Notes · Environmental ScienceUK · A-Levels

Global climate change

This chapter examines the human enhancement of the greenhouse effect and its consequences. It covers the main anthropogenic greenhouse gases and their potency, the evidence for climate change and the crucial distinction between correlation and causation, the feedback mechanisms and tipping points that can amplify change, the impacts and the mitigation and adaptation responses, and, kept firmly separate, the depletion of stratospheric ozone and its successful international control.

5 sections~17 min reading time3 competenciesLevel Standard 3 · Advanced 2

T·0555 / 16

Exam profile

AO1 · Describe the causes, evidence and mechanisms of climate change and ozone depletionAO2 · Apply greenhouse and feedback concepts to interpret trend data such as the Keeling curveAO3 · Analyse climate data, distinguishing correlation from causation, and evaluate mitigation and adaptation strategies

Operators:describeexplainanalyseevaluateinterpretdiscuss

basic level

AS-Level expects you to describe the enhanced greenhouse effect, the main greenhouse gases and their sources, the evidence for climate change and the idea of mitigation and adaptation.

higher level

The full A-Level requires analysis of climate data and feedbacks, the evaluation of mitigation and adaptation strategies, and a rigorous separation of climate change from ozone depletion.

Depth

Text

Contents · 5 sections

Global climate change

The enhanced greenhouse effect and greenhouse gases#

StandardAQA 7447 3.2.1

Global warming potentials of greenhouse gases

Fig. 1Approximate 100-year global warming potentials relative to carbon dioxide (illustrative values); the potent gases dwarf CO2 per unit mass.

The enhanced greenhouse effect is the additional warming caused by human activities raising the concentration of greenhouse gases above their natural levels. The natural greenhouse effect, described in the previous chapter, is essential; the concern is the extra absorption of outgoing long-wave radiation as concentrations rise, which shifts the energy budget towards warming. The principal anthropogenic greenhouse gases are carbon dioxide, methane, nitrous oxide and the halocarbons, alongside water vapour, whose concentration rises as a feedback rather than as a direct emission.

Each gas has characteristic sources. Carbon dioxide comes chiefly from burning fossil fuels and from deforestation, and is the largest single contributor because it is emitted in such vast quantities. Methane comes from livestock and rice paddies, landfill, and the extraction of fossil fuels; nitrous oxide comes largely from nitrogen fertilisers and combustion; and the halocarbons are entirely synthetic. Because the gases differ, tackling climate change means tackling several different human activities, not just one.

Greenhouse gases differ greatly in their warming effect per molecule and in how long they persist. The global warming potential (GWP) expresses the warming caused by a mass of a gas relative to the same mass of carbon dioxide over a given period, so that carbon dioxide has a GWP of 1 by definition while methane and nitrous oxide have much higher values. A gas with a high GWP but a short atmospheric lifetime, such as methane, has a strong but shorter-lived effect, whereas carbon dioxide is less potent per molecule but persists for a very long time, so its effect accumulates. Comparing gases therefore requires both their GWP and their lifetime.

Understanding these differences matters for policy. It explains why cutting methane can slow warming quickly, why carbon dioxide is the central long-term problem, and why the halocarbons are doubly damaging as both greenhouse gases and ozone-depleting substances. When interpreting emissions data, it is important to weight gases by their global warming potential rather than by mass alone, so that a small mass of a potent gas is not overlooked.

Worked example

Weighting emissions by GWP

A farm emits 1000 kg of carbon dioxide and 20 kg of methane. Using an approximate methane GWP of 28, calculate the carbon dioxide equivalent of the methane and the total, and comment.

Convert the methane
Carbon dioxide equivalent = mass x GWP = $20 \times 28 = 560$ kg CO2-equivalent.
Add the carbon dioxide
Total = $1000 + 560 = 1560$ kg CO2-equivalent.
Comment
Although the methane is a small fraction of the mass, its high GWP means it accounts for over a third of the warming effect, so it cannot be ignored.

Result: The methane is equivalent to 560 kg of CO2, giving 1560 kg CO2-equivalent in total; the small mass of methane has a large effect.

Typical mistakes

Confusing the natural and enhanced greenhouse effects; only the human enhancement is the environmental problem.
Comparing gases by emitted mass alone and ignoring their very different global warming potentials.

