Reading Earth's Diary

How do scientists know the climate is changing if reliable thermometer records only go back about 150 years? They read proxy records — natural archives that preserve climate information across millennia. Ice cores trapped ancient atmosphere in air bubbles. Tree rings recorded growing conditions year by year.

The evidence comes from multiple independent sources that were not designed to agree with each other. Ice cores from Greenland and Antarctica. Coral growth bands from tropical oceans. Cave formations from every continent. Pollen records from lake sediments.