Climate change

The Quaternary is the most recent geological period, covering approximately the last 2.6 million years up to the present. It has been characterised by repeated cycles of glacials (cold periods) and interglacials (warmer periods).

We are currently living in an interglacial — the Holocene — which began about 11,700 years ago.

Three sources (other than ice cores) are:

• Tree rings (dendrochronology) — the width of each annual growth ring records the growing conditions (warmth and moisture) of that year.
• Ocean-sediment cores — the shells of tiny marine organisms (foraminifera) preserved in the layers, and their oxygen-isotope ratios, record past ocean temperature.
• Pollen analysis — pollen grains preserved in peat and lake sediments show which plant species were growing, and hence the climate they lived in.

(Historical records — harvest dates, ship's logs, paintings of frozen rivers — would also be credited.)

Pollen grains have tough, resistant outer walls, so they are preserved for thousands of years in oxygen-poor sediments such as peat bogs and lake beds, building up in dated layers.

Because the pollen of different plant species has a distinctive shape, scientists can identify which species were growing at each level in the sediment. As each species is suited to particular climatic conditions (for example, certain trees indicate a warm climate while grasses or tundra species indicate cold conditions), the changing mix of pollen up through the layers shows how the vegetation, and therefore the climate, changed over time.

For example, a shift from tree pollen to grass and sedge pollen indicates a change from a warmer, forested climate to a colder, more open (glacial) climate, providing clear proxy evidence of climate change.

Overall pattern. Both temperature and CO₂ show a cyclical (oscillating) pattern rather than a steady trend, with peaks recurring approximately every 100,000 years — this matches the timing of the glacial–interglacial cycles.

Manipulating the data. Temperature varies over a range of about 10°C (from roughly −8°C during glacials to +2°C during interglacials), while CO₂ varies by about 100 ppm (from ~180 ppm in glacials to ~280 ppm in interglacials).

The relationship. The two traces are positively correlated and move almost in step: when temperature is high (interglacials) CO₂ is high (~280 ppm), and when temperature is low (glacials) CO₂ is low (~180 ppm). Every warm peak in temperature is matched by a peak in CO₂, and every cold trough by a CO₂ minimum.

Interpretation and a note of caution. This close link is strong evidence that temperature and greenhouse-gas concentrations changed together naturally through the Quaternary. However, a correlation on its own does not prove which caused which — in these natural cycles warming (triggered by orbital cycles) and rising CO₂ reinforced one another through feedbacks. The record clearly shows, though, that climate has changed repeatedly and naturally in the past.

Milankovitch cycles are regular, long-term variations in the Earth's orbit and axis that change the amount and distribution of solar radiation (insolation) reaching the Earth.

Eccentricity (~100,000 years). The shape of the Earth's orbit changes from more circular to more elliptical and back over about 100,000 years, altering the total insolation received at different points in the orbit. This ~100,000-year rhythm closely matches the timing of glacial–interglacial cycles in the ice-core record.

Obliquity / axial tilt (~41,000 years). The tilt of the Earth's axis varies between about 21.8° and 24.4° over roughly 41,000 years. A smaller tilt gives milder summers at high latitudes, so winter snow is less likely to melt and can build up into ice sheets.

Precession (~26,000 years). The Earth's axis wobbles over about 26,000 years, changing which hemisphere faces the Sun when the Earth is closest to it, and so altering the intensity of the seasons.

Triggering glaciation. When these cycles combine to produce cool northern-hemisphere summers, snow survives the summer and accumulates. The fresh snow and ice have a high albedo, reflecting more sunlight and causing further cooling — a positive feedback that grows the ice sheets and pushes the Earth into a glacial. When the cycles later favour warmer summers, the ice melts and an interglacial begins.

Milankovitch cycles therefore act as the natural 'pacemaker' of the Quaternary's glacial–interglacial rhythm.

Before thermometers (instrumental records begin around 1850), climate must be reconstructed from proxy (indirect) data such as ice cores, tree rings and ocean sediments. These are valuable but each has strengths and limitations, so their reliability must be judged.

Ice cores — strong but geographically limited. Ice cores from Antarctica extend back about 800,000 years, giving trapped air bubbles (direct samples of past CO₂ and methane) and an oxygen-isotope temperature record. They are highly valued because the air bubbles are direct physical samples of the ancient atmosphere. However, they only exist where thick ice has survived (polar regions and high mountains), so they represent high-latitude conditions, not the whole planet, and dating deep, compressed layers becomes less precise with depth.

