Climate Change: Evidence and Causes

Climate is the long-term average of weather conditions — typically measured over at least 30 years. Climate change is a significant, lasting shift in global or regional climate patterns.

The Quaternary period — the last 2.6 million years of geological time — is the timeframe AQA requires for this topic. During this period, Earth has cycled between cold glacial periods (ice ages) and warmer interglacials driven by natural factors. The last major glacial peaked around 20,000 years ago; we are currently in an interglacial.

What makes current climate change scientifically distinctive is not that Earth is warming — it has warmed before — but the speed:

Change	Timescale
Natural transition from glacial to interglacial (~5–6°C warming)	~10,000–20,000 years
Current warming (~1.2°C above pre-industrial levels)	~150 years

The recent rate of warming is far faster than natural glacial–interglacial transitions in the Quaternary record. This speed implicates a new forcing factor: human activity.

Multiple independent lines of evidence confirm that global climate is changing. Their convergence — each using different methods, different samples, and different scientists — is why the scientific consensus is so robust.

Instrumental temperature records: The UK's Central England Temperature series, the world's longest continuous thermometer record (from 1659), shows clear warming since the industrial era. Global records from 1850 show the warmest years on record all occurring in the 2010s and 2020s.

Ice cores: Ice drilled from Antarctica and Greenland contains ancient air bubbles — samples of Earth's past atmosphere. Analysis reveals atmospheric CO₂ and temperature fluctuating together over 800,000 years of ice core records. CO₂ levels in the early/mid-2020s (~422 ppm) are far above any natural level in those 800,000 years.

Sea level records: Global average sea level has risen approximately 20 cm since 1900, with the rate accelerating to ~3.7 mm per year since 2006. Both tide gauges and satellite altimetry independently confirm this.

Retreating glaciers: Satellite imagery and historical photographs document glacier retreat worldwide — Glacier National Park (USA), the Alps, and Himalayan glaciers are all losing mass at measurable rates. Arctic sea ice minimum extent has declined roughly 40% since the late 1970s.

Biological indicators: Spring events are occurring earlier — earlier flowering dates, earlier arrival of migratory birds, earlier emergence of insects. Species ranges are shifting poleward. Coral bleaching from warming seas is increasing in frequency and severity.

Natural factors explain the climate cycles of the Quaternary — the sequence of ice ages and interglacials that preceded and set the context for current change.

Milankovitch cycles (orbital changes):

Earth's orbit around the Sun varies in three ways:

• Eccentricity (~100,000 years): the orbit shifts from nearly circular to more elliptical. A more elliptical orbit means greater variation in Earth's distance from the Sun across a year.
• Axial tilt (~41,000 years): Earth's tilt (currently 23.5°) varies between 22.1° and 24.5°. Greater tilt produces more extreme seasons.
• Precession (~26,000 years): Earth's axis wobbles slowly, like a spinning top, affecting which hemisphere faces the Sun during which part of the orbit.

These three cycles interact to determine how much solar energy reaches different latitudes and seasons. Their combination explains the regular pattern of ice ages and interglacials visible in the ice core record.

Solar output: The Sun's energy output varies slightly over an 11-year sunspot cycle and over longer periods. Reduced solar output during the Maunder Minimum (~1645–1715) may have contributed to the Little Ice Age in Europe. However, solar output has been stable or slightly declining since the 1980s — the period of fastest warming — so solar variation cannot explain recent temperature rises.

Volcanic eruptions can influence climate, but in a more complex way than is often assumed.

Short-term cooling: Large eruptions inject sulphur dioxide (SO₂) into the stratosphere. There, it forms sulphate aerosols that reflect incoming solar radiation back to space before it warms the surface. The effect lasts 1–3 years before the aerosols settle out.

Mount Pinatubo (Philippines) erupted in June 1991 — the second largest volcanic eruption of the 20th century. It ejected ~20 million tonnes of SO₂. Global average temperature fell by approximately 0.5°C over the following 1–2 years before recovering.

Long-term context: Volcanoes do also release CO₂. Over geological timescales (millions of years), volcanic CO₂ has maintained the greenhouse effect. But modern annual volcanic CO₂ output is approximately 50 to 100 times less than human annual CO₂ emissions. Volcanic activity cannot explain current warming.

