net rate of photosynthesis, so crop yields can fall. Warmer water can also speed up photosynthesis in cyanobacteria, increasing harmful algal blooms.
Catalase temperature method
Equilibrate buffered hydrogen peroxide tubes in water baths at several temperatures, add equal volumes of catalase, and collect oxygen over water in a measuring cylinder at fixed intervals. Plot volume against time; the tangent at the start gives the initial rate for each temperature.
Carbon cycle
Photosynthesis removes CO2; carbon passes along food chains; respiration by organisms and decomposers returns it.
  • CO2 also dissolves in oceans and is used to build calcium carbonate shells.
  • Incompletely decomposed matter forms peat and fossil fuels;
    • combustion or disturbing soils and peat releases this stored carbon.
Renewable energy
Wind, solar, tidal and geothermal power generate electricity without releasing CO2 and are becoming cheaper, but each has limits: solar depends on sunshine, wind on wind speed (and may harm birds and bats), tidal on coastal sites and geothermal on nearby volcanic activity.
Model limitations
Climate predictions are uncertain because future emissions and carbon-capture success are unknown, climate systems are complex, tipping points (e.g. permafrost releasing methane) could accelerate warming suddenly, and natural events such as volcanic ash can cool the planet unpredictably.
Sea level and currents
Sea level rises because warmer water expands and land ice melts. Warming may also alter ocean currents, which could cool some regions locally, and warmer air holds more moisture, so some areas receive heavier rain while others suffer drought.

Section 2 of 40Process & sequence

1
Short-wave radiation from the Sun passes through the atmosphere and warms the Earth's surface.
2
The Earth's surface re-emits energy as longer-wavelength infrared radiation.
3
Greenhouse gases (CO2, CH4, water vapour) absorb infrared and re-radiate it in all directions, including back to Earth.
4
Burning fossil fuels and deforestation raise CO2; livestock, rice paddies and landfill raise CH4.
5
Higher concentrations trap more infrared, raising mean global temperature (enhanced greenhouse effect).
6
Warming causes melting ice, sea-level rise, changes in rainfall and more extreme weather, altering species' distributions and development.

Effect of temperature on brine shrimp hatchingwrite these steps in this order; each step is a marking point.

1
Add a fixed mass of brine shrimp eggs (e.g. 0.05 g) to equal volumes of salt water of the same concentration in several containers.
2
Place each container in a water bath or incubator at a different temperature (e.g. 15, 20, 25, 30 °C), with equal light exposure.
3
After a set time (e.g. 48 h), count the hatched larvae in a sample using a pipette and hand lens/microscope (larvae are attracted to a light).
4
Repeat at each temperature to calculate means; handle live organisms ethically and return/dispose responsibly.
5
Plot number hatched (or rate) against temperature and calculate Q10 where appropriate.

Section 3 of 40Compare

Evidence for climate change
SourceWhat it measuresLimitation
Temperature recordsDirect thermometer readings since c. 1850Short time span; early records fewer and less reliable
DendrochronologyRing width reflects growing conditionsOnly where trees grow; other factors (e.g. nutrients) affect ring width
Pollen in peat bogsPlant species present in pastPollen may be blown in; species' climate tolerance may have changed
Ice coresCO2 in air bubbles; temperature from isotopesLimited locations (polar)
Atmospheric CO2 (e.g. Mauna Loa)Direct CO2 since 1958Single site, short period; shows seasonal fluctuation
Effects of rising temperature on organisms
EffectExample/explanation
Changed distributionSpecies move towards poles or higher altitude
Changed timing (phenology)Earlier flowering, egg laying, migration; mismatch with food supply
Enzyme activityMetabolic rates rise up to optimum (Q10); above optimum enzymes denature
Sex ratioIn some reptiles temperature determines sex of offspring
Development rateFaster development of ectotherms, e.g. insects; more life cycles per year

Section 4 of 40Worked examples

2.25 (2.3 to 2 s.f.)
The rate of an enzyme reaction is 12 units at 20 °C and 27 units at 30 °C. Calculate Q10.
  • Q10 = rate at 30 °C ÷ rate at 20 °C = 27 ÷ 12 = 2.25 (2.3 to 2 s.f.); no units.
32.9%
Atmospheric CO2 rose from 316 ppm in 1960 to 420 ppm in 2023. Calculate the percentage increase and the mean rate of increase.
  • % increase = (420 − 316) ÷ 316 × 100 = 104 ÷ 316 × 100 = 32.9% (33%).
    • Rate = 104 ÷ 63 years = 1.65 ppm yr−1 (1.7).
20 days
Q10 for development of an insect is 2.0. Development takes 40 days at 15 °C. Predict the time at 25 °C.
  • Rate doubles, so time halves40 ÷ 2.0 = 20 days.

