Climate Change Evidence and Projections

Spec mapping: AQA 7037, Paper 1 (Physical), §3.1.1 — evidence for, and the causes and consequences of, changing carbon and water cycles, including the role of carbon in the atmosphere as a climate driver and the use of scenarios to project future change. This depth lesson assumes the feedback theory of the previous lesson and develops the proxy and instrumental evidence base, natural vs anthropogenic attribution, and the IPCC SSP/RCP scenario framework. AOs exercised: AO1 (proxy methods; Milankovitch theory; scenario definitions; ECS), AO2 (explaining how proxies are calibrated; why scenarios are not predictions), AO3 (manipulating Keeling-Curve, ice-core and sea-level data; reading scenario projections). Synoptic links run to Hazards (changing hazard regimes) and Global systems (climate governance).

Distinguishing evidence of past and present change from projections of future change — and understanding the very different epistemology of each — is central to this part of the specification. Proxy and instrumental records tell us what has happened; scenario-driven models tell us what could happen under specified assumptions. The depth treatment requires fluency with the methods, their uncertainties, and the crucial idea that scenarios are conditional "if–then" pathways, not forecasts.

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Proxy Records of Past Climate

Direct instrumental records reach back only to the mid-nineteenth century. Earlier climates are reconstructed from proxies — natural archives that record past conditions.

Ice cores

• Source: Antarctic (Vostok, EPICA Dome C) and Greenland (GRIP, NEEM) ice sheets.
• What they record: trapped air bubbles give direct samples of past atmospheres (CO₂, CH₄); the oxygen-isotope ratio $\delta^{18}\text{O}$δ18O of the ice records past temperature (lighter $^{16}\text{O}$16O evaporates and precipitates preferentially, so colder periods leave isotopically distinct snow).
• Range: EPICA Dome C extends ~800,000 years, capturing eight glacial–interglacial cycles.
• Key finding: CO₂ oscillated between ~180 ppm (glacials) and ~280 ppm (interglacials) for 800,000 years; today's ~420 ppm is unprecedented in the entire record — and rising far faster than any natural transition.

Tree rings (dendrochronology)

• Principle: annual rings in temperate/boreal trees vary in width and density with growing-season temperature and moisture.
• Range: living trees give centuries; overlapping fossil/subfossil sequences (e.g. the European oak chronology) extend records ~12,000 years.
• Limitations: record only the growing season; require calibration against instrumental data; not all species ring clearly. (These limitations are exactly the kind of AO3 evaluation examiners want.)

Ocean sediment cores

• Source: sea-floor drilling (e.g. JOIDES Resolution / IODP).
• What they record: $\delta^{18}\text{O}$δ18O in the shells of foraminifera records a combination of ocean temperature and global ice volume.
• Range: hundreds of millions of years, though resolution falls with age.
• Key finding: corroborates ice cores and ties glacial cycles to orbital forcing.

Proxy	What it records	Approx. range
Pollen (palynology)	Vegetation/climate zones	up to ~1 Myr
Coral growth bands	SST, ocean chemistry	~500 yr/colony; fossils extend it
Speleothems	Temperature, rainfall via $\delta^{18}\text{O}$δ18O	up to ~600,000 yr
Historical records	Harvests, frost fairs, diaries	centuries
Lake varves	Seasonal layering, pollen, diatoms	up to ~50,000 yr

Why multiple proxies matter: each proxy has different strengths, biases and resolution. Confidence comes from convergence — when ice cores, ocean sediments and corals independently agree, the reconstruction is robust. Triangulating across proxies, and acknowledging each one's limits, is a hallmark of top AO3 work.

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Instrumental Records

Temperature

• Reliable global records begin in the 1850s.
• Global mean surface temperature has risen ~1.1 °C above the 1850–1900 baseline (IPCC AR6 and subsequent updates), reaching ~1.45 °C in the record-warm year 2023.
• The warmest years on record cluster overwhelmingly in the most recent decades.

Atmospheric CO₂ — the Keeling Curve

• Continuous measurement began at Mauna Loa Observatory, Hawaii in 1958 (Charles David Keeling).
• The Keeling Curve climbs from ~315 ppm (1958) to ~424 ppm (mid-2020s), with a sawtooth annual cycle reflecting Northern-Hemisphere vegetation drawing down CO₂ in summer and releasing it in winter.
• Pre-industrial CO₂ (ice cores) was ~280 ppm.

Sea level

• Tide gauges (mid-19th C) and satellite altimetry (1993–) show rise accelerating from ~1.3 mm/yr (1901–1971) to ~3.7 mm/yr (2006–2018) (IPCC AR6), as ice-sheet mass loss adds to thermal expansion.

