• 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. For periods of 30 years or more, however, distinct weather patterns occur. A desert might experience a rainy week, but over the long term, the region receives very little rainfall. It has a dry climate.

    Because climates are mostly constant, living things can adapt to them. Polar bears have adapted to stay warm in polar climates, while cacti have evolved to hold onto water in dry climates. The enormous variety of life on Earth results in large part from the variety of climates that exist.

    Climates do change, however—they just change very slowly, over hundreds or even thousands of years. As climates change, organisms that live in the area must adapt, relocate, or risk going extinct.

    Earth’s Changing Climate

    Earth’s climate has changed many times. For example, fossils from the Cretaceous period (144 to 65 million years ago) show that Earth was much warmer than it is today. Fossilized plants and animals that normally live in warm environments have been found at much higher latitudes than they could survive at today. For instance, breadfruit trees, now found on tropical islands, grew as far north as Greenland.

    Earth has also experienced several major ice ages—at least four in the past 500,000 years. During these periods, Earth’s temperature decreased, causing an expansion of ice sheets and glaciers. The most recent Ice Age began about 2 million years ago and peaked about 20,000 years ago. The ice caps began retreating 18,000 years ago. They have not disappeared completely, however. Their presence in Antarctica and Greenland suggests the Earth is still in a sort of ice age. Many scientists believe we are in an interglacial period, when warmer temperatures have caused the ice caps to recede. Many centuries from now, the glaciers may advance again.

    Climatologists look for evidence of past climate change in many different places. Like clumsy criminals, glaciers leave many clues behind them. They scratch and scour rocks as they move. They deposit sediment known as glacial till. This sediment sometimes forms mounds or ridges called moraines. Glaciers also form elongated oval hills known as drumlins. All of these geographic features on land that currently has no glaciers suggest that glaciers were once there.

    Scientists also have chemical evidence of ice ages from sediments and sedimentary rocks. Some rocks only form from glacial material. Their presence under the ocean or on land also tells scientists that glaciers were once present in these areas.

    Scientists also have paleontological evidence—fossils. Fossils show what kinds of animals and plants lived in certain areas. During ice ages, organisms that are adapted to cold weather can increase their range, moving closer to the Equator. Organisms that are adapted to warm weather may lose part of their habitat, or even go extinct.

    Climate changes occur over shorter periods, as well. For example, the so-called Little Ice Age lasted only a few hundred years, peaking during the 16th and 17th centuries. During this time, average global temperatures were 1 to 1.5 degrees Celsius (2-3 degrees Fahrenheit) cooler than they are today.
    A change of one or two degrees might not seem like a lot, but it was enough to cause some pretty massive effects. For instance, glaciers grew larger and sometimes engulfed whole mountain villages. Winters were longer than usual, limiting the growing seasons of crops. In northern Europe, people deserted farms and villages to avoid starvation.

    One way scientists have learned about the Little Ice Age is by studying the rings of trees that are more than 300 years old. The thickness of tree rings is related to the amount of the trees’ annual growth. This in turn is related to climate changes. During times of drought or cold, trees could not grow as much. The rings would be closer together.

    Some climate changes are almost predictable. One example of regular climate change results from the warming of the surface waters of the tropical eastern Pacific Ocean. This warming is called El Niño—The Child—because it tends to begin around Christmas. In normal years, trade winds blow steadily across the ocean from east to west, dragging warm surface water along in the same direction. This produces a shallow layer of warm water in the eastern Pacific and a buildup of warm water in the west.

    Every few years, normal winds falter and ocean currents reverse. This is El Niño. Warm water deepens in the eastern Pacific. This, in turn, produces dramatic climate changes. Rain decreases in Australia and southern Asia, and freak 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 a variety of 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 atmospheric changes.

    Natural climate change can also be affected by forces outside Earth’s atmosphere. For instance, the 100,000-year cycles of ice ages are probably related to changes in the tilt of Earth’s axis and the shape of its orbit around the sun. Those planetary factors change slowly over time and affect how much of the sun’s energy reaches different parts of the world in different seasons.

    Small changes in the amount of sunlight that reaches the Earth can also have an effect on global temperatures. One popular theory, which has not been proved, is that the Earth’s climate is related to the number of sunspots on the sun’s surface. Sunspots are storms that appear as dark marks on the sun. Some scientists say the number of sunspots has doubled in the past 100 years, making the sun brighter to us—and making it warmer here on Earth. Scientists have noticed that an unusually quiet period of sunspot activity coincided with the Little Ice Age.