Section 5 of 40Keep going

Climate Change Edexcel A Level Biology SNAB: what to do next

These Climate Change Edexcel A Level Biology SNAB notes covered the key ideas, worked examples and exam technique for this topic. Check your Climate Change Edexcel A Level Biology SNAB understanding with the practice questions below, then reread any Climate Change Edexcel A Level Biology SNAB point you were unsure about before moving on.

Climate Change Edexcel A Level Biology SNAB: revision checklist

1
Revise the main Climate Change Edexcel A Level Biology SNAB definitions until you can write them from memory.
2
Practise Climate Change Edexcel A Level Biology SNAB exam questions under timed conditions.
3
Link each Climate Change Edexcel A Level Biology SNAB idea to a diagram or a worked example.
4
List the common Climate Change Edexcel A Level Biology SNAB mistakes and how to avoid them.
5
Explain one Climate Change Edexcel A Level Biology SNAB idea out loud as if teaching a friend.
6
Summarise Climate Change Edexcel A Level Biology SNAB in five key points on a revision card.
7
Check which command words appear most often in Climate Change Edexcel A Level Biology SNAB questions.
8
Come back to Climate Change Edexcel A Level Biology SNAB after a week for spaced practice.
1
Practice: Climate Change Edexcel A Level Biology SNAB practice questions
2
Previous lesson: 5.2 Ecosystems & Energy Transfer — 9BI0 Notes
3
Next lesson: 5.4 Evolution — 9BI0 Notes
4
Course: Edexcel A Level Biology A (snab) 9BI0 — all units
5
Official syllabus: Pearson Edexcel A Levels

Section 6 of 40What climate and climate change are

Climate is the weather conditions in a region over a long period of time, i.e. several decades.

Climate change
When these conditions change significantly over a long period, this is climate change.
Climate has changed many times during Earth's history because of changes in atmospheric gas concentrations, volcanic activity, orbital changes and solar output,
  • but the changes since the mid-1800s are thought to be the result of human activities.
Scientists have long hypothesised that increasing greenhouse gases cause global warming, and global warming is currently taking place.

Section 7 of 40Carbon dioxide and temperature

Atmospheric CO2 levels have fluctuated throughout Earth's history, but recent increases have been faster and greater than ever before.

Carbon dioxide and temperature
Data show a correlation between changing CO2 and temperature over thousands of years, for example between atmospheric CO2 concentrations and average Antarctic temperatures from ice cores (air bubbles trapped in ice reveal past CO2).
A correlation does not prove causation, however.
CO2 is not the only factor affecting climate: solar winds and sunspots can also affect climate,
  • but scientists think their effects are small compared with atmospheric CO2.

Section 8 of 40Records of temperature

Thermometers measure air temperature, and records from different places around the world over long periods can be combined to show average global temperature over time. Records since the mid-1800s show an overall trend of increasing average global temperature. There are short periods within this window when temperatures declined, but the overall trend is upward.

Section 9 of 40Indirect (proxy) evidence from the past

Peat cores. Peat accumulates in waterlogged places called peat bogs (peatlands).

Indirect (proxy) evidence from the past
It builds up in layers, with recent peat at the top and older peat deeper down.
Cores can be taken and the layers analysed for pollen grains trapped in the peat.
Pollen from each plant species is unique,
  • so under a microscope the plant species growing around the bog at different times can be identified,
    • and since plant species have particular temperature and moisture needs, past climate can be inferred.

Dendrochronology uses the growth in a tree trunk each year as a measure of climate.

Indirect (proxy) evidence from the past
Each year a tree adds a ring; wide rings indicate warm, wet years with good growth, while narrow rings indicate cold or dry years.
Rings can be counted and compared between trees and with old timber to extend the record back thousands of years.
Limitations include that tree growth also depends on other factors, such as disease, light and competition.

Section 10 of 40The greenhouse effect

When radiation from the Sun hits the Earth, it is re-radiated from the surface as infrared.

Greenhouse gas
A greenhouse gas absorbs this re-radiated radiation and traps heat in the atmosphere so that it is not lost to space.
Greenhouse effect
This is like the glass in a greenhouse, hence the name greenhouse effect.
Without it the Earth would be too cold for life; the problem is the enhanced effect caused by increased concentrations of gases such as CO2 and methane.

