Scientists say that the Earth’s climate is changing much faster now than in the past. How do scientists know about past climates on Earth?

Quick Answer

Scientists know about ancient climates from climate proxies: biological, chemical, physical, and geological records that are chronologically dated, calibrated, and analyzed with great care.

People existed thousands of years ago, but our distant ancestors didn’t have equipment like we do today to measure climate variables like temperature and rainfall.  We can imagine that our ancestors sat around fires and talked about weather events such as severe storms and drought, but we don’t have records of those conversations.

Luckily, we have something else to tell us about climates of the past: climate proxies, which are biological, physical, and chemical records in nature that preserve Earth’s climate history and substitute for direct measurements.  Examples of these are ice cores from glaciers and ice sheets, sediment cores from the bottom of the ocean and lakes, tree rings, coral skeletons, clam shells, leaf fossils, and other natural records.

For example, some of the most finely detailed climate records come from ice cores: cylinders of ice that are extracted from the Antarctic and Greenland ice sheets, and from mountain glaciers around the world. Glacial ice forms from layer upon layer of seasonal snow that eventually gets compressed into ice, and the deeper you drill in the ice, the farther back in time you go. Scientists can date the cores from annual layers that are preserved in the ice.

To learn about past climates from ice cores, scientist use the fact that there is a known relationship between the ratios of stable oxygen and hydrogen isotopes in the ice and the temperature at the time the snow fell. Since the isotopes are stable, they don’t change over even billions of years, so the isotope ratios preserve this temperature information.

Carbon dioxide levels come from measurements of tiny air bubbles that were trapped in the ice at the time the ice formed. The ancient air—and its chemical composition—is preserved in these bubbles, and in this way we can learn about the concentration of greenhouse gases in the atmosphere from hundreds of thousands of years ago.

Learn More

• Learn more about climate proxies in the Museum of the Earth's online exhibit Changing Climate: Our Future, Our Choice.
• Digital Atlas of Ancient Life: Paleoenvironmental Reconstruction
• You can find many climate proxy data sets, freely available, on a website from the National Centers for Environmental Information.