Scientists study tree rings like these to figure out what climates of the past were like. Each year that the tree was alive it grew another ring, making its trunk wider. The thickness of a ring depends on what the weather was like during the year in which it grew. Click on image for full size (154 Kb) Image courtesy of UCAR Digital Image Library

Earth's climate has been changing for billions of years. It warmed and cooled many times long before humans were around.

There weren’t any people on Earth millions and billions of years ago to describe what climate was like, but the Earth kept records of past climates in special ways. Sediments and fossils deposited millions of years ago provide a record of ancient environments. Thin layers of mud and sand that form at the bottom of lakes record seasonal changes. Bubbles of ancient air trapped inside glaciers record what the atmosphere was like. Tree rings show what climate was like for each year of a tree’s life.

If climate has always changed, then why are people concerned about global warming? People are concerned because the Earth is warming faster now than it has in the past as more greenhouse gases are released into the atmosphere. Warming is happening faster and life on Earth, including humans, may not have time to get used to the warming planet.

Click on the links below to learn more about how climate has changed in the past.

• Happenings During the Paleozoic Era (545-248 Million Years Ago)
• Happenings During the Mesozoic Era (248-65 Million Years Ago)
• Happenings During the Cenozoic (65 Million Years Ago to Present)
• The Little Ice Age (1350-1850)