    The impact of large meteors on Earth could also cause climate change. The impact of a meteor would send millions of tons of debris into the atmosphere. This debris would block at least some of the sun’s rays, making it cold and dark. This climate change would severely limit what organisms could survive. Many paleontologists believe the impact of a meteor or comet contributed to the extinction of the dinosaurs. Dinosaurs simply could not survive in a cool, dark climate. Their bodies could not adjust to the cold, and the dark limited the growth of plants on which they fed.

    Plate tectonics also play a role in climate changes. 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, and so did the movements of ocean currents. Both of these changes had effects on climate.

    Changes in greenhouse gases in the atmosphere also have an impact on climate change. Gases like carbon dioxide trap the sun’s heat in Earth’s atmosphere, causing temperatures on the surface to rise. Volcanoes—both on land and under the ocean—release greenhouse gases, so periods of high volcanic activity tend to be warmer.

    Volcanoes contribute to the greenhouse effect. The greenhouse effect happens when carbon dioxide, water vapor, and other gases in the atmosphere absorb some of the sun’s heat as it is radiated back from the surface of the Earth. This keeps the Earth warm. Without any greenhouse gases in the atmosphere, most life on Earth would freeze to death. However, adding too many of these gases to the atmosphere can intensify the greenhouse effect and slowly make the Earth warmer.

    Human Causes of Climate Change

    Some human activities release greenhouse gases—burning fossil fuels for transportation and electricity, or using agricultural technology that increases meat production, for instance. Trees absorb carbon dioxide, so cutting down forests for timber or development contributes to the greenhouse effect. So do factories that emit pollutants into the atmosphere.

    Many scientists are worried that these activities are causing dramatic and dangerous changes in Earth’s climate. Average temperatures around the world have risen since the late 1870s, when scientists began tracking them. The seven warmest years of the 20th century occurred in the 1980s and 1990s. This warming trend may be a sign that the greenhouse effect is increasing because of human activity. This climate change is often referred to as “global warming.”

    Global warming is often linked to the burning of fossil fuels—coal, oil, and natural gas—by industries and cars. Warming is also linked to the destruction of tropical forests. Such human activity has increased the amount of carbon dioxide in the atmosphere by more than 20 percent in the past 100 years.

    The amount of methane, a chemical produced by decomposing plant and animal matter, is also increasing. Increased amounts of methane in Earth’s atmosphere are usually linked to agricultural development and industrial technology. As economies grow, populations consume more goods and throw away more materials. Large landfills, filled with decomposing waste, release tons of methane into the atmosphere.

    Chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), and hydrofluorocarbon (HFC) chemicals are used in refrigeration and aerosol sprays. These chemicals are also greenhouse gases. Many countries are working to phase out their use, and some have laws to prevent companies from manufacturing them.

    Global Warming

    As the proportion of greenhouse gases in the atmosphere rises, so does the temperature of Earth. Climatologists worry that the global temperature will increase so much that ice caps will begin melting within the next several decades. This would cause the sea level to rise. Coastal areas, including many low-lying islands, would be flooded. Severe climate change may bring more severe weather patterns—more hurricanes, typhoons, and tornadoes. More precipitation would fall in some places and far less in others. Regions where crops now grow could become deserts.

    As climates change, so do the habitats for living things. Animals that live in an area may become threatened. Many human societies depend on specific crops for food, clothing, and trade. If the climate of an area changes, the people who live there may no longer be able to grow the crops they depend on for survival. Some scientists worry that as the Earth warms, tropical diseases such as malaria, West Nile virus, and yellow fever will expand into more temperate regions.

    The temperature will continue to rise unless preventive steps are taken. Most climatologists 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 absorb carbon dioxide, keeping it out of the atmosphere.
    • Use less electricity.
    • Eat less meat. Cows are one of the biggest methane producers.
    • Support alternative energy sources that don’t burn fossil fuels, such as solar power and wind energy.

    The climate has changed many times during Earth’s history, but the changes have occurred slowly, over thousands of years. Only in this century have human activities begun to influence climate—and scientists are still struggling to understand what the consequences might be.

    Earth's Changing Climate
    The most recent phase of climate change has resulted in warmer, drier weather.

    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.

    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.

