Reading Past Climates: Ice Cores, Tree Rings, Lake Cores and Fossils

Let's start on the Niagara Escarpment

Clinging to the cliffs of the Niagara Escarpment in southern Ontario are small, twisted eastern white cedars. They don't look impressive, but researchers from the University of Guelph found that some of them are more than 1000 years old, among the oldest trees in eastern North America. Inside each one is a ring for every year it has lived, and every ring is a tiny weather report.

Thermometer records only go back about 150 years in most places. To know what the climate was like before that, and to decide whether today's changes are unusual, scientists read natural archives: trees, ice, lake mud, ocean sediments and fossils. This lesson shows you how.

What is a proxy?

A proxy is something that stands in for a direct measurement. Nobody measured the temperature 10 000 years ago, but the plants that grew then, the chemistry of the snow that fell then, and the width of the tree rings that formed then all depend on the climate. By measuring them, scientists can reconstruct past climate.

Good proxy science always:

1. Calibrates the proxy against modern measurements (for example, comparing tree-ring widths with thermometer records for the last 100 years).
2. Dates the record accurately (by counting layers, radiocarbon dating, or matching known events such as volcanic eruptions).
3. Combines many independent proxies, because each one has limits.

Ice cores

On ice sheets in Antarctica and Greenland, and on ice caps in Canada's High Arctic, snow builds up year after year and is slowly squeezed into ice. Scientists drill out long cylinders called ice cores.

• Layers: near the top, yearly layers can be counted like tree rings.
• Air bubbles: as snow turns to ice, it traps tiny bubbles of air, real samples of the ancient atmosphere. Scientists measure their CO₂ and methane directly.
• Isotopes: the ratio of heavy to light isotopes of oxygen and hydrogen in the ice depends on the temperature when the snow fell, giving a temperature record.
• Dust and ash: more dust means drier, windier times; volcanic ash layers mark known eruptions and help with dating.

The longest continuous ice-core record, from Antarctica, goes back about 800 000 years. It shows CO₂ and temperature rising and falling together through eight or so ice-age cycles.

Tree rings

Each year a tree in a seasonal climate adds a layer of wood: light, fast-growing wood in spring and denser, darker wood later in summer. Together they form one annual ring.

• Wide rings usually mean good growing conditions (enough warmth and water).
• Narrow rings mean stress: drought, cold, insect attacks.
• Fire scars record past forest fires.
• Density of the late-summer wood is often closely linked to summer temperature in northern forests.

By cross-dating (matching the pattern of wide and narrow rings between living trees, old buildings, and logs preserved in lakes and bogs), scientists can build records thousands of years long. Tree rings give year-by-year detail, but they mostly record conditions during the growing season and only where trees grow.

Lake sediment cores

Every year, sediment settles to the bottom of lakes: mineral grains washed in by spring runoff, plus organic material, pollen and tiny shells. Scientists push a tube into the lake bottom to pull up a sediment core.

• Varves: in some lakes, each year leaves a light layer (spring and summer) and a dark layer (winter), so the core can be counted year by year.
• Pollen: pollen grains from surrounding plants are tough and well preserved. Lots of spruce pollen means a cold, boreal climate; lots of oak, maple and beech pollen means a warmer climate. Ontario lake cores show tundra and spruce forest after the ice sheet melted, gradually replaced by today's forests as the climate warmed.
• Charcoal records past fires; diatoms (algae with glass shells) and midge larvae record past water temperature and chemistry.

Crawford Lake, in a conservation area near Milton, Ontario, is unusually deep for its size, and its bottom water doesn't mix, so its varves are undisturbed. Pollen in its sediments records corn fields planted by an Iroquoian village that stood nearby several hundred years ago; the conservation area includes a reconstructed longhouse village. In 2023, a scientific working group chose Crawford Lake's sediments as the proposed reference site to mark the start of a new geological epoch, the Anthropocene, because they clearly record human impacts from the 1950s, such as plutonium from nuclear weapons tests and particles from burning fossil fuels. In 2024, a vote by a committee of the International Union of Geological Sciences rejected making the Anthropocene a formal epoch, so the debate continues. Either way, the lake's mud shows how visible human influence has become.

Fossils and preserved organisms

• Ocean sediment cores contain the shells of tiny organisms called foraminifera. The oxygen isotope ratio in their shells records ocean temperature and how much water was locked in ice sheets, giving records millions of years long.
• Plant and insect fossils: beetles are useful because many species live only within narrow temperature ranges. Finding fossils of a cold-adapted beetle in an Ontario sediment layer tells you the climate was much colder then.
• Corals grow yearly bands like tree rings and record tropical ocean temperatures.
• Preserved organisms in permafrost: in the Klondike region of Yukon, frozen ground has preserved Ice Age animals and plants for tens of thousands of years. In 2016, gold miners found a mummified wolf pup, about 57 000 years old, on the traditional territory of the Tr'ondëk Hwëch'in, who named her Zhùr. Remains like these show which animals and plants lived there and what the climate was like.

Other archives: historical documents (ship logs, diaries, Hudson's Bay Company records of freeze-up and break-up dates) and the knowledge held by Indigenous communities about long-term changes in ice, animals and seasons add detail that instruments missed.

What proxies show: natural change and human influence

Taken together, proxies show that:

• Climate has changed naturally many times: ice ages and warm periods, paced by Milankovitch cycles (Lesson 9); shorter events linked to volcanoes and the Sun.
• During natural changes, CO₂ and temperature rose and fell together, and CO₂ stayed between about 180 and 300 ppm for at least 800 000 years.
• Since the Industrial Revolution, CO₂ has risen to over 420 ppm, far outside the natural range, and much faster than in any natural change recorded in ice cores.
• Reconstructions combining hundreds of tree-ring, coral, ice and sediment records show that recent global temperatures are higher than at any time in at least the last 2000 years, and the warming is happening faster.

This is how proxies help separate natural variation from human influence.