Ocean water changes with climate. If the climate becomes colder, ocean water also becomes colder. Some animals make shells one way in cold water and another way in warm water. When these animals died long ago, their shells sank down onto the ocean floor. Shells in the ocean floor give us a record of climates over 100 million years ago.
We can also learn about past climates from large areas of ice called glaciers. The amount of oxygen trapped in the ice is different in different parts of the glacier. The amount depends on the temperature of the air when the glacier formed. Pieces of ice have shown what the climate was in some places for the past 100,000 years.
Some of the best information about past climates comes from trees. Each tree ring shows one year’s growth. How much a tree grows each year depends on the climate. By studying how wide the tree rings are, scientists can learn about the climate of a place during the time the trees grew. They can also find out about how old the wood is. Using wood from trees, they have been able to piece together the world’s climate to about 4000 years ago. What kinds of climates would cause trees to grow faster or more slowly?
From what they have learned, scientists think that the earth’s climate has changed a great deal. They think the average world temperature at one time million of years ago was about 23oC. Because of this warm climate, the poles were free of ice! But at other times, climates were much colder than today. Within the last one million years, there were four different ice ages. Ice ages were times when large parts of the world were covered with ice. Ice sheets may have once covered the land where you live.
What is happening right now that may change the earth’s climates in the future? For one thing, scientists know that the amount of carbon dioxide in the atmosphere has gone up. This added carbon dioxide comes from burning of fuels such as oil and coal. Some scientists believe that the larger amount of carbon dioxide will make climate warmer. Others do not agree. They believe that carbon dioxide will stop some of the sun’s energy from reaching the earth’s surface in the first place. This would make the atmosphere cooler. If just a little less energy from the sun reaches the earth, another ice age could begin!
The climates of the earth have always changed and will change in the future. What will the climates be like in 50, 100, and 1000 years? Scientists have different ideas of what they think will happen. The answer will come through discovering what causes changes in climates.