Tree rings — precise but short and regional. Dendrochronology gives annually-resolved, precisely-dated evidence, and overlapping timbers extend the record back several thousand years. It is excellent for detecting events such as the Little Ice Age. Its limitations are that trees only grow in certain climates, the record is comparatively short, and ring width can be affected by non-climatic factors (disease, competition, soil), which can blur the climate signal.

Ocean-sediment cores — very long but low-resolution. Foraminifera in sediment cores extend the record back millions of years, far beyond ice or trees. But sediments accumulate slowly, so the resolution is coarse (thousands of years, not single years), and disturbance of the layers can reduce accuracy.

Judgement. No single proxy is completely reliable: each is limited in coverage, timescale or resolution. However, reliability is greatly increased when several independent proxies agree — for example ice cores, tree rings and sediments all showing the same warm and cold periods. This cross-checking means that, used together, proxy data give a broadly reliable picture of past climate, even if any one source alone is uncertain.

Over the Quaternary period — the last 2.6 million years — the Earth's climate has repeatedly alternated between cold glacials and warmer interglacials, and we are now in an interglacial (the Holocene). A wealth of proxy (indirect) evidence demonstrates that these changes happened long before human industrial activity, showing that climate has changed naturally in the past. This answer examines the main lines of evidence and weighs how convincing they are.

Ice cores provide perhaps the strongest evidence. Cores drilled from the Antarctic ice sheet reach back about 800,000 years. Each annual layer traps bubbles of ancient air, giving a direct measurement of past CO₂ and methane concentrations, while the oxygen-isotope ratio (¹⁸O/¹⁶O) of the ice records past temperature. The records show CO₂ oscillating between roughly 180 ppm (glacials) and 280 ppm (interglacials) and temperature swinging by around 10°C, with peaks recurring about every 100,000 years. Because the atmosphere-sampling bubbles are direct physical evidence, ice cores are highly convincing; their main limitation is that they only exist in polar and high-mountain regions.

Tree rings (dendrochronology) give precisely-dated, annual evidence over the last few thousand years. Wide rings indicate warm, wet years and narrow rings cold or dry years, allowing scientists to detect events such as the cold decades of the Little Ice Age (roughly 1300–1850). This evidence is strong because it is annually resolved and can be cross-matched between many trees, though the record is comparatively short and ring width can be affected by non-climatic factors.

Ocean-sediment cores extend the picture much further back — millions of years. The shells of foraminifera and their oxygen-isotope ratios record past ocean temperatures and ice volume. Their strength is their enormous time-span; their weakness is low resolution, because sediment accumulates slowly.

Pollen analysis and historical records add further support. Pollen preserved in peat and lake sediments shows shifting vegetation — for example a change from tree pollen to tundra species marks a move into colder conditions. Historical evidence, such as records of the Thames freezing during frost fairs, vineyard and harvest dates, and paintings, documents the Little Ice Age and the earlier Medieval Warm Period. Physical evidence such as retreating glaciers and the landforms they left behind also record past ice advance and retreat.

Weighing the evidence. No single source is decisive on its own — ice cores are geographically limited, tree rings are short, sediments are coarse, and historical records are patchy and subjective. However, the evidence is powerful precisely because these independent proxies agree: ice cores, sediments, pollen and historical records all point to the same warm and cold periods and to a regular, cyclical pattern that matches the Milankovitch orbital cycles. This consistency makes the case very strong.

Conclusion. Taken together, the proxy evidence overwhelmingly shows that climate has changed naturally and repeatedly throughout the Quaternary, driven by natural causes such as orbital cycles, solar variation and volcanic activity. The reliability comes not from any one proxy but from the way multiple, independent records corroborate one another. This natural variability is the essential baseline against which the much more rapid, recent warming — widely attributed to human activity — must be judged.

Climate change during the Quaternary has several natural causes, operating over very different timescales. Assessing their relative importance means judging which best explains the observed pattern of glacials and interglacials.

Milankovitch (orbital) cycles — the main long-term driver. The most important cause of the long-term, cyclical glacial–interglacial rhythm is the set of Milankovitch cycles: eccentricity (~100,000 years), obliquity (axial tilt 21.8–24.4°, ~41,000 years) and precession (~26,000 years). These change the amount and distribution of insolation; when they produce cool northern summers, snow accumulates and the albedo feedback amplifies the cooling into a full glacial. Crucially, the ~100,000-year eccentricity cycle matches the timing of the glacial cycles in the ice-core record, so orbital forcing is widely regarded as the fundamental pacemaker of Quaternary climate.