Section 11 of 40Worked examples

  1. Explain how peat cores provide evidence of past climate. [4]4 marks

  2. State one limitation of using tree rings as evidence. [1]1 mark

  3. Why is a correlation between CO2 and temperature not proof of causation? [2]2 marks

Section 12 of 40Greenhouse gases and human activity

Greenhouse gases absorb radiation re-emitted from the Earth's surface and trap it in the atmosphere.

Greenhouse gases and human activity
Atmospheric CO2 levels have fluctuated through history due to volcanic eruptions and the weathering of limestone rocks,
  • but since the industrial revolution they have risen to the highest in Earth's history, mainly because of the burning of fossil fuels.
Methane (from cattle, rice paddies, landfill and thawing permafrost) is another important greenhouse gas.
Both CO2 and methane have increased since the industrial revolution due to human activities.

Carbon dioxide is also released when natural stores of carbon, called carbon sinks, are damaged or destroyed by human activities. Sinks include trees, soils, peat bogs and the oceans. Deforestation, soil degradation, peat harvesting and ocean warming all add CO2 to the atmosphere.

Section 13 of 40Describing graphs of climate data

Example task

  • a graph shows average global temperature and atmospheric CO2 since the year 1000.
  • Describe the data and explain what can be concluded.

1
Describe the trend and any anomalies, and include numbers to support the description.
2
Atmospheric CO2 concentration and average global temperature have both increased since the year 1000: CO2 from around 280 ppm to around 380 ppm,
  • and temperature from around 13.8 °C to around 14.4 °C.
3
Quote changes with units (ppm = parts per million, ppb = parts per billion) and note the period during which the rise was fastest (since about 1850).
4
Conclusions must follow from the data: the data show a positive correlation,
  • which is consistent with CO2 causing warming but does not by itself prove it;
    • other variables may play a role.

Section 14 of 40Models of future climate change

Existing data on global warming can be used to predict future temperatures.

Extrapolating
Using data this way is extrapolating: extending trends beyond the measured range.
Extrapolated data produce models showing how climate may change in future, based on a range of scenarios (for example, different rates of greenhouse gas emission).
Limitations of climate models:
  • The IPCC has produced models for several emissions scenarios, and we do not know which is most likely.
  • Models rely on assumptions about future human behaviour, population and technology.
  • There are complex feedback effects, such as ice melting and reducing reflectivity (albedo), more water vapour, or thawing peat releasing more methane.
  • Some factors cool the Earth, such as volcanic ash and aerosols, which reflect radiation back into space.
  • Uncertainty increases the further into the future the prediction goes.

Section 15 of 40Feedback and uncertainty in models

Positive feedbacks speed up warming

  • melting ice exposes darker land or sea that absorbs more heat;
    • warmer oceans hold less dissolved CO2;
    • thawing permafrost releases methane.

Feedback and uncertainty in models
Negative feedbacks may reduce it: more CO2 may increase plant growth and photosynthesis, and more cloud may reflect sunlight.
The balance of these is hard to predict,
  • so models give a range of possible outcomes rather than a single value, and each should be compared against past data to test its accuracy.

Section 16 of 40Worked examples

  1. Describe how human activity has increased atmospheric CO2. [3]3 marks

  2. State two limitations of climate models based on extrapolation. [2]2 marks

  3. A graph shows CO2 rising from 280 to 380 ppm. Calculate the percentage increase. [2]2 marks

Section 17 of 40Warming and its consequences

Greenhouse gases absorb infrared radiation emitted by the Earth, causing the atmosphere to warm.

1
The higher the concentration, the more infrared is absorbed, and the warmer the atmosphere becomes.
2
Increased warming has had and will have multiple impacts on climate patterns,
  • such as changes in rainfall, more extreme weather,
  • and effects on the timing of seasonal events.
3
Earlier plant growth in spring alters invertebrate life cycles,
  • so when a migrant bird species arrives in summer, its usual food source may not be available.
    • (loss of synchrony)
4
The distribution of species changes as they move towards the poles or higher altitude.
5
Polar ice and glaciers are retreating: there may soon be no summer ice in the Arctic if the warming continues.
6
Loss of glacier ice from mountain ranges may affect water supplies for many people and wildlife.
7
Sea levels have been rising faster in recent years (thermal expansion of water and melting land ice), putting many people and animals at risk of being flooded out of their homes.

