How do scientists know temperatures from millions of years ago?

Scientists use climate proxies, indirect natural archives that record the temperature conditions of the time they formed. The main proxies are ice cores from Greenland and Antarctica, tree rings, pollen grains, fossil leaves, coral skeletons, cave deposits (speleothems), and the chemistry of lake and ocean sediments. Combining several proxies for the same period gives a calibrated estimate of past temperatures.

How far back can we estimate Earth's temperature?

Direct instrumental temperature records go back about 175 years (to the 1850s). Tree rings extend a few thousand years, ice cores from Antarctica reach around 800,000 years (with newer EPICA Beyond cores recovering ice estimated at ~1.2 million years), and ocean-sediment foraminifera let scientists reconstruct temperatures back tens of millions of years, into the age of the dinosaurs.

What is a climate proxy?

A climate proxy is a preserved physical or chemical feature of the natural environment whose properties depend on the climate at the time it formed. Examples include the oxygen-isotope ratio in ice cores, the width of tree rings, the species mix in pollen samples, the shape and stomatal density of fossil leaves, and the shells of marine plankton in seafloor sediments.

How do ice cores reveal past temperatures?

Each annual snowfall locks in a chemical fingerprint of the temperature at which it fell, most importantly the ratio of oxygen-18 to oxygen-16 (delta-18O) and of deuterium to ordinary hydrogen. Colder snow contains relatively less of the heavy isotopes. Tiny trapped air bubbles also preserve samples of the actual ancient atmosphere, including its CO2 and methane content.

Are these paleotemperature estimates accurate?

They are not perfect, but they are calibrated. Each proxy has known uncertainties, and scientists cross-check independent proxies (e.g., ice-core isotopes against ocean-sediment foraminifera against tree-ring widths) to make sure they agree. The further back you go, the larger the error bars, but the broad temperature history of Earth over hundreds of millions of years is well established.

How Do We Know The Temperature On Earth Millions Of Years Ago?

Table of Contents (click to expand)

Climate Proxies
Ice Sheets
Tree Rings
Pollen Grains
Fossil Leaves
Lake And Ocean Sediments

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Researchers estimate ancient temperatures using data from climate proxy records, i.e., indirect methods to measure temperature through natural archives, such as coral skeletons, tree rings, glacial ice cores and so on.

Finding out what the temperature is is ridiculously simple these days. However, ascertaining temperature was more challenging two centuries ago. The modern instrumental temperature record only stretches back about 175 years (to the 1850s), which is fine for tracking recent warming, but useless if you want to know what the climate was doing in the time of the dinosaurs.

In that case, how do scientists and researchers discuss the climatic conditions thousands or even millions of years ago? How can they tell what the temperatures of Earth were in the ancient past?

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How Do We Know Temperatures from Thousands of Years Ago?

Global temperature in present times is estimated by utilizing specialized thermometers installed on ships, buoys, and several weather stations that are operational worldwide.

However, determining the temperature of previous centuries is more complex than it seems due to our limited resources to ascertain past weather conditions conclusively with any degree of certainty.

Climate Proxies

In order to deduce the ancient temperatures of this planet, scientists rely on a number of indirect methods and techniques called climate proxies. (Source)

temperature trend — The global temperature variation over the last 2000 years (Image Credit: Wikimedia Commons)

These are preserved physical characteristics of the ancient past that help scientists estimate the corresponding weather conditions of that particular era. Since reliable records of the Earth’s historical temperatures only began to appear in the 1860s, climate proxies are the only way researchers can estimate our planet’s weather conditions before that era.

Some common examples of climate proxies are rocks, ice cores, tree rings, fossils, lake and sea sediments etc. They act as “natural climatic archives” as they contain imprints of the ambient temperature conditions on them.

Ice Sheets

The two great ice sheets in the polar regions, Greenland and Antarctica (plus high-altitude tropical glaciers like Peru's Quelccaya), can provide us with valuable information about historical temperatures. Each year, snowfall forms a new layer of ice on top of the previous one. These layers are preserved and, when drilled out as a long cylindrical "ice core," they can be counted and dated almost like tree rings.

Snow that fell at different temperatures has subtly different chemical signatures. Specifically, the ratio of heavy oxygen-18 to ordinary oxygen-16 in the ice (δ¹⁸O) shifts with the temperature at which the snow originally fell — colder air precipitates relatively less of the heavier isotope. Tiny air bubbles trapped between the snowflakes also lock in samples of the actual ancient atmosphere, letting scientists measure past concentrations of CO₂ and methane directly. The deepest cores from East Antarctica, drilled by the European EPICA project, have produced a continuous climate record stretching back about 800,000 years, and a successor project announced in 2025 has already recovered ice estimated to be around 1.2 million years old.

Tree Rings

The rings visible in a horizontal cross-section cut through the trunk of a tree are commonly called tree rings. It’s interesting to note that tree rings can be wider or narrower depending on the existing climatic conditions when the tree was growing.

Therefore, fossils of trees can help scientists estimate the trends of changing weather conditions.

Pollen Grains

The best thing about the pollen produced by plants is that it can help identify the parent plant species and is highly resistant to decomposition.

Since pollen production largely depends on the existing weather conditions, their abundance or absolute absence in certain geographical regions can help us establish how warm/cold those areas were in the past.

Fossil Leaves

leaf-stoma — An ultra-magnified image of a stoma on the leaf of a tomato plant (Image Credit: Photohound / Wikimedia Commons)

The carbon dioxide content of the ancient atmosphere can be determined by studying fossil leaves’ isotope composition and stomata (tiny pores found in leaves and stems that assist in gas exchange).

Lake And Ocean Sediments

Deep sediments that are found at the bottom of water bodies, such as lakes and oceans, are a great source of knowledge regarding ancient temperatures. The most important of those sediments are the layers formed by the shells of small, surface-living animals that are deposited over millions of years.

Microfossils from marine sediments (Image Credit: Hannes Grobe / Wikimedia Commons)

Scientists examine the oxygen isotopes present in these sediments, which gives us some solid quantitative information about the weather conditions dating back to the age of the dinosaurs!

And there are still other proxies we haven't even covered, like coral skeletons (whose Sr/Ca ratios and oxygen isotopes record sea surface temperatures going back hundreds of thousands of years), cave deposits called speleothems, and the boron-isotope chemistry of fossilized plankton. Although none of these techniques give an absolute, perfect-thermometer reading, together they provide enough overlapping information for scientists to make pretty well-calibrated guesses about the climate of the planet millions of years ago.

References (click to expand)