Climate
It may seem hard to believe that people can actually change the Earth’s climate. But scientists think that the things people do that send greenhouse gases into the air are making our planet warmer. Atmosphere
The atmosphere covers the Earth. It is a thin layer of mixed gases which make up the air we breathe. This thin layer also helps the Earth from becoming too hot or too cold, much like clothing does for us. Weather systems, which develop in the lower atmosphere, are driven by heat from the sun, the rotation of the Earth, and variations in the Earth's surface.
Oceans
Oceans cover about 70 percent of Earth's surface. Their large mass and thermal properties, enable them to store vast quantities of heat. Oceans buffer and regulate temperature – energy absorbed or lost by the oceans results in a smaller surface temperature change than would occur over land. The atmosphere and ocean constantly exchange energy and matter. For example, water evaporates from the oceans into the atmosphere. This moisture then falls back to the Earth as precipitation – rain, snow, sleet, and even the morning dew on the grass.
See an animation of how the planet's water cycle is likely affected by climate change.(Macromedia Flash Version 5 or higher plug-in required)
Land
Land covers 27 percent of Earth's surface, and land topography influences weather patterns. For example, the weather in areas covered by mountains can be completely different than the weather in areas where the land is mostly flat.
Ice
Ice is the world's largest supply of freshwater. It covers the remaining 3 percent of Earth's surface including most of Antarctica and Greenland. Because ice is highly reflective and because of its insulating properties, ice plays an important role in regulating climate.
Biosphere
The biosphere is that part of Earth's atmosphere, land, oceans that supports any living plant, animal, or organism. It is the place where plants and animals, including humans, live. Large quantities of carbon dioxide are exchanged between the land-based biosphere and the atmosphere as plants take in carbon dioxide and give off oxygen, and animals inhale oxygen and exhale carbon dioxide.
See an animation of how the planet's carbon cycle is likely affected by global warming. (Macromedia Flash 
Version 5 or higher plug-in required)
Earth's climate has been changing constantly over its 5-billion-year history. |
Sometimes, the climate has warmed so that the oceans have risen and covered much of the Earth. Each of the changes may seem extreme, but they usually occurred slowly over many thousands of years. |
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The first people arrived in America between 15,000 and 30,000 years ago. During that time, much of North America was covered by great ice sheets. Some 14,000 years ago, the last ice sheet began to melt very quickly. By 7,000 years ago, the ice was gone.
This end to the ice ages caused big changes on the Earth. The changes caused many kinds of plants and animals to die. For example, mastodons – elephant-like animals – and other large mammals that preferred cold climates may not have been able to live in the warmer, drier conditions.
Starting in the 14th century, Europeans lived through what is known as the "Little Ice Age." The Little Ice Age lasted for several hundred years. During the Little Ice Age, the advance of glaciers along with hard winters and famines caused some people to starve and others to leave their homes.
Recent 
The Earth has warmed about 1
Melting Glaciers: a glacier is a large sheet of ice that moves very, very slowly. Many glaciers in the world are now melting. For example, glaciers are melting in Montana's Glacier National Park. Some scientists think the glaciers are melting partly because the Earth is getting warmer.
Rising Sea Level: have you ever built a sandcastle on the beach, close to the ocean on wet sand? If you have, you probably know that the sandcastle won't last very long. Chances are the waves will wash away the sandcastle as soon as the tide comes in. The water goes higher up the beach when the tide comes in. At most shores throughout the world, two high tides and two low tides occur every day. But now the level of the sea is rising, so high tides are higher than they were before. Over the last 100 years, the level of the sea has risen about 6-8 inches worldwide. When the sea level rises, the tide goes farther up the beach.
Scientists think the sea has risen partly because of melting glaciers and sea ice. When some glaciers melt, they release water into the sea and make it higher than it was before. Scientists also think that warmer temperatures in the sea make it rise even more. Heat makes water expand. When the ocean expands, it takes up more space.
What Might Happen?
Scientists are not fortune-tellers. They don't know exactly what will happen in the future. But they can use special computer programs to find out how the climate may change in the years ahead. And the computer programs tell us that the Earth may continue to get warmer.
Together, the melting glaciers, rising seas, and computer models provide some good clues. They tell us that the Earth's temperature will probably continue to rise as long as we continue increasing the amount of greenhouse gases in the 
Scientists have to think like detectives. They look for clues to help them understand how the world works. Then they investigate the clues to find evidence – real facts that can give them a better idea of what is going on. Here are some of the ways that scientists gather evidence about climate, both past and present:
Weather Stations
Weather stations help us find out the temperature on the surface of the Earth. Weather stations use special thermometers that tell us the temperature. They can be set up almost anywhere on land. Weather stations also can tell us how fast the wind is moving and how much rain falls on the ground during a storm.
Weather Balloons
Almost everyone likes balloons – including scientists! Weather balloons are released to float high up into the atmosphere. They carry special instruments that send all kinds of information about the weather back to people on the ground.
Ocean Buoys
A buoy is an object that floats on water, and is often used to 
warn boats away from dangerous places in the ocean or on a river. But some buoys have special instruments on them. These buoys can tell us the temperature and other things about the conditions of the atmosphere.
Weather Satellites
Humans send satellites into space to travel around the Earth. The satellites send back information to scientists on the ground. Some of the information they give us is about the weather and the Earth’s temperature.
Ice Cores
Some scientists who want to find out more about climate study ice for clues. Not just any ice – they are studying the ice from glaciers that have been around for a very long time. They cut pieces of ice and look for air bubbles that were trapped in the ice hundreds or even thousands of years ago. The air bubbles help them discover what the climate used to be like on Earth. The evidence they uncover is creating a historical record of regional temperatures and greenhouse gas concentrations dating back 160,000 years.
Sediment Analyses
Sediment is the earth and rock that has built up in layers over time. Scientists are learning a great deal about past climate from studying these layers. Sediment layering provides information about where glaciers have been in the past. Ocean sediments provide a map of how ocean currents have flowed in the past. And fossilized pollen found in sediment layers tells us about where different plants have grown in the past.
Tree Rings
You can tell how old a tree is by counting its rings because it grows a new ring every year. Tree rings also can tell us how much precipitation fell each year in the place where the tree lives. Precipitation is rain or snow or any other moisture that falls to the Earth. Scientists study the sizes of tree rings. The different sizes of the rings tell us about changes in temperature and precipitation.
What Does All of This Mean?
Weather stations, balloons, ocean buoys, and satellites tell us the Earth’s temperature today. Ice cores, sediment layers, and tree rings tell us about what the Earth’s climate has been like in the past. With this evidence, scientists are learning how climate changes over time.
What Are Scientists Still Unsure About?
How do clouds respond to changes in temperature and precipitation? How do oceans transport heat? How do climate and intense weather events like hurricanes affect each other? As scientists try to answer these and other questions, they will discover many more clues about how the Earth’s climate system works.
