Thermometers have only been read carefully for about 170 years. To see how climate behaved before that, scientists turn to nature's own archives. The study of past climates is called paleoclimatology, and it offers something models and short records cannot: real-world tests of how the planet responded when its climate changed before.

Nature's record keepers

Many natural materials record the conditions in which they formed. These stand-ins for direct measurements are called proxies.

• Tree rings grow wider in good years and narrower in poor ones, recording temperature and rainfall for centuries, sometimes millennia.
• Corals and cave formations lay down yearly layers whose chemistry tracks ocean temperature or rainfall.
• Ocean sediments hold the shells of tiny plankton called foraminifera. The ratio of heavy to light oxygen in their shells reveals ocean temperatures and how much water was locked up in ice, stretching back tens of millions of years.
• Pollen in lake mud shows which plants grew nearby, and so what the climate allowed.

Ice cores: bottled ancient air

The most famous archive is ice. In Greenland and Antarctica, snow piles up year after year and compresses into ice, trapping tiny bubbles of air. Drill a core several kilometers deep and you can measure the actual atmosphere of the past. The chemistry of the ice itself acts as a thermometer, because the proportion of heavy hydrogen and oxygen in snow depends on how cold it was. The European EPICA core from Dome C in Antarctica reaches back about 800,000 years.

The cores show a striking pattern. Over that span, Earth swung through eight or so ice ages separated by short warm periods. CO₂ rose and fell in near lockstep with temperature, ranging from about 180 ppm in the cold depths to about 280 ppm in the warm periods. Today's level, above 420 ppm, lies far outside that entire range.

What paces the ice ages

In the early twentieth century the Serbian engineer Milutin Milanković spent decades calculating, by hand, how slow changes in Earth's orbit alter the sunlight reaching high northern latitudes in summer. Earth's orbit stretches and rounds over about 100,000 years, its tilt shifts over about 41,000 years, and its axis wobbles over roughly 23,000 years. In 1976 a landmark study of seafloor sediments found these same rhythms in the climate record, confirming that orbital changes are the pacemaker of the ice ages.

But the orbital changes alone are too small to explain such large swings. They act as a trigger. As northern summers warm, ice sheets shrink and reflect less sunlight, and the warming ocean releases CO₂, which warms the planet further. In Antarctic records, CO₂ rises slightly after local temperature at the start of a warming. Some people cite this as proof that CO₂ does not drive temperature. In fact it shows CO₂ working as an amplifier. Studies combining records from around the world find that, averaged globally, temperature rose mostly after CO₂ during the last deglaciation. Today the order is different again: we are adding the CO₂ first.

A colder world, and a warmer one

At the height of the last ice age, about 20,000 years ago, global average temperature was roughly 6°C colder than before industry. Ice sheets up to several kilometers thick covered much of North America and northern Europe, and sea level was about 120 meters lower. A change of a few degrees in the global average means a different world.

Going further back, about three million years ago in the Pliocene, CO₂ was similar to today's levels, global temperatures were roughly 2.5 to 4°C warmer than pre-industrial, and sea level was many meters higher. About 56 million years ago, during the Paleocene-Eocene Thermal Maximum, a huge release of carbon warmed the planet by around 5 to 8°C, acidified the oceans and drove many deep-sea species extinct. Recovery took over a hundred thousand years. Scientists estimate today's carbon is being released around ten times faster than during that event.

The stable stage for civilization

For the past 11,700 years, an unusually steady warm period called the Holocene has framed all of recorded history, including the invention of farming and the growth of cities. Proxy reconstructions show that the warming of recent decades is faster than any global change in at least the last 2,000 years.

Worked example: reading an ice core chart

Imagine a chart from an Antarctic core with two lines: temperature and CO₂, over 400,000 years.

• Look at the shape: both lines show a sawtooth, with slow slides into cold and rapid climbs into warmth, about every 100,000 years.
• Compare the lines: they rise and fall together, which tells you CO₂ and temperature are tightly coupled.
• Check the scale: CO₂ never leaves the range of about 180 to 280 ppm. Mark today's 420 ppm and it sits far above the top of the chart.
• Interpret carefully: in the past, orbits started the change and CO₂ amplified it. The chart shows how sensitive the system is, not that CO₂ is harmless.

The past cannot tell us exactly what will happen next, but it shows that Earth's climate responds strongly to modest pushes, and that carbon has always been part of the story.