Some 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

Climate change has affected the Earth throughout its 4.6 billion year history. The Earth has repeatedly warmed and cooled and the changes affected the types of environments on Earth’s surface and the organisms that could survive.

Evidence of past climate change comes from many sources. The sediments 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 glacial ice record the characteristics of the atmosphere. Fossil plant pollen and tiny marine plankton provide clues the temperatures in the oceans and on the land. Tree rings show what climate was like over the life of the tree. The scientists who decipher these clues to ancient climates are called paleoclimatologists.

If climate has always changed, then why is global warming such a concern today? Humans are adding large amounts of greenhouse gas to our atmosphere by burning fossil fuels. This change in the concentration of greenhouse gases is increasing the speed of global warming. The rate of change today is increasing so rapidly that scientists are concerned that life on Earth, including humans, will not have time to adapt to the changing conditions.

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)

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)

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

The climate of Earth has been changing for billions of years! It warmed and cooled many times long before humans were on the planet.

How do we know what the climate was like a thousand years ago, a million years ago, or even a billion years ago? To figure out what climates used to be like, scientists work like detectives and look for clues around the planet to solve the mystery. They find clues inside sedimentary rocks, glaciers, fossils, trees and corals.

Today, people are worried because the Earth is warming faster than it has in the past. Faster warming is happening because more greenhouse gases are released into the atmosphere.

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)