The critical exam point: volcanic eruptions produce short-term cooling (aerosols), not long-term warming. They are a natural climate forcing that explains some past variability — they are not a driver of current change.

The greenhouse effect is a natural process that makes Earth habitable:

1. Short-wave solar radiation passes through the atmosphere and warms Earth's surface
2. Earth radiates heat upward as long-wave (infrared) radiation
3. Greenhouse gases (CO₂, CH₄, N₂O, water vapour) absorb this outgoing radiation and re-emit it in all directions — including back toward Earth
4. This traps heat, maintaining Earth's average temperature at ~15°C (without the greenhouse effect it would be approximately –18°C)

The diagram below shows the process: short-wave radiation reaches the surface, the warmed Earth re-emits long-wave radiation, and greenhouse gases trap some of it and send it back down.

The enhanced greenhouse effect occurs when human activity increases greenhouse gas concentrations, trapping more heat than before:

Greenhouse gas	Pre-industrial level	Approx. level in 2024
Carbon dioxide (CO₂)	~280 ppm	~422 ppm
Methane (CH₄)	~722 ppb	~1,900 ppb

Recent CO₂ levels are the highest in at least 800,000 years of ice core records.

Burning fossil fuels (coal, oil, natural gas) for electricity generation, transport, heating, and industry accounts for approximately 75% of global greenhouse gas emissions. Combustion converts stored carbon — locked in geological deposits over millions of years — into CO₂ released into the atmosphere in seconds.

Agriculture:

• Livestock (particularly cattle and sheep) produce methane (CH₄) through enteric fermentation — the digestive process. Methane is approximately 80 times more potent as a greenhouse gas than CO₂ over a 20-year timeframe.
• Rice paddies emit methane from the anaerobic decomposition of organic matter in flooded fields.
• Nitrogen fertilisers decompose in soil, releasing nitrous oxide (N₂O) — a greenhouse gas ~270 times more potent than CO₂ over 100 years.

Deforestation:

• Trees absorb CO₂ through photosynthesis and store it as biomass. Cutting them releases stored carbon as CO₂ (or methane in waterlogged conditions).
• Deforestation also destroys future absorption capacity — every felled tree is a carbon absorber permanently removed.
• Brazil's Amazon has in some recent years become a net carbon emitter rather than absorber due to deforestation and degradation.

Overview of effects on people and the environment:

• Rising temperatures: more frequent, longer heatwaves; heat stress on crops and livestock
• Sea level rise: coastal communities and low-lying islands at risk of inundation; the Maldives and Bangladesh Delta face existential threats
• Changing precipitation patterns: drier regions becoming drier; already-wet regions experiencing more intense rainfall; unpredictability increasing
• Loss of biodiversity: coral bleaching, species range shifts poleward, habitat loss
• Food insecurity: crop failures from heat, drought, and flooding affecting hundreds of millions
• Climate migration: communities displaced from areas that become uninhabitable

1. Confusing the greenhouse effect with ozone depletion

The ozone layer (in the stratosphere) protects Earth from UV radiation. The greenhouse effect traps heat via atmospheric gases. These are different processes, different gases, different atmospheric layers. Ozone depletion does not cause global warming. Mixing these up seriously undermines an extended answer.

2. Saying climate has "never changed before"

It has — the Quaternary ice ages prove it. The scientific argument is not that climate never changes naturally, but that current change is occurring far faster than any natural transition in the geological record. Speed, not the fact of change, is what makes the current situation distinctive.

3. Claiming volcanic activity drives current warming

Volcanoes produce short-term cooling (via SO₂ aerosols), not long-term warming. Modern volcanic CO₂ is trivially small compared to human emissions. Volcanoes help explain some natural variability in the Quaternary record; they cannot explain a warming trend that has continued despite no unusual increase in volcanic activity.

4. Confusing CO₂ sources with methane sources

Fossil fuels primarily emit CO₂. Livestock digestion and rice paddies primarily emit methane (CH₄). Mixing these up in an answer about human causes suggests the concepts haven't been properly distinguished. Identify the correct gas for each source.

5. Treating orbital cycles as an explanation for current warming

Milankovitch cycles operate over tens of thousands of years. They cannot drive the 1.2°C warming observed in the last 150 years. Solar and orbital factors explain ice ages; they cannot explain a warming trend occurring far faster than natural Quaternary transitions.