  • Term Part of Speech Definition Encyclopedic Entry
    adapt Verb

    to adjust to new surroundings or a new situation.

    aerosol can Noun

    container of liquid material under high pressure. When released through a small opening, the liquid becomes a spray or foam.

    agricultural development Noun

    modern farming methods that include mechanical, chemical, engineering and technological methods. Also called industrial agriculture.

    agricultural technology Noun

    the art and science of complex machines used to perform tasks associated with farming and ranching.

    animal Noun

    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.

    annual Adjective


    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    axis Noun

    an invisible line around which an object spins.

    Encyclopedic Entry: axis
    breadfruit Noun

    large fruit native to trees found on islands in the south Pacific Ocean.

    carbon dioxide Noun

    greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

    carpool Noun

    system of transportation where one car transports several riders.

    century Noun

    100 years.

    chlorofluorocarbon (CFC) Noun

    chemical compound mostly used in refrigerants and flame-retardants. Some CFCs have destructive effects on the ozone layer.

    climate Noun

    all weather conditions for a given location over a period of time.

    Encyclopedic Entry: climate
    climate change Noun

    gradual changes in all the interconnected weather elements on our planet.

    Encyclopedic Entry: Earth's Changing Climate
    climatologist Noun

    person who studies long-term patterns in weather.

    coal Noun

    dark, solid fossil fuel mined from the earth.

    coincide Verb

    to occur at the same time.

    consequence Noun

    result or outcome of an action or situation.

    continental plate Noun

    tectonic plate found beneath continents.

    crop Noun

    agricultural produce.

    Encyclopedic Entry: crop
    current Noun

    steady, predictable flow of fluid within a larger body of that fluid.

    Encyclopedic Entry: current
    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    decade Noun

    10 years.

    decompose Verb

    to decay or break down.

    decrease Verb

    to lower.

    deposit Verb

    to place or deliver an item in a different area than it originated.

    desert Noun

    area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

    Encyclopedic Entry: desert
    development Noun

    construction or preparation of land for housing, industry, or agriculture.

    dinosaur Noun

    very large, extinct reptile chiefly from the Mesozoic Era, 251 million to 65 million years ago.

    dramatic Adjective

    very expressive or emotional.

    drought Noun

    period of greatly reduced precipitation.

    Encyclopedic Entry: drought
    drumlin Noun

    earthen mound shaped by glaciers.

    dry climate Noun

    climate group that experiences low precipitation.

    electricity Noun

    set of physical phenomena associated with the presence and flow of electric charge.

    El Nino Noun

    irregular, recurring weather system that features a warm, eastern-flowing ocean current in the eastern Pacific Ocean.

    Encyclopedic Entry: El Niño
    emission Noun

    discharge or release.

    emit Verb

    to give off or send out.

    engulf Verb

    to surround or submerge.

    enormous Adjective

    very large.

    environment Noun

    conditions that surround and influence an organism or community.

    Equator Noun

    imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

    Encyclopedic Entry: equator
    expansion Noun

    process of enlarging.

    extinct Adjective

    no longer existing.

    factory Noun

    one or more buildings used for the manufacture of a product.

    food Noun

    material, usually of plant or animal origin, that living organisms use to obtain nutrients.

    Encyclopedic Entry: food
    forest Noun

    ecosystem filled with trees and underbrush.

    fossil Noun

    remnant, impression, or trace of an ancient organism.

    Encyclopedic Entry: fossil
    fossil fuel Noun

    coal, oil, or natural gas. Fossil fuels formed from the remains of ancient plants and animals.

    fossilize Verb

    to become a solid mineral.

    glacier Noun

    mass of ice that moves slowly over land.

    Encyclopedic Entry: glacier
    global warming Noun

    increase in the average temperature of the Earth's air and oceans.

    Encyclopedic Entry: The Greenhouse Effect and our Planet
    greenhouse effect Noun

    phenomenon where gases allow sunlight to enter Earth's atmosphere but make it difficult for heat to escape.

    Encyclopedic Entry: Greenhouse Effect
    greenhouse gas Noun

    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.

    habitat Noun

    environment where an organism lives throughout the year or for shorter periods of time.

    Encyclopedic Entry: habitat
    hurricane Noun

    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.

    hydrochlorofluorocarbon Noun

    greenhouse gas often used as an industrial cooling material.

    hydrofluorocarbon Noun

    greenhouse gas often used as an industrial cooling material.

    ice age Noun

    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.

    ice sheet Noun

    thick layer of glacial ice that covers a large area of land.

