Climate is the long-term pattern of weather in a particular area. Weather can change from hour to hour, day to day, month to month or even from year to year. Climate refers to what the weather is generally like over 30 years or more. A desert might experience a rainy week, but over the long term, it receives very little rainfall. It has a dry climate.

Living things adjust to climates. Polar bears (Ursus maritimus) have adjusted to stay warm in polar climates. Over time, cacti have evolved to hold onto water in dry climates. The number of different kinds of life on Earth is partially due to the number of different climates.

Climates do change. They just change very slowly, over hundreds or even thousands of years. As climates change, organisms that live in the area must adjust, relocate, or risk dying out.

Earth's Changing Climate

Earth's climate has changed many times. For example, fossils from the Cretaceous period (144 million to 65 million years ago) show that Earth was much warmer than it is today. Breadfruit (Artocarpus altilis—also called "jackfruit"—trees are now found on tropical islands. However, millions of years ago they even grew on Greenland.

Earth has also experienced several major ice ages. There have been at least four in the past 500,000 years. During these periods, Earth's temperature decreased, causing ice sheets and glaciers to expand. The most recent ice age began about two million years ago and only started ending about 18,000 years ago.

Warmer temperatures have caused the glaciers to shrink. The glaciers have not disappeared completely, however—they still exist in Antarctica and Greenland. Scientists think we live in an "interglacial period," or a time between glaciers. They have gone away somewhat for now, but hundreds of years from now, the glaciers may grow again.

Scientists who study climate look for proof of past climate change in many different places. Like clumsy criminals, glaciers leave many clues behind. They scratch and rub rocks as they move. They leave little bits of material behind known as "glacial till." This sometimes forms mounds or ridges. Glaciers also form long, oval-shaped hills. If you see a piece of land with any of these signs, it suggests that a glacier was once there.

Some types of rocks only form from materials left behind from glaciers. When scientists find these rocks, it tells them that glaciers were once there.

Scientists also have proof of glaciers from fossils. Fossils show what kinds of animals and plants lived in certain areas. Looking for fossils of animals that lived in the cold can show scientists how far across the planet the glaciers reached.

Climate changes happen over shorter periods, as well. For example, there was a "Little Ice Age" that lasted only a few hundred years. It peaked during the 1500s and 1600s. During this time, average temperatures around the world were two to three degrees Fahrenheit cooler (about one to 1.5 degrees Celsius) than they are today. A change of one or two degrees might not seem like much, but it was enough to cause major changes. Glaciers grew larger and sometimes engulfed whole mountain villages. Winters were longer than usual, limiting the growing seasons of crops. In northern Europe, people left their farms and villages to avoid starving.

One way scientists have learned about the Little Ice Age is by studying the rings of trees. The thickness of tree rings is related to how much the tree grew each year. During times when it was very dry or very cold, trees could not grow as much and rings would be closer together.

Some climate changes are almost predictable. El Niño, which means "The Child" in Spanish, is a good example of this. El Niño refers to the warming of the surface waters in the Pacific Ocean around the equator. In normal years, winds blow across the ocean from east to west. This drags warm water along in the same direction.

Every few years, normal winds change and ocean currents reverse. This is El Niño. Warm water deepens in the eastern Pacific, near South America. This, in turn, produces big climate changes. Rain decreases in Australia and southern Asia, and crazy storms may pound Pacific islands and the west coast of the Americas. Within a year or two, El Niño ends, and climate systems return to normal.

Natural Causes of Climate Change

Climate changes happen for many reasons. Some of these reasons have to do with Earth's atmosphere. The climate change brought by El Niño, which relies on winds and ocean currents, is an example of natural changes in the atmosphere.

Natural climate change can also be affected by forces outside Earth's atmosphere. Earth's relationship to the sun also affects climate. This includes how Earth is tilted and how it orbits around the sun. These change slowly over time and affect how much of the sun's light reaches different parts of the world at different times. The 100,000-year cycles of ice ages are most likely caused by changes in these things.