Active revision

Explain why reducing methane emissions can slow warming more quickly than reducing carbon dioxide, even though carbon dioxide is the larger overall contributor.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Evidence for climate change and interpreting the data#

StandardAQA 7447 3.2.1AQA 7447 3.7

The rise in atmospheric carbon dioxide

Fig. 2The observed upward trend in atmospheric carbon dioxide (illustrative of the Keeling-curve record); values are approximate.

The evidence for a changing climate comes from several independent lines. Direct instrumental records of temperature, sea level and atmospheric composition span roughly the last century and a half; the continuous measurement of atmospheric carbon dioxide, whose rising, saw-toothed record is often called the Keeling curve, is a famous example. Longer-term evidence comes from proxies: ice cores trap ancient air bubbles and record past temperature and gas concentrations over hundreds of thousands of years, tree rings (dendrochronology) record past growth conditions, and ocean sediments and pollen records extend the picture further back. That these independent sources agree strengthens the conclusion.

It is essential to distinguish weather from climate. Weather is the state of the atmosphere at a place over hours or days and is highly variable, whereas climate is the long-term average pattern over decades. A cold winter or a heatwave is weather and proves nothing on its own; climate change is a trend detected by averaging out the year-to-year variability over a long period. Confusing the two, for example by citing a single cold spell as evidence against warming, is a classic error.

The most important analytical skill in this topic is distinguishing correlation from causation. The record shows that carbon dioxide concentration and global temperature have risen together, a strong correlation, but a correlation alone does not prove that one causes the other; both could be driven by a third factor, or the link could be coincidental. Causation is established by having a physical mechanism, here the well-understood absorption of long-wave radiation by greenhouse gases, and by ruling out alternatives. When interpreting climate data you should state whether the data show correlation or support causation, and identify the mechanism.

Good interpretation also attends to the quality of the data. Proxy records have uncertainties and are calibrated against instrumental data; averaging reduces the effect of natural variability; and trends must be judged over a long enough period to be meaningful. Being able to describe a trend from a graph, distinguish it from short-term fluctuation, and comment on reliability and uncertainty, is exactly what the data-response questions reward, and it guards against both overstating and dismissing the evidence.

Worked example

Correlation versus causation

A student claims that because carbon dioxide and temperature have risen together, the graph proves carbon dioxide causes warming. Evaluate this claim.

Identify the correlation
The data show a strong positive correlation: as carbon dioxide rises, temperature rises.
State the limitation
Correlation alone does not prove causation; the two could be linked by another factor or the relationship could be coincidental.
Establish causation
Causation is supported here by a known physical mechanism, the absorption of long-wave radiation by carbon dioxide, confirmed by physics and by many independent datasets, so the claim is justified but must be argued from mechanism, not correlation alone.

Result: The correlation is real, but causation is established by the physical mechanism and multiple datasets, not by the correlation on its own.

Typical mistakes

Citing a single cold or hot spell (weather) as evidence for or against climate change (a long-term trend).
Treating a correlation between carbon dioxide and temperature as proof of causation without stating the physical mechanism.

Active revision

A graph shows global temperature and atmospheric carbon dioxide both rising over the twentieth century. Explain what this does and does not prove, and how causation is established.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Feedback mechanisms and tipping points#

AdvancedAQA 7447 3.2.1

The ice-albedo positive feedback

Fig. 3A positive feedback: each step reinforces the original warming, so the loop amplifies change.

Feedback mechanisms can amplify or dampen an initial warming and are the main source of uncertainty in projecting climate change. A positive feedback reinforces the original change, making warming worse; a negative feedback opposes it, moderating warming. Because several powerful positive feedbacks exist, the response to a given rise in greenhouse gases can be larger than the direct effect alone, and understanding these loops is essential to reasoning about the pace and risk of change.

The ice-albedo feedback is the clearest positive feedback: warming melts reflective ice and snow, exposing darker ocean or land with a lower albedo, which absorbs more solar radiation and warms further, melting more ice. Others include the release of methane from thawing permafrost and warming wetlands, and the rise in atmospheric water vapour as the air warms, since water vapour is itself a greenhouse gas. A potential negative feedback is that more warming and carbon dioxide can increase plant growth, drawing down some carbon dioxide, though this is limited and can be offset by drought and by the warming-driven release of carbon from soils.