Section 18 of 40Temperature and enzyme activity

Changing air temperature affects the metabolism of living organisms because of its effect on enzyme activity.

1
Each enzyme has an optimum temperature, at which it catalyses a reaction at the maximum rate.
2
Below the optimum, molecules have less kinetic energy, so there are fewer collisions and fewer enzyme-substrate complexes.
3
Above it the hydrogen and ionic bonds holding the tertiary structure break; the active site changes shape and the enzyme denatures.
4
Note that 35 °C is not the optimum for every enzyme-controlled reaction.

Section 19 of 40Effects on photosynthesis and food webs

Photosynthesis converts carbon dioxide into carbohydrates, producing food for producers and for organisms higher up the food chain.

It depends on proteins in the electron transport chain and on enzymes such as rubisco.
In warmer water cyanobacteria (blue-green algae) photosynthesise faster because enzyme activity increases, which can lead to harmful algal blooms.
Photorespiration
Some chemical reactions slow down at higher temperatures: at high temperatures plants carry out photorespiration faster, in which rubisco binds oxygen instead of carbon dioxide.
This reduces the efficiency of carbon fixation, slowing photosynthesis and growth.
Crop yields may therefore fall in some regions, while others may benefit from longer growing seasons.

Section 20 of 40Data question: seasonal timing

In a typical data question, records show that the date of first leaf-out has become about two weeks earlier over 50 years while the arrival date of a migratory bird has not changed.

Data question: seasonal timing
The mismatch means the birds' peak food demand (feeding chicks) no longer coincides with the peak in caterpillars, so breeding success falls.
Suggest that evidence is correlational and other factors, such as habitat change, also affect bird numbers.

Section 21 of 40Worked examples

  1. Explain how global warming could disrupt a food chain. [4]4 marks

  2. Explain why a very high temperature reduces the rate of photosynthesis. [3]3 marks

  3. Give two reasons sea level is rising. [2]2 marks

Section 22 of 40Investigating temperature and catalase

The rate of catalase activity is measured as the volume of oxygen produced.

1
Set out apparatus: hydrogen peroxide solution, buffer solution, catalase (e.g. from liver or yeast), a gas syringe or an inverted measuring cylinder,
  • and water baths.
2
Method: set up water baths at a range of temperatures, e.g. 10, 20, 30, 40 and 50 °C.
3
Place equal volumes of hydrogen peroxide and buffer in a boiling tube in each water bath and let them reach the temperature; add the same volume of catalase; seal and measure the volume of oxygen collected over a set time.
4
Rate = change in volume / time taken.
5
Repeat three times at each temperature and calculate the mean.
6
A tangent can be drawn to the volume-time curve to calculate the initial rate of the reaction.
7
Control: pH (buffer), concentration and volume of enzyme and substrate.
8
Safety: hydrogen peroxide is an irritant.

Section 23 of 40The temperature coefficient Q10

The temperature coefficient (Q10) compares the rate of reaction when the temperature is increased by 10 °C: Q10 = rate at (T + 10) °C / rate at T °C.

1
A Q10 of about 2 means the rate doubles for each 10 °C rise.
2
Example: in an enzyme-catalysed reaction, 3.5 cm3 s-1 of product was recorded at 30 °C and 6.8 cm3 s-1 at 40 °C.
3
Q10 = 6.8 / 3.5 = 1.94, which is about 2.
  • If the rise is not exactly 10 °C, a different formula is used, but A-level questions normally use 10 °C intervals.
4
Above the optimum temperature Q10 falls below 1 because the enzyme denatures.

Section 24 of 40Temperature and development

Evidence from the natural world indicates that global warming affects the development of organisms, and this can be investigated in the laboratory. Seedling growth: grow seedlings (e.g. cress) at a range of temperatures; growth rate increases as temperature rises up to 25 °C, after which it decreases, which could be due to cellular enzymes denaturing at high temperatures. Brine shrimp hatching: place an equal number (e.g. 40) of brine shrimp eggs in saltwater in a series of water baths at different temperatures, and count the number hatching after a fixed time.

1
Hatch rate rises with temperature up to 25 °C and decreases above it.
2
Keep salinity, light and volume constant.

Brine shrimps are living organisms, so welfare must be considered. Handle them gently, and return hatched shrimps to a suitable environment that replicates their natural habitat at the end of the experiment.

Section 25 of 40Worked example: a rate-temperature graph

Suppose volumes of oxygen after 60 s are 4, 9, 18, 30 and 8 cm3 at 10, 20, 30, 40 and 50 °C.