Large meteorites hitting Earth could also cause climate change. If a meteor hit Earth, it would send millions of tons of dirt and dust into the atmosphere. This would block some of the sun's rays, making it cold and dark. Many plants and animals would die. Many paleontologists believe that dinosaurs went extinct partially due to a meteor or comet hitting Earth. Dinosaurs could not survive in a cool, dark climate. Their bodies could not adjust to the cold, and the dark killed many plants that they ate. Without the plants, the plant-eating dinosaurs died. And without those plant-eating dinosaurs, the dinosaurs that ate them died too.

Plate tectonics also play a role in climate changes. Earth is made of many layers. The top part is the crust, and just beneath that is the mantle. Together, these make up the "plates" in plate tectonics. We now know there are 15 major plates that cover the planet's surface. They move about as fast as our fingernails grow.

Earth's continental plates have moved a great deal over time. More than 200 million years ago, the continents were merged together as one giant landmass called Pangaea. As the continents broke apart and moved, their positions on Earth changed. The movements of ocean currents also changed. Both of these changes affected climate.

Another cause of climate change is called the greenhouse effect. The greenhouse effect happens when gases like carbon dioxide trap the sun's heat in the atmosphere. Gases that do this are called greenhouse gases. They keep Earth warm. Without any greenhouse gases in the atmosphere, most life on Earth would freeze to death. However, adding too much of these gases to the atmosphere slowly makes the planet warmer.

Human Causes of Climate Change

Some human activities release greenhouse gases. For example, humans burn fossil fuels such as coal, oil and natural gas. People often use them for transportation and electricity. Burning fossil fuels releases greenhouse gases, like carbon dioxide. Trees soak up carbon dioxide, so cutting down forests also adds to the greenhouse effect. Factories send greenhouse gases into the atmosphere too.

Many scientists are worried that these activities are causing dangerous changes in Earth's climate. Average temperatures around the world have risen since about 1880. The seven warmest years of the 1900s happened in the 1990s. This warming trend may be a sign that the greenhouse effect is increasing because of human activity. This is often referred to as "global warming." It is estimated that humans have increased the amount of carbon dioxide in the atmosphere by about 30 percent in the past 150 years.

Other greenhouse gases are increasing, as well. Methane is an example. Methane is a greenhouse gas produced by rotting plants and animals. As populations grow, they use more goods and throw away more. Large landfills, filled with rotting waste, release tons of methane into the atmosphere.

Some chemicals that are used in refrigeration, air conditioning, and aerosol sprays are also greenhouse gases. Many countries are working to get rid of them. Some have laws to prevent companies from manufacturing them.

Global Warming

As the amount of greenhouse gases in the atmosphere rises, so does the temperature of Earth. Scientists worry that the temperature will increase so much that ice caps will begin seriously melting within the next several decades. This would cause the sea level to rise. Coastal areas and small islands would be flooded. Severe climate change may bring more severe weather patterns. This could include more hurricanes, typhoons, and tornadoes. More rain and snow would fall in some places and far less in others. Places where crops now grow could become deserts.

As climates change, so do the homes for many living things. Animals may not be able to survive in their current homes. Many human societies depend on specific crops for food, clothing, and trade. If the climate of an area changes, the same crops may not grow. Some scientists worry that as the planet warms, tropical diseases will spread further.

The temperature will continue to rise unless steps are taken to stop it. Most scientists agree that we must reduce the amount of greenhouse gases released into the atmosphere. There are many ways to do this, including:

• Drive less. Use public transportation, carpool, walk, or ride a bike.
• Fly less. Airplanes produce huge amounts of greenhouse gas emissions.
• 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.
• Support alternative power sources that don't burn fossil fuels. These include power that comes from the sun and from wind.

The climate has changed many times during Earth's history. However, those changes have happened slowly, over thousands of years. Only since the Industrial Revolution have human activities begun to influence climate. Scientists are still working to understand what the consequences might be.