    Encyclopedic Entry: ice sheet
    impact Noun

    meaning or effect.

    interglacial period Noun

    time between ice ages, when global temperatures are warmer and sea levels rise.

    island Noun

    body of land surrounded by water.

    Encyclopedic Entry: island
    landfill Noun

    site where garbage is layered with dirt and other absorbing material to prevent contamination of the surrounding land or water.

    latitude Noun

    distance north or south of the Equator, measured in degrees.

    Encyclopedic Entry: latitude
    Little Ice Age Noun

    (~1600-~1850) period of cooling climate (~1° Celsius), documented largely in the Northern Hemisphere.

    malaria Noun

    infectious disease caused by a parasite carried by mosquitoes.

    massive Adjective

    very large or heavy.

    merge Verb

    to combine.

    meteor Noun

    rocky debris from space that enters Earth's atmosphere. Also called a shooting star or falling star.

    Encyclopedic Entry: meteor
    methane Noun

    chemical compound that is the basic ingredient of natural gas.

    moraine Noun

    material, such as earth, sand, and gravel, transported by a glacier.

    Encyclopedic Entry: moraine
    natural gas Noun

    type of fossil fuel made up mostly of the gas methane.

    Encyclopedic Entry: natural gas
    oil Noun

    fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.

    orbit Noun

    path of one object around a more massive object.

    organism Noun

    living or once-living thing.

    paleontologist Noun

    person who studies fossils and life from early geologic periods.

    Pangaea Noun

    supercontinent of all the Earth's landmass that existed about 250 million years ago.

    phase out Verb

    to get rid of in stages, or stop using over time.

    plant Noun

    organism that produces its own food through photosynthesis and whose cells have walls.

    plate tectonics Noun

    movement and interaction of the Earth's plates.

    polar climate Noun

    climate group found within the Arctic and Antarctic Circles.

    pollutant Noun

    chemical or other substance that harms a natural resource.

    predictable Adjective

    regular or able to be forecasted.

    preventive Adjective

    slowing or stopping.

    proportion Noun


    public transportation Noun

    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.

    radiate Verb

    to move outward from a central spot.

    rainfall Noun

    amount of precipitation that falls in a specific area during a specific time.

    range Noun

    agricultural land where livestock graze.

    recede Verb

    to retreat or withdraw.

    refrigerant Noun

    substance used to keep materials cool.

    relocate Verb

    to move a residence or business from one place to another.

    retreat Verb

    to go back to a familiar or safe place.

    rock Noun

    natural substance composed of solid mineral matter.

    scour Verb

    to rub harshly, often to polish.

    sea level Noun

    base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

    Encyclopedic Entry: sea level
    season Noun

    period of the year distinguished by special climatic conditions.

    Encyclopedic Entry: season
    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    Encyclopedic Entry: sediment
    sedimentary rock Noun

    rock formed from fragments of other rocks or the remains of plants or animals.

    severe Adjective


    solar power Noun

    rate of producing, transferring, or using solar energy.

    starvation Noun

    dying from lack of food.

    storm Noun

    severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

    sunspot Noun

    dark, cooler area on the surface of the sun that can move, change, and disappear over time.

    technology Noun

    the science of using tools and complex machines to make human life easier or more profitable.

    temperate Adjective


    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    till Noun

    rock, earth, and gravel left behind by a retreating or melting glacier.

    timber Noun

    wood in an unfinished form, either trees or logs.

    tornado Noun

    a violently rotating column of air that forms at the bottom of a cloud and touches the ground.

    trade Noun

    buying, selling, or exchanging of goods and services.

    trade wind Noun

    winds that blow toward the Equator, from northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere.

    transportation Noun

    movement of people or goods from one place to another.

    tree ring Noun

    layered formation in the trunk of a tree that marks its growth at least once a year.

    tropical Adjective

    existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

    tropical disease Noun

    sickness that usually occurs in a warm, humid climate.

    typhoon Noun

    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.

    vapor Noun

    visible liquid suspended in the air, such as fog.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: Plate Tectonics and Volcanic Activity
    weather Noun

    state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

    Encyclopedic Entry: weather
    weather pattern Noun

    repeating or predictable changes in the Earth's atmosphere, such as winds, precipitation, and temperatures.

    West Nile virus Noun

    infectious disease spread by mosquitoes, with symptoms ranging from mild flu to possible death.

    wind energy Noun

    kinetic energy produced by the movement of air, able to be converted to mechanical power.

    yellow fever Noun

    infectious disease spread by mosquitoes, primarily affecting the liver.