Weather can change from hour to hour or day to day. It can change month to month, or even from year to year. Climate is different. Climate is the pattern of weather in a certain area over a long period of time. Climate is usually the weather pattern over 30 years or more. A desert might have a rainy week. Still, it usually gets very little rain. It has a dry climate.

Living things change so that they can survive in different climates. Polar bears (Ursus maritimus) have ways to keep warm in the cold. Cactus plants in the desert can hold in water. There are many different climates in the world. That is one reason there are so many different animals. Climates change very slowly. It takes hundreds or even thousands of years. As climates change, plants and animals that live in the area must change, too. If they do not, they might die out.

Earth's Changing Climate

Earth's climate has changed many times. At some points, it has been much warmer than it is now. It has also had periods where it is very cold. These are called ice ages. Glaciers are big sheets of ice. They grow much bigger during ice ages.The last ice age began about two million years ago. It ended about 18,000 years ago. Warmer temperatures have caused the glaciers on Earth to get smaller. The glaciers have not disappeared completely, though. They still exist in Antarctica and Greenland. Hundreds of years from now, the glaciers may grow again.

Scientists who study climate look for proof of past climate change. They can find proof in many different places. Glaciers leave many clues behind them. They scratch and rub rocks as they move. They leave little bits of rocks behind. This sometimes forms mounds or ridges. They also form long, oval-shaped hills. Finding certain rocks can also show that a glacier was there. Scientists also look at fossils. Fossils show the kinds of animals and plants that were around at different times. This tells scientists how big the glaciers were. Tree rings also tell us about the climate. The size of each tree ring shows how much a tree has grown each year. Trees do not grow as much when it is very cold or dry. When this happens, the rings are closer together.

Climate changes happen over shorter periods, too. There was a "Little Ice Age" that lasted only a few hundred years. It happened mostly during the 1500s and 1600s. During this time, the world was about two to three degrees Fahrenheit (about one to 1.5 degrees Celsius) cooler than it is today. This may not seem like much, but it was enough to cause major changes.

Some climate changes are almost predictable. El Niño is a good example of this. It is a weather pattern that happens every few years. First, very warm waters move around in the Pacific Ocean. They head toward South America. Then, the air there becomes warmer and strong storms and winds happen. A year or two later it goes back to normal. Then the cycle repeats.

Natural Causes of Climate Change

Climate changes happen for many reasons. Sometimes wind and oceans change naturally. This can change the climate. Earth's relationship to the sun also affects climate. How the sun's rays hit Earth can change how hot it is. How far away Earth is from the sun also makes a difference. These things change slowly over time.

Sometimes, large rocks from space fall onto Earth. These are called meteorites. They can also cause a change in the climate. If a meteorite hit Earth, it would send huge amounts of dirt and dust into the air. This would block some of the sun's rays. Earth would become cold and dark. Many scientists believe that this is how dinosaurs died out. Dinosaurs could not survive in a cool, dark climate. The continents on Earth have moved over millions of years. The movements of oceans have also changed. Both of these changes had effects on climate.

Another cause of climate change is called the greenhouse effect. Earth is surrounded by a cloud of gases. This is called the atmosphere. You can think of it as a blanket of air. It holds in heat, keeping the planet warm. The greenhouse effect happens when gases trap the sun's heat in the atmosphere. Gases that do this are called greenhouse gases. Adding too many of these gases to the atmosphere slowly makes the planet warmer.

Human Causes of Climate Change

Humans do many things that create greenhouse gases. People burn coal, oil, and gas. They do this to power their cars and homes. This creates greenhouse gases. Trees soak up carbon dioxide. Carbon dioxide is a greenhouse gas. This means that cutting down forests also adds to the greenhouse effect.

Landfills release greenhouse gases, like methane. This happens when garbage rots. Many scientists worry about the changes in Earth's climate. The planet has been getting warmer and warmer. It is getting warmer faster than it would naturally. This is called "global warming."

Global Warming

Scientists worry Earth will get so warm that ice caps will begin seriously melting. This could happen in the next 20 years. This would cause the sea level to rise. Major floods would happen. Climate change could also cause more storms, even hurricanes and tornadoes. Some places would get much more rain and snow. Other places would get much less. Places where crops now grow could become deserts.

Some kinds of animals may not be able to survive if the climate changes. Humans will have a hard time too. Certain crops are very important to some countries. The same crops may not grow if the climate gets warmer. Some diseases could spread further, too.

Earth will continue to get warmer unless steps are taken to stop it. Most scientists say that we must put less greenhouse gases into the atmosphere. There are many ways to do this. Some are listed below.

• Drive less. Use trains and buses, carpool, walk, or ride a bicycle.
• Fly less. Airplanes release huge amounts of greenhouse gas.
• Reduce, reuse, and recycle.
• Plant a tree. Trees soak up carbon dioxide, keeping it out of the atmosphere.
• Use less electricity.
• Eat less meat. Cows are one of the biggest methane producers.
• Use types of power that do not burn coal, oil, or gas. Instead, use power from the wind or the sun.

The climate has changed many times during Earth's history. These changes happened slowly, over thousands of years. Humans started changing the climate about 200 years ago. Scientists are still trying to understand what the consequences might be.