Thermometers go back only about 150 years worldwide. To test ideas like the orbital cycles, scientists read climate from natural records called proxies: layers that build up year by year and keep a chemical memory of the climate when they formed.
Key idea
Ice cores from Greenland and Antarctica hold bubbles of ancient air (a direct sample of past carbon dioxide and methane), and the oxygen in the ice records past temperature; Antarctic cores reach back about 800,000 years. Tree rings record growing conditions year by year, for thousands of years. Ocean sediment cores hold the shells of tiny plankton whose chemistry records ocean temperature and ice volume over millions of years. Corals, cave formations and pollen add more records.
Why it works
Many proxies are layered, so they can be counted like pages: one tree ring or one band of ice for each year, where snow or growth changes with the seasons. Their chemistry acts as a thermometer. Water molecules with the heavier oxygen-18 atom evaporate a little less easily and condense a little more easily, so by the time vapor reaches the poles in a cold climate, more of the heavy oxygen has already rained out, and polar snow is "lighter." Bubbles trapped as snow turns to ice seal in samples of the air itself. Records from different proxies, places and methods agree with each other, which is what makes them trustworthy.
Worked example: Counting layers
A Greenland ice core has yearly layers about 2.5 cm thick near the top. About how many years does the top 50 m cover?
- 50 m = 5000 cm.
- $5000 \div 2.5 = 2000$ layers: about 2000 years.
- Deeper down the layers are squeezed thinner, so the same length of core covers more years.
Answer: About 2000 years
Worked example: Then and now
Through the last 800,000 years, ice cores show carbon dioxide between about 180 ppm (ice ages) and 300 ppm (warm times). Today it is about 425 ppm. How much higher is today than the highest level in the cores?
- $425 - 300 = 125$ ppm higher.
- As a percentage of 300: $125 \div 300 \approx 42\%$ higher.
- The IPCC (2021) concluded that carbon dioxide is now higher than at any time in at least 2 million years.
Answer: About 125 ppm (about 42%) higher
Common mistake
Thinking we know past climates only from written records. Writing covers a few thousand years at most, and thermometers about 150 years worldwide; bubbles in Antarctic ice hold samples of air from about 800,000 years ago, and ocean sediments reach back tens of millions of years. Check: ask what a record is made of, and how its layers are dated.