The idea of a tipping point captures the danger of positive feedbacks. A tipping point is a threshold beyond which change becomes self-sustaining and effectively irreversible on human timescales, because the feedbacks take over. Examples discussed include the loss of the great ice sheets, the dieback of major forests, and the release of stored carbon from permafrost. Once a tipping point is crossed, reducing emissions may no longer be enough to reverse the change, which is the central argument for acting before thresholds are reached.

Feedbacks and tipping points explain why climate projections carry ranges rather than single figures and why the precautionary principle is invoked: because the feedbacks are incompletely understood and their thresholds uncertain, the risk of severe, irreversible change justifies action despite the uncertainty. When evaluating climate policy, the existence of positive feedbacks and possible tipping points strengthens the case for early and substantial mitigation.

Worked example

Reasoning about a feedback

Explain why the melting of Arctic sea ice is described as a positive feedback and why it is of particular concern.

Trace the loop
Warming melts sea ice; the exposed ocean is darker and has a lower albedo, so it absorbs more solar radiation and warms further, melting more ice.
Classify
The response reinforces the original warming, so it is positive feedback.
Concern
Positive feedbacks accelerate change and can drive the system towards a tipping point where ice loss becomes self-sustaining, making the change hard to reverse.

Result: The loop is positive feedback that accelerates warming and risks an irreversible tipping point, strengthening the case for early action.

Typical mistakes

Labelling a feedback as negative when it reinforces the change; a positive feedback amplifies, a negative feedback opposes.
Treating tipping points as ordinary gradual change rather than thresholds beyond which change is self-sustaining.

Active revision

Explain how the thawing of permafrost could act as a positive feedback on climate change, and why this is a concern for policy.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Impacts, mitigation and adaptation#

StandardAQA 7447 3.2.1

Mitigation and adaptation responses

Fig. 4Mitigation reduces the causes of climate change; adaptation copes with its unavoidable effects.

The impacts of climate change are wide-ranging and uneven. Rising temperatures shift the ranges of species and the timing of seasonal events, disrupting ecosystems and threatening specialists that cannot move or adapt quickly; warming and melting ice raise sea levels, threatening low-lying land and coastal cities; and changes in circulation alter rainfall patterns, increasing drought in some regions and flooding in others, with the frequency of some extreme weather events expected to rise. These physical changes have consequences for agriculture, water supply, health and the displacement of people, and they fall hardest on those least able to cope.

Responses fall into two complementary categories. Mitigation means reducing the causes of climate change by cutting greenhouse-gas emissions or removing gases from the atmosphere: switching from fossil fuels to low-carbon energy, improving energy efficiency, reducing deforestation and restoring forests, changing agricultural practices, and capturing and storing carbon. Adaptation means adjusting to the changes that can no longer be avoided: building flood defences and sea walls, developing drought-resistant crops, changing water management, and planning for the movement of species and people. Both are needed, because some change is already committed while worse change can still be avoided.

Mitigation strategies vary in cost, feasibility and effectiveness, which makes them a natural subject for evaluation. Renewable energy and efficiency cut emissions at source but require investment and infrastructure; carbon capture and sequestration can remove carbon dioxide but is costly and unproven at scale; reforestation stores carbon and brings other benefits but takes time and land. Because emissions are global, mitigation also requires international cooperation, which is difficult to achieve and enforce, so evaluating a strategy means weighing its technical potential against its economic and political realism.

A balanced answer recognises the trade-offs and the distribution of costs and benefits. Mitigation now reduces future harm but imposes present costs, and the countries that have emitted most are often not those that will suffer most, raising questions of fairness. Adaptation protects against unavoidable change but can be expensive and cannot prevent the loss of ecosystems that cannot adapt. The strongest responses combine ambitious mitigation to limit the ultimate change with adaptation to cope with what is already unavoidable, and they acknowledge the uncertainty that the feedbacks introduce.

Worked example

Classifying and evaluating responses

A low-lying country builds sea walls and also invests in solar power. Classify each response and comment on why both are needed.

Classify the sea walls
Sea walls cope with rising sea level, an effect of climate change, so they are adaptation.
Classify the solar power
Solar power reduces greenhouse-gas emissions, a cause of climate change, so it is mitigation.
Explain the combination
Mitigation limits future change but cannot undo the change already committed, so adaptation is needed now; both together give the best protection.

