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 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 (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. Some volcanic eruptions can add to the greenhouse effect, since they release carbon dioxide.
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.
Photograph by Felipe Mesa, MyShot
Early Squirrels
The North American red squirrel has started breeding earlier in the year as a result of climate change. Food becomes available to the squirrels earlier because of warmer winters.
Cool Warming
Could the current phase of climate changeglobal warmingcause another Little Ice Age? As strange as it sounds, some scientists believe it could. If melting glaciers release large amounts of freshwater into the oceans, this could disrupt the ocean conveyor belt, an important circulation system that moves seawater around the globe. Stopping this cycle could possibly cause cooling of 3 to 5 degrees Celsius (5-9 degrees Fahrenheit) in the ocean and atmosphere.
to adjust to new surroundings or a new situation.
container of liquid material under high pressure. When released through a small opening, the liquid becomes a spray or foam.
modern farming methods that include mechanical, chemical, engineering and technological methods. Also called industrial agriculture.
the art and science of complex machines used to perform tasks associated with farming and ranching.
organisms that have a well-defined shape and limited growth, can move voluntarily, acquire food and digest it internally, and can respond rapidly to stimuli.
yearly.
large fruit native to trees found on islands in the south Pacific Ocean.
greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.
system of transportation where one car transports several riders.
100 years.
chemical compound mostly used in refrigerants and flame-retardants. Some CFCs have destructive effects on the ozone layer.
person who studies long-term patterns in weather.
to occur at the same time.
result or outcome of an action or situation.
tectonic plate found beneath continents.
remains of something broken or destroyed; waste, or garbage.
10 years.
to decay or break down.
to lower.
to place or deliver an item in a different area than it originated.
area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.
construction or preparation of land for housing, industry, or agriculture.
very large, extinct reptile chiefly from the Mesozoic Era, 251 million to 65 million years ago.
very expressive or emotional.
earthen mound shaped by glaciers.
climate group that experiences low precipitation.
set of physical phenomena associated with the presence and flow of electric charge.
irregular, recurring weather system that features a warm, eastern-flowing ocean current in the eastern Pacific Ocean.
discharge or release.
to give off or send out.
to surround or submerge.
very large.
conditions that surround and influence an organism or community.
imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.
process of enlarging.
no longer existing.
one or more buildings used for the manufacture of a product.
material, usually of plant or animal origin, that living organisms use to obtain nutrients.
ecosystem filled with trees and underbrush.
coal, oil, or natural gas. Fossil fuels formed from the remains of ancient plants and animals.
to become a solid mineral.
phenomenon where gases allow sunlight to enter Earth's atmosphere but make it difficult for heat to escape.
gas in the atmosphere, such as carbon dioxide, methane, water vapor, and ozone, that absorbs solar heat reflected by the surface of the Earth, warming the atmosphere.
environment where an organism lives throughout the year or for shorter periods of time.
tropical storm with wind speeds of at least 119 kilometers (74 miles) per hour. Hurricanes are the same thing as typhoons, but usually located in the Atlantic Ocean region.
greenhouse gas often used as an industrial cooling material.
greenhouse gas often used as an industrial cooling material.
long period of cold climate where glaciers cover large parts of the Earth. The last ice age peaked about 20,000 years ago. Also called glacial age.
meaning or effect.
time between ice ages, when global temperatures are warmer and sea levels rise.
site where garbage is layered with dirt and other absorbing material to prevent contamination of the surrounding land or water.
(~1600-~1850) period of cooling climate (~1° Celsius), documented largely in the Northern Hemisphere.
infectious disease caused by a parasite carried by mosquitoes.
very large or heavy.
to combine.
rocky debris from space that enters Earth's atmosphere. Also called a shooting star or falling star.
chemical compound that is the basic ingredient of natural gas.
fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.
path of one object around a more massive object.
living or once-living thing.
person who studies fossils and life from early geologic periods.
supercontinent of all the Earth's landmass that existed about 250 million years ago.
to get rid of in stages, or stop using over time.
organism that produces its own food through photosynthesis and whose cells have walls.
movement and interaction of the Earth's plates.
climate group found within the Arctic and Antarctic Circles.
chemical or other substance that harms a natural resource.
regular or able to be forecasted.
slowing or stopping.
amount.
methods of movement that are available to all community members for a fee, and which follow a fixed route and schedule: buses, subways, trains and ferries.
to move outward from a central spot.
amount of precipitation that falls in a specific area during a specific time.
agricultural land where livestock graze.
to retreat or withdraw.
substance used to keep materials cool.
to move a residence or business from one place to another.
to go back to a familiar or safe place.
natural substance composed of solid mineral matter.
to rub harshly, often to polish.
base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.
rock formed from fragments of other rocks or the remains of plants or animals.
harsh.
rate of producing, transferring, or using solar energy.
dying from lack of food.
severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.
dark, cooler area on the surface of the sun that can move, change, and disappear over time.
the science of using tools and complex machines to make human life easier or more profitable.
moderate.
rock, earth, and gravel left behind by a retreating or melting glacier.
wood in an unfinished form, either trees or logs.
a violently rotating column of air that forms at the bottom of a cloud and touches the ground.
buying, selling, or exchanging of goods and services.
winds that blow toward the Equator, from northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere.
movement of people or goods from one place to another.
layered formation in the trunk of a tree that marks its growth at least once a year.
existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.
sickness that usually occurs in a warm, humid climate.
tropical storm with wind speeds of at least 74 miles (119 kilometers) per hour. Typhoons are the same thing as hurricanes, but usually located in the Pacific or Indian Ocean region.
visible liquid suspended in the air, such as fog.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.
repeating or predictable changes in the Earth's atmosphere, such as winds, precipitation, and temperatures.
infectious disease spread by mosquitoes, with symptoms ranging from mild flu to possible death.
kinetic energy produced by the movement of air, able to be converted to mechanical power.
infectious disease spread by mosquitoes, primarily affecting the liver.
Articles & Profiles
- National Geographic News: Climate Change Pushing Tropics Farther, Faster
- National Geographic News: Did Climate Change Trigger Human Evolution?
- National Geographic Magazine: Climate Connections
Images
- National Geographic Environment: Arctic Climate Change Photos
- National Geographic Environment: Food, Drought, and Climate Change Photos
Worksheets & Handouts
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