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AO3 skills exemplar: rates of change from the Keeling Curve and ice cores

Manipulate. Mean rate of CO₂ rise at Mauna Loa, 1958–2023:

$\frac{424 - 315}{2023 - 1958} = \frac{109\ \text{ppm}}{65\ \text{yr}} \approx 1.7\ \text{ppm/yr}.$2023−1958424−315​=65 yr109 ppm​≈1.7 ppm/yr.

But the rate is accelerating: in the 1960s it was ~0.8 ppm/yr; in recent years it exceeds ~2.4 ppm/yr.

Compare with natural change. The last glacial-to-interglacial CO₂ rise was ~100 ppm (180→280) over roughly 10,000 years, i.e.

$\frac{100\ \text{ppm}}{10{,}000\ \text{yr}} = 0.01\ \text{ppm/yr}.$10,000 yr100 ppm​=0.01 ppm/yr.

Explain and evaluate. The modern rate (~1.7–2.4 ppm/yr) is therefore on the order of 100–200 times faster than the fastest natural deglacial change — a decisive piece of attribution evidence, because ecosystems and the slow carbon cycle cannot adjust at this pace. The comparison must be made carefully: ice-core resolution is decadal-to-centennial in older sections, so very brief natural spikes could be smoothed out; even so, the sustained modern rate is unambiguously anomalous. Stating both the headline ratio and the resolution caveat is what secures full AO3 credit.

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Natural Causes of Climate Variability

Establishing the anthropogenic signal requires understanding the natural drivers it must be distinguished from.

Milankovitch cycles

Milutin Milankovitch (1920s–40s) identified three orbital variations that redistribute insolation:

Cycle	Period	Description	Climatic effect
Eccentricity	~100,000 yr	Orbit shape: near-circular ↔ more elliptical	Modulates total insolation; pacemaker of recent glacial cycles
Obliquity (axial tilt)	~41,000 yr	Tilt varies 22.1°–24.5° (now ~23.4°)	Greater tilt → stronger seasons → more summer melt
Precession	~19,000–26,000 yr	Axial wobble changes the season of perihelion	Alters seasonal intensity per hemisphere

How they drive ice ages. Orbital cycles change the distribution (not greatly the total) of solar energy. The trigger is reduced summer insolation at high northern latitudes: if summers are too cool to melt the winter snow, ice sheets grow. Once growth begins, positive feedbacks (ice-albedo and the CO₂ drawdown recorded in ice cores) amplify the small orbital forcing into the large temperature swings observed — a direct application of the previous lesson's feedback theory.

Key point — orbital forcing cannot explain modern warming. Milankovitch cycles operate over tens of thousands of years and currently favour gradual cooling; they cannot account for rapid 20th–21st-century warming. The pacing of past change is natural; the present, fast change is not.

Other natural factors

• Volcanic eruptions inject stratospheric sulphate aerosols that reflect sunlight, causing short-lived (1–3 yr) cooling. Mt Pinatubo (1991) cooled global mean temperature by ~0.5 °C for 1–2 years — a natural "experiment" that also validated climate models.
• Solar variability is small: output varies ~0.1% over the 11-year sunspot cycle (~0.1 °C effect). Crucially, solar activity has been flat or slightly declining since the 1980s even as warming accelerated — ruling out the Sun as the cause of recent warming.

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Attribution: separating human from natural causes

IPCC AR6 concludes it is "unequivocal that human influence has warmed the atmosphere, ocean and land." The attribution rests on several independent strands:

• Fingerprinting: observed patterns (stratospheric cooling while the surface warms; greater Arctic and night-time/winter warming) match the greenhouse signature, not the solar one (which would warm the whole atmosphere).
• Rate: the ~100–200× anomaly in CO₂ rise (above) and the corresponding warming rate.
• Isotopic evidence: the declining $^{13}\text{C}/^{12}\text{C}$13C/12C ratio of atmospheric CO₂ (the Suess effect) shows the added carbon is from old, $^{13}\text{C}$13C-depleted fossil sources, not volcanoes or the ocean.
• Model–observation agreement: models reproduce observed warming only when anthropogenic forcings are included.

Marshalling this multi-strand attribution — rather than asserting "humans cause climate change" — is a key AO2 discriminator.

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IPCC AR6 Scenarios — projections, not predictions

AR6 (2021–23) projects future change using Shared Socioeconomic Pathways (SSPs): internally consistent "if–then" storylines of demographic, economic and policy development that yield different emissions and forcing.

Scenario	Storyline	CO₂ by 2100	Warming by 2100 (best estimate)
SSP1-1.9	Very low emissions; rapid mitigation; sustainability	~350 ppm	~1.4 °C
SSP1-2.6	Low emissions; strong mitigation	~440 ppm	~1.8 °C
SSP2-4.5	"Middle of the road"; current-ish policies	~600 ppm	~2.7 °C
SSP3-7.0	High emissions; regional rivalry	~870 ppm	~3.6 °C
SSP5-8.5	Very high emissions; fossil-intensive	~1,135 ppm	~4.4 °C

The number after the dash is the approximate radiative forcing in W/m² in 2100 (e.g. 8.5 W/m²), so the scenarios are ordered by forcing.