Worked example: a rate-temperature graph
The rate at 30 °C is 18 / 60 = 0.30 cm3 s-1, and at 40 °C it is 0.50 cm3 s-1, so Q10 from 30 to 40 °C is 0.50 / 0.30 = 1.7.
  • The optimum lies near 40 °C, and the low value at 50 °C shows denaturation.
To find the optimum more precisely, test more temperatures between 35 and 50 °C.

Section 26 of 40Worked examples

  1. Calculate Q10 for a reaction with a rate of 2.0 at 20 degrees C and 4.4 at 30 degrees C. [2]2 marks

  2. Suggest why brine shrimp hatch rate falls above 25 degrees C. [2]2 marks

  3. Name two variables to control in the catalase experiment. [2]2 marks

Section 27 of 40Consensus and evaluating claims

There is a consensus in the scientific community, i.e. general agreement, that increasing concentrations of greenhouse gases cause global warming and that human activity is a major cause.

Consensus and evaluating claims
When claims are made that human activities are not the cause of climate change, evaluate them by asking:
  • Who made the claim, and are they an expert in the field?
  • Are they independent, or do they have a financial interest in the outcome (for example linked to fossil fuel technologies)?
  • Has the work been published and peer reviewed?
  • Is the evidence reliable and reproducible?
  • Does it explain the data better than the existing hypothesis?
There is a great deal of scientific evidence, tested and checked by other scientists, that supports the hypothesis that burning fossil fuels causes climate change,
  • which makes further claims of this kind more likely to be correct.
If predictions are correct, the potential impacts on the future of Earth are huge,
  • so scientists have a responsibility to help others assess the evidence thoroughly.

Section 28 of 40The carbon cycle

The CO2 that contributes to the greenhouse effect is part of the global carbon cycle: the processes by which carbon is transferred and stored.

1Carbon pools
Locations where carbon is stored are carbon pools (atmosphere, oceans, soils, living organisms, fossil fuels, rocks),
  • and the processes by which it moves are carbon fluxes.
2
Fluxes into the atmosphere include respiration of all organisms, decomposition, combustion of fossil fuels, peat and biomass such as wood,
  • and volcanic activity.
3
Fluxes out of the atmosphere include photosynthesis and dissolving in the oceans (where it forms carbonates in shells and, over time, limestone).

Section 29 of 40Using the carbon cycle to reduce climate change

A good understanding of the carbon cycle is essential in fighting global warming.

Using the carbon cycle to reduce climate change
It shows the points at which carbon enters the atmosphere,
  • so reducing transfer at these points can prevent further increases: for example, burn less fossil fuel, stop burning peat and biomass,
    • and stop deforestation.
Models using extrapolation show that the consequences could be extremely serious for humans and global biodiversity if current activities continue,
  • so we must act quickly both to reduce emissions and to increase the rate at which carbon is removed from the atmosphere.
Approaches are described in the next lesson.

Section 30 of 40Applying evaluation criteria

Use the following checklist for a claim in an exam: Is the source independent or funded by someone with an interest?

1
Was the data published in a peer-reviewed journal?
2
Was the sample size, time span and sampling method adequate?
3
Has it been repeated by others?
4
Does the claim use evidence correctly (for example, does it confuse short-term variation with long-term trends)?
5
A claim that cites a short cooling period within a long warming trend, for example, selects data to support its view and ignores the overall trend.

Section 31 of 40Worked examples

  1. Suggest two reasons to treat a claim by an oil company scientist with caution. [2]2 marks

  2. Distinguish between a carbon pool and a carbon flux. [2]2 marks

  3. Name two processes that add carbon dioxide to the atmosphere. [2]2 marks

Section 32 of 40Biofuels

Biofuels are fuels made from recently grown plants, such as ethanol from sugar cane or biodiesel from oilseed.

Biofuels
They are considered carbon neutral because the CO2 released when burned was recently absorbed in photosynthesis.
Arguments against: the vast amounts of land required to grow biofuels could otherwise have been used for food production; creating land for biofuel growth often involves the loss of other habitats such as rainforest,
  • which is bad for biodiversity;
    • and cutting down mature trees reduces the removal of carbon from the atmosphere by photosynthesis.
The fuel needed for farming, processing and transport also produces CO2.

Section 33 of 40Renewable energy

Wind, solar, geothermal, tidal and hydroelectric energy do not release CO2 when generating electricity.

