How We Know What Earth’s Climate Was Like Long Ago

Part Three

In Part One, we explored how scientists measure climate change right here on Earth’s surface.
In Part Two, we looked at how satellites in space track changes across the whole planet.

Now comes Part Three—one of the coolest parts of climate science:
How scientists learn about Earth’s climate long before thermometers and satellites existed.

So how do we know what Earth’s climate was like hundreds, thousands, or even millions of years ago?

The answer: Earth keeps records of its own past. Scientists call this paleoclimate data—natural clues that reveal what the climate used to be like.

Ice Cores: Climate Time Capsules

One of the most powerful climate records comes from ice cores.

In places like Antarctica and Greenland, snow falls year after year and slowly gets packed into ice. Over time, this creates layers, much like a layer cake. Scientists drill deep into the ice and pull out long cylinders called ice cores.

Each layer holds clues, including:

• Tiny bubbles of ancient air
• Past levels of carbon dioxide (CO₂)
• Signs of temperature changes

In Greenland, ice cores go back about 400,000 years. In Antarctica, they reach back 800,000 years! These ice cores reveal a clear pattern: when carbon dioxide (CO₂) levels go up, temperatures go up too.

Long ago, small changes in Earth’s orbit started warming the oceans. As the oceans warmed, they released CO₂ into the air. That extra CO₂ trapped more heat and caused most of the warming.

Today, the story is different. Humans are adding CO₂ directly to the atmosphere, and that CO₂ is now the main driver of climate change.

Tree Rings: Nature’s Climate Diary

Trees are another amazing climate recorder.

Most trees grow one ring per year.

• Wide rings usually mean warm, wet years
• Narrow rings often mean cold or dry years

By studying tree rings, scientists can learn about:

• Droughts
• Heat waves
• Rainfall patterns

Some trees live for thousands of years, and overlapping records from many trees help scientists build climate histories stretching far into the past.

For hundreds of years, tree rings told a clear story. In many places—especially cold, northern regions—warmer years usually meant thicker and denser tree rings. When scientists compared tree rings to temperature records, they matched really well.

But something unusual started happening in the mid-1900s.

Even though temperatures kept rising, tree growth in some areas slowed down or even declined. The tree rings stopped keeping up with the warming climate.

Scientists call this the “divergence problem” because the tree-ring record starts to diverge, or go in a different direction, from actual measured temperatures.

This doesn’t mean tree rings are unreliable. Instead, it tells scientists that today’s rapid climate change is affecting trees in new ways, such as heat stress, drought, air pollution, or changing seasons—things trees didn’t face in the same way in the past.

Ocean Mud and Lake Sediments: Clues From Underwater

At the bottom of oceans and lakes, layers of mud slowly build up over time. Different layers tell scientists whether the climate was warmer, colder, wetter, or drier at different times in Earth’s history. These layers contain:

• Tiny shells of sea creatures
• Pollen from plants
• Dust blown in by winds
• Plant remains
• Algae
• Charcoal

Some of the oldest sedimentary rocks on Earth are about 3.9 billion years old. That means sediment records let scientists study Earth’s climate through most of our planet’s history!

Fossils and Coral Reefs: Signs of Past Life

Coral reefs help scientists reconstruct climate change during the Holocene (the past 10,000 years). These fossils show where plants and animals lived long ago. Since different species need specific climates, fossils help scientists understand:

• Past temperatures
• Sea level
• Ocean conditions

Coral reefs are especially helpful. As corals grow, they record changes in ocean temperature and chemistry—like tree rings, but underwater.

Why Paleoclimate Data Matters Today

You might wonder: Why study ancient climate if we’re worried about today?

Because paleoclimate data helps scientists:

• See how Earth’s climate naturally changed over time
• Understand what’s normal—and what’s not
• Compare today’s changes to the past

Here’s the big takeaway:
Earth’s climate has changed before—but never this fast, and never because of humans until now.

When scientists combine paleoclimate data with modern measurements, all the evidence points to the same conclusion:
Human activities are warming the planet.