Relationship to the AR5 RCPs: SSPs broadly map onto the earlier Representative Concentration Pathways — SSP1-2.6 ≈ RCP2.6, SSP2-4.5 ≈ RCP4.5, SSP5-8.5 ≈ RCP8.5 — but add the socioeconomic narrative behind each forcing level.

Key AR6 headline findings

• Warming of 1.09 °C (0.95–1.20) in 2011–2020 vs 1850–1900.
• Human influence on warming is unequivocal.
• Every additional increment of warming intensifies extremes (heatwaves, heavy rain, drought).
• Limiting warming to 1.5 °C requires global CO₂ to reach net zero ~2050 (SSP1-1.9 territory); 2 °C requires net zero ~2070.

Critical AO2 point — a scenario is not a forecast. SSPs are conditional: each says "if society follows this pathway, then this warming results." They are deliberately spread to bracket plausible futures, and the realised future depends on choices not yet made. Treating SSP5-8.5 as a prediction (or dismissing SSP1-1.9 as impossible) misreads their purpose. This distinction is one of the most reliable discriminators in the whole topic.

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Equilibrium Climate Sensitivity (ECS)

ECS is the long-term equilibrium warming for a doubling of CO₂ (280→560 ppm).

• AR6 best estimate 3.0 °C (likely range 2.5–4.0 °C), narrowed from AR5's 1.5–4.5 °C thanks to better-constrained cloud feedbacks.
• ECS integrates all feedbacks (water vapour, ice-albedo, lapse-rate, cloud) — directly the $\Delta T = \Delta T_{0}/(1-f)$ΔT=ΔT0​/(1−f) logic of the previous lesson.
• It assumes equilibrium; in reality the deep ocean delays full warming by decades to centuries, so the transient response is smaller than ECS at any given moment (the basis of "committed" warming).

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Located evidence: the EPICA ice core and the 800,000-year context

The EPICA Dome C core from the East Antarctic Plateau is the single most important palaeoclimate archive, providing a continuous ~800,000-year record of temperature and greenhouse gases. Its findings frame the entire modern debate:

• Atmospheric CO₂ oscillated between ~180 ppm (glacials) and ~280 ppm (interglacials) through eight full glacial cycles, never once exceeding ~300 ppm in 800,000 years — yet today it stands at ~424 ppm, roughly 50% above the highest natural interglacial value of that entire span.
• Methane tracked the same cycles, between ~350 and ~700 ppb; today it exceeds ~1,900 ppb, far outside the natural envelope.
• Temperature and CO₂ moved in lockstep, evidencing the tight coupling and the amplifying feedbacks (ice-albedo, CO₂) that turn small orbital triggers into large climate swings.

The EPICA record thus does two things at once: it demonstrates the natural range and pacing of change (Milankovitch-driven), and it shows by direct comparison that current concentrations are unprecedented in nearly a million years and rising orders of magnitude faster than any natural transition. Quoting the 800,000-year, 180–280 ppm envelope against today's 424 ppm is one of the most decisive single pieces of evidence a candidate can deploy.

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Consequences of projected change: linking the science to impacts

Projections matter because of their consequences, and the specification expects candidates to connect warming pathways to physical and human impacts.

Pathway (2100 warming)	Illustrative physical/human consequences
~1.5 °C (SSP1-1.9)	~70–90% of warm-water coral reefs lost; more frequent heatwaves; some ice loss committed
~2 °C (SSP1-2.6)	~99% of warm-water reefs lost; substantially more extreme heat and heavy rain; greater sea-level commitment
~2.7 °C (SSP2-4.5)	Major intensification of droughts, floods and heat; widening food- and water-security stress
~4+ °C (SSP5-8.5)	Risk of crossing multiple tipping points; very large sea-level rise over coming centuries; widespread ecosystem collapse

The recurring AR6 finding is that risks increase with every increment of warming and that impacts scale non-linearly — the jump from 1.5 to 2 °C roughly doubles many risk metrics. This is the scientific basis for the Paris Agreement's distinction between "well below 2 °C" and "pursuing 1.5 °C", and it is the direct bridge to the management lesson, where the question becomes how — and whether — these pathways can be altered.

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Sea-level rise: a worked illustration of committed and projected change

Sea-level rise is the consequence that most clearly exposes the difference between projected and committed change. AR6 projects global mean sea-level rise by 2100 of roughly 0.3–0.6 m under low emissions (SSP1-2.6) and 0.6–1.0 m under very high emissions (SSP5-8.5), with the upper end higher still if ice-sheet instabilities engage. But the committed rise extends far beyond 2100: because ice sheets respond over centuries to millennia (their long residence time, from Lesson 1), even holding warming at today's level locks in metres of eventual rise.