Evidence for Past Climate Change: Ice Cores and Tree Rings

Evidence for past climate change comes from proxies, natural records such as ice cores, tree rings, sediments, pollen, corals and glaciers that preserve information about temperature, rainfall and atmospheric composition long before instruments were used. In brief: ice cores drilled in Antarctica and Greenland contain trapped air bubbles that reveal past carbon dioxide levels, and their oxygen isotopes show temperature, going back about 800,000 years. Tree rings record yearly growth conditions for thousands of years. These records show that the climate has always changed, but that today’s carbon dioxide level and rate of warming are unusual.

Reconstructing past climates gives context for the warming of today and helps to test climate models. This guide explains how ice cores, tree rings and other proxies work, what they show about ice ages and recent history, compares the main proxies in a table, works through calculations, outlines the history of the field and corrects common mistakes.

What do ice cores show?

In the polar ice sheets, snow falls year after year and is compressed into ice that keeps a layered record. Scientists drill long cylinders of ice, called cores, and analyze them in the laboratory.

• Trapped air: as the snow turns to ice, small bubbles of the atmosphere are sealed in. Measuring the carbon dioxide and methane in them gives the composition of the air from the past.
• Isotopes: water molecules contain oxygen-16 and the heavier oxygen-18 (and hydrogen-2, deuterium). The ratio in the snow depends on the temperature when it fell, so the ice provides a thermometer.
• Layers and impurities: annual layers can be counted in the upper part, as with tree rings. Dust, volcanic ash, sea salt and sulfate indicate winds, aridity and eruptions.

The Vostok core in Antarctica, drilled in the 1980s and 1990s, reached back about 420,000 years, and the EPICA Dome C core extended the record to about 800,000 years. The deeper the ice, the older it is, but the layers become thin and compressed, so time resolution becomes poorer.

What do tree rings show?

Dendrochronology is the study of tree rings. In regions with distinct seasons, a tree adds one ring each year. A wide ring generally indicates a favorable year with good growth, and a narrow ring indicates cold, drought or stress, depending on what limits the growth where the tree lives. Trees near the tree line or in arid areas respond most clearly to temperature or rainfall. By matching ring patterns from living trees with those from dead wood, called cross-dating, scientists build continuous records that extend back thousands of years, for example from bristlecone pines in California (more than 4,000 years in individual living trees) and oak chronologies in Europe going back over 10,000 years. Rings also provide absolute dates for archaeology and for events such as volcanic eruptions that caused cold years. Wood density and chemical isotopes in the rings add further information.

What other evidence is there?

• Ocean and lake sediments: the shells of tiny animals (foraminifera) hold oxygen isotopes that reflect the temperature and ice volume, and layers of sediment, called varves, can be counted.
• Pollen: grains preserved in peat and lake mud show which plants lived in the area and thus the climate.
• Corals: growth bands and chemistry record sea temperature in the tropics.
• Glacial landforms: moraines and erratic boulders show where ice extended in the past.
• Historical records and fossils: harvest dates, diaries, ship logs, paintings and ancient fossils show conditions in earlier times.
• Boreholes and stalagmites: ground temperature profiles and cave deposits.
• Instrumental records: thermometers from the 1700s and 1800s, and direct measurements of carbon dioxide at Mauna Loa since 1958.

What do the records show?

Over the last 800,000 years, ice cores show cycles of cold glacial periods about 100,000 years long, separated by shorter warm interglacial periods, with carbon dioxide rising and falling in step with temperature, between about 180 ppm (parts per million) during ice ages and about 280 ppm in interglacials. These cycles are paced by regular changes in Earth’s orbit, known as Milankovitch cycles, with periods of about 100,000 years (shape of the orbit), 41,000 years (axial tilt) and 23,000 years (wobble). The changes in orbit start the cycles, and the carbon dioxide and the ice-albedo feedback amplify them.

For the last thousand years, tree rings and other proxies show regional warm and cool periods, such as the Medieval Climate Anomaly (roughly 900–1300) and the Little Ice Age (roughly 1300–1850), though these were less uniform across the globe than the present warming. Since 1958, direct measurements have shown that carbon dioxide has risen from about 315 ppm to over 420 ppm, which is far above the 800,000-year range of 180–300 ppm and has risen much faster than at any time in the ice core record.

Proxies compared

Worked example: reading the records

The numbers are hypothetical or simplified and show the logic of the methods.

1. Counting tree rings: a cut stump has 85 rings. If the tree was felled in 2024, the oldest ring formed in 2024 − 85 + 1 = 1940, so it germinated around 1940.
2. Interpreting ring widths: rings in a pine are 2.1 mm wide in a warm year, 0.6 mm in a cold year. The warm-year ring is 2.1 ÷ 0.6 = 3.5 times wider.
3. Ice core age: suppose that the annual layers near the surface are 25 cm thick. If the layers kept this thickness, a depth of 500 m would contain 500 ÷ 0.25 = 2,000 layers and be about 2,000 years old. In reality, layers become thinner with depth because of the weight of the ice above, so more years fit into the same depth, and the true age at 500 m is greater than 2,000 years. Glaciologists use flow models to correct for the thinning.
4. CO₂ change: the pre-industrial concentration was about 280 ppm and today it is above 420 ppm. The increase is 420 − 280 = 140 ppm, or 140 ÷ 280 = 50%.
5. Glacial-interglacial range: the carbon dioxide level varied between 180 ppm and 280 ppm over 800,000 years, a range of 100 ppm. In the last 170 years, the increase of about 140 ppm is larger than that whole natural range.
6. Carbon-14 dating: carbon-14 has a half-life of 5,730 years. A wood sample with 25% of the original carbon-14 has gone through two half-lives, so its age is 2 × 5,730 = 11,460 years.

How did paleoclimate research develop?

In the 1830s and 1840s, Louis Agassiz proposed that large ice sheets had once covered much of Europe. A. E. Douglass developed tree-ring dating in the early 20th century in Arizona, first to study sunspot effects on climate. Milutin Milanković calculated the effect of orbital changes on sunshine in the 1920s and 1930s, and ocean sediment records confirmed these cycles in 1976 (Hays, Imbrie and Shackleton). Willi Dansgaard and others began analyzing isotopes in Greenland ice in the 1950s and 1960s. The Vostok core in 1987 showed the close link between carbon dioxide and temperature, and the EPICA core in 2004 extended it to about 800,000 years. Further work with many proxies allowed reconstructions of global temperature over the last 2,000 years.

Which mistakes are common?

Mistake 1: Saying that past changes prove that today’s warming is natural

Past changes show that the climate is sensitive to forcing, such as orbital changes or greenhouse gases. The causes of today’s warming are identified from measurements of gases, isotopes and energy flows, not from the fact that climate has changed before.

Mistake 2: Believing that a tree ring shows only temperature

A ring responds to temperature, rainfall, sunlight and disease, depending on the site. Scientists select trees from places where a single factor dominates and calibrate them with instrumental data.

Mistake 3: Assuming that ice cores show current CO₂

Ice cores preserve air from the past, and the modern level is measured directly in the atmosphere, for example at Mauna Loa.

Mistake 4: Mixing up the order of cause and effect in ice ages

In the natural ice-age cycles, orbital changes began the warming and CO₂ amplified it. Today, extra CO₂ from human activity is the driver.