Renewable energy
Arguments in favour: they do not depend on fossil fuels, which are finite; they reduce emissions of greenhouse gases; and they do not run out.
Drawbacks include high set-up costs, dependence on weather or location, visual impact and effects on wildlife, and the need for energy storage.

Section 34 of 40Carbon removal: photosynthesis and reforestation

There is much excitement over new carbon capture technologies, but technology to remove vast quantities of carbon from the atmosphere and store it does not yet exist at scale.

We need to rely on existing methods of removal, and the main existing mechanism is photosynthesis.
Reforestation
Humans can increase the global rate of photosynthesis by planting trees, i.e. reforestation (and afforestation, planting where there was no forest), by protecting existing forests and restoring peatlands, mangroves and other sinks,
  • and by reducing deforestation.
If trees are allowed to grow to maturity they can store huge amounts of carbon in their biomass.
Some countries have shown that restoring lost forest is possible: Costa Rica now plants seven times more trees than it cuts down.
Achievements like this need huge government inputs, such as payments to landowners for protecting or planting forest.

Section 35 of 40Other approaches

Reducing energy use and improving efficiency, using public transport, reducing meat consumption (livestock produce methane), capturing methane from landfill and using it as fuel, and international agreements that set targets for emissions all contribute. No single strategy is enough, so a combination is needed.

Section 36 of 40Weighing up strategies

In an extended answer, give one advantage and one disadvantage of each strategy and then reach a conclusion.

1
Renewable energy removes the source of emissions but needs investment and storage.
2
Biofuels use existing engines but compete for land.
3
Reforestation removes CO2 but only while forests grow and requires land and long-term protection.
4
Reducing demand for energy, for example by insulation, is cheap and effective.
5
A combined approach with international agreement is needed because carbon emissions are global.

Section 37 of 40Worked examples

  1. Evaluate the use of biofuels to reduce atmospheric CO2. [4]4 marks

  2. Explain why planting trees reduces atmospheric CO2. [2]2 marks

  3. Give one reason why reforestation requires government support. [1]1 mark

Section 38 of 40Key terms

TermMeaning
ClimateWeather conditions over several decades.
Greenhouse gasGas that absorbs re-radiated infrared and traps heat.
DendrochronologyUsing tree rings to study past climate.
PeatPartially decayed plant material accumulating in waterlogged areas.
Proxy evidenceIndirect evidence for past conditions, such as pollen or tree rings.
TermMeaning
Carbon sinkStore that takes in more carbon than it releases.
ExtrapolationExtending a data trend beyond measured values.
IPCCIntergovernmental Panel on Climate Change; produces climate scenarios.
ppmParts per million.
TermMeaning
Optimum temperatureTemperature at which an enzyme works fastest.
DenaturationIrreversible change in the shape of a protein.
PhotorespirationRubisco reaction with oxygen, reducing photosynthetic efficiency.
Algal bloomRapid increase in algae population.
TermMeaning
Q10Factor by which rate increases for a 10 degrees C rise.
Initial rateRate at the start of a reaction.
Optimum temperatureTemperature of maximum enzyme activity.
Brine shrimpSmall crustacean often used to investigate hatching.
TermMeaning
ConsensusGeneral agreement among scientists.
Carbon poolStore of carbon.
Carbon fluxTransfer of carbon between pools.
Peer reviewEvaluation of research by independent experts before publication.
TermMeaning
BiofuelFuel made from recently grown biomass.
ReforestationReplanting of forest.
Renewable energyEnergy from sources that are replaced naturally.
Carbon neutralReleasing as much CO2 as is absorbed.

Section 39 of 40Command terms

Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.

Section 40 of 40Exam technique

Avoid these mistakes
Confusing weather and climate.
Saying all climate change is caused by humans.
Saying a correlation proves cause.
Avoid these mistakes
Giving conclusions that go beyond the data.
Describing a graph without numbers.
Saying methane is not a greenhouse gas.
Avoid these mistakes
Saying enzymes are killed by heat.
Saying melting sea ice raises sea level — melting floating ice does not, land ice does.
Thinking all organisms are harmed equally.
Avoid these mistakes
Calculating Q10 as the difference in rates.
Ignoring animal welfare.
Using a temperature range that is too narrow.
Avoid these mistakes
Saying a consensus means no uncertainty.
Dismissing a claim just because of who made it without evaluating the evidence.
Saying the atmosphere is the only carbon store.
Avoid these mistakes
Saying biofuels have no environmental cost.
Claiming carbon capture technology already removes vast amounts of CO2.
Saying young trees store the most carbon.

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