Result: The sea walls are adaptation and the solar power is mitigation; both are needed because some change is unavoidable while more can still be prevented.

Typical mistakes

Confusing mitigation (reducing the causes) with adaptation (coping with the effects).
Presenting one strategy as a complete solution rather than recognising that mitigation and adaptation are complementary and involve trade-offs.

Active revision

Evaluate the replacement of fossil-fuel power stations with renewable energy as a strategy for mitigating climate change.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Ozone depletion and its control#

AdvancedAQA 7447 3.2.1

Catalytic destruction of ozone by chlorine

Fig. 5Chlorine acts as a catalyst: it is regenerated, so one atom destroys many ozone molecules.

Ozone depletion is a separate problem from climate change, though both involve the atmosphere and both were caused by industrial gases. The natural balance of stratospheric ozone, described in the previous chapter, is disturbed by ozone-depleting substances, above all the chlorofluorocarbons (CFCs) once widely used as refrigerants, aerosol propellants and solvents, and the related halons. These are stable and unreactive in the lower atmosphere, so they persist and slowly drift up into the stratosphere.

In the stratosphere, ultraviolet radiation breaks the CFCs apart and releases chlorine atoms. A chlorine atom reacts with ozone to form chlorine monoxide and oxygen, and the chlorine monoxide then reacts with an oxygen atom to regenerate the chlorine atom and release oxygen. Because the chlorine is regenerated, a single chlorine atom can destroy many thousands of ozone molecules before it is eventually removed; it acts as a catalyst. This catalytic destruction upsets the natural equilibrium of the Chapman cycle, so ozone is destroyed faster than it is formed and the layer thins, most dramatically over Antarctica in the polar spring, giving the ozone hole.

The consequence of a thinner ozone layer is that more ultraviolet radiation reaches the surface, increasing rates of skin cancer, cataracts and immune suppression in humans, harming other organisms, and damaging the phytoplankton at the base of marine food webs. It is important to keep this quite separate from the greenhouse effect: a common and heavily penalised error is to state that the ozone hole causes global warming or that greenhouse gases cause the ozone hole. They are distinct problems with distinct causes, mechanisms and consequences.

Ozone depletion is often cited as an environmental success story and a model for international action. Under the Montreal Protocol, an international agreement, the production and use of CFCs and other ozone-depleting substances were phased out and replaced with safer alternatives. Because the agreement was widely adopted and enforced, atmospheric concentrations of the offending gases have fallen and the ozone layer is expected to recover slowly over the coming decades, although the long lifetimes of the gases mean recovery is gradual. This contrast with the far slower progress on greenhouse gases is itself a valuable point for evaluation.

Worked example

Explaining catalytic destruction

Explain why a small amount of CFC in the stratosphere can destroy a very large amount of ozone.

Release of chlorine
Ultraviolet radiation breaks the CFC molecule and releases a chlorine atom in the stratosphere.
The catalytic cycle
The chlorine reacts with ozone to form chlorine monoxide and oxygen; the chlorine monoxide then reacts with an oxygen atom, regenerating the chlorine atom.
Why the effect is large
Because the chlorine is regenerated at the end of each cycle, one atom can destroy many thousands of ozone molecules before it is removed, so a little CFC causes a lot of destruction.

Result: Chlorine acts as a catalyst that is regenerated each cycle, so a small amount destroys a very large amount of ozone.

Typical mistakes

Stating that ozone depletion causes global warming, or that greenhouse gases cause the ozone hole; these are distinct problems.
Forgetting that chlorine is a catalyst that is regenerated, so a little causes a lot of destruction.

Active revision

Explain why the Montreal Protocol has been more successful in tackling ozone depletion than international agreements have so far been in tackling climate change.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

Global climate change

5 sections~17 min reading time3 competenciesLevel Standard 3 · Advanced 2

Global climate change

Key points

Weighting emissions by GWP

Key points

Correlation versus causation

Key points

Reasoning about a feedback

Key points

Classifying and evaluating responses

Key points

Explaining catalytic destruction

Sources

Global climate change

Key points

Weighting emissions by GWP

Key points

Correlation versus causation

Key points

Reasoning about a feedback

Key points

Classifying and evaluating responses

Key points

Explaining catalytic destruction

Sources