Variations in solar output — important on shorter timescales. The Sun's output varies with the ~11-year sunspot cycle and over longer periods. Reduced output during the Maunder Minimum (about 1645–1715), when sunspots almost vanished, coincided with the coldest part of the Little Ice Age. Solar variation is therefore significant for decadal-to-centennial changes, but the changes in output are relatively small, so it is a secondary rather than primary driver.

Volcanic eruptions — significant but short-lived. Large eruptions inject SO₂ into the stratosphere, forming aerosols that reflect sunlight and cause short-term global cooling. After Mount Pinatubo (1991) global temperatures fell about 0.5°C for a year, and Tambora (1815) caused the "year without a summer" in 1816. These effects are dramatic but last only a few years, so volcanism causes short-term perturbations rather than the long-term trend.

Other causes. Changes in ocean circulation (e.g. the thermohaline conveyor) can redistribute heat and amplify change, and rare asteroid impacts can throw dust into the atmosphere and cause cooling, but these are either amplifying feedbacks or very infrequent events.

Judgement. The causes are not equally important, and their importance depends on timescale. Over the long term of the Quaternary, Milankovitch cycles are the dominant cause, setting the fundamental rhythm of glacials and interglacials. Solar and volcanic factors are important for shorter-term fluctuations superimposed on that rhythm — for instance the Little Ice Age — while ocean circulation and feedbacks such as albedo and CO₂ release amplify the orbital trigger. The most convincing view is that no cause acts alone: orbital cycles provide the trigger, and the other factors and feedbacks determine its magnitude and shorter-term detail.

The Quaternary period (the last ~2.6 million years) has swung between cold glacials and warm interglacials roughly every 100,000 years, and we now live in an interglacial, the Holocene. Milankovitch cycles are widely regarded as the fundamental cause of this rhythm, but the question of whether they explain it alone requires the role of feedbacks and of other natural causes to be weighed against them.

The strong case for Milankovitch cycles. Milankovitch cycles are regular variations in the Earth's orbit and axis that change the amount and distribution of insolation: eccentricity (orbit shape, ~100,000 years), obliquity (axial tilt 21.8–24.4°, ~41,000 years) and precession (axial wobble, ~26,000 years). The most compelling evidence is the match in timing: the ~100,000-year eccentricity cycle closely mirrors the spacing of glacials seen in the ice-core and ocean-sediment records, and the ~41,000-year obliquity signal appears clearly in earlier parts of the Quaternary. This correspondence is why orbital forcing is described as the pacemaker of the ice ages, and to a large extent the cycles do explain when glacials begin and end.

Why Milankovitch cycles are not a complete explanation on their own. The changes in total insolation produced by the orbital cycles are too small to produce the ~10°C temperature swings recorded in the ice cores. The full change is only reached because the orbital trigger is amplified by feedbacks:

• The albedo feedback — cool northern summers let winter snow survive; fresh snow and ice reflect more sunlight (high albedo), causing further cooling and more ice growth.
• The greenhouse-gas feedback — as the oceans cool they absorb more CO₂, and warming releases it; ice cores show CO₂ falling to ~180 ppm in glacials and rising to ~280 ppm in interglacials in step with temperature, reinforcing the change.

Without these feedbacks, the modest orbital forcing could not generate a full glacial, so Milankovitch cycles are best seen as a trigger rather than the whole mechanism.

Other natural causes add further detail. Superimposed on the orbital rhythm are shorter-term causes that the cycles cannot explain. Variations in solar output — for example the Maunder Minimum (~1645–1715), a period of very few sunspots — help explain colder spells such as the Little Ice Age. Volcanic eruptions inject sulfate aerosols that reflect sunlight and cause short-term cooling, as after Pinatubo (1991) (~0.5°C) and Tambora (1815). Shifts in ocean (thermohaline) circulation also redistribute heat and can amplify or moderate change. These operate over decades to centuries — far shorter than the orbital cycles — and account for fluctuations that Milankovitch cycles alone do not.

Judgement. To a large extent, Milankovitch cycles explain the long-term, ~100,000-year rhythm of the Quaternary glacial–interglacial cycles, and no other cause matches that timing so precisely. However, they cannot explain it alone: the size of the temperature change depends on albedo and greenhouse-gas feedbacks that amplify the orbital trigger, while solar and volcanic factors and ocean circulation explain shorter-term variations layered on top. The most convincing view is therefore that Milankovitch cycles are the primary driver and pacemaker, but the observed pattern is the product of orbital forcing working together with feedbacks and secondary causes.