• Global warming describes the current rise in the average temperature of Earth’s air and oceans. Global warming is often described as the most recent example of climate change.

    Earth’s climate has changed many times. Our planet has gone through multiple ice ages, in which ice sheets and glaciers covered large portions of the Earth. It has also gone through warm periods when temperatures were higher than they are today.

    Past changes in Earth’s temperature happened very slowly, over hundreds of thousands of years. However, the recent warming trend is happening much faster than it ever has. Natural cycles of warming and cooling are not enough to explain the amount of warming we have experienced in such a short time—only human activities can account for it. Scientists worry that the climate is changing faster than some living things can adapt to it.

    In 1988, the World Meteorological Organization and the United Nations Environment Programme established a committee of climatologists, meteorologists, geographers, and other scientists from around the world. This Intergovernmental Panel on Climate Change (IPCC) includes thousands of scientists who review the most up-to-date research available related to global warming and climate change. The IPCC evaluates the risk of climate change caused by human activities.

    According to the IPCC’s most recent report (in 2007), Earth’s average surface temperatures have risen about 0.74 degrees Celsius (1.33 degrees Fahrenheit) during the past 100 years. The increase is greater in northern latitudes. The IPCC also found that land regions are warming faster than oceans. The IPCC states that most of the temperature increase since the mid-20th century is likely due to human activities.

    The Greenhouse Effect

    Human activities contribute to global warming by increasing the greenhouse effect. The greenhouse effect happens when certain gases—known as greenhouse gases—collect in Earth’s atmosphere. These gases, which occur naturally in the atmosphere, include carbon dioxide, methane, nitrogen oxide, and fluorinated gases sometimes known as chlorofluorocarbons (CFCs).

    Greenhouse gases let the sun’s light shine onto the Earth’s surface, but they trap the heat that reflects back up into the atmosphere. In this way, they act like the insulating glass walls of a greenhouse. The greenhouse effect keeps Earth’s climate comfortable. Without it, surface temperatures would be cooler by about 33 degrees Celsius (60 degrees Fahrenheit), and many life forms would freeze.

    Since the Industrial Revolution in the late 1700s and early 1800s, people have been releasing large quantities of greenhouse gases into the atmosphere. That amount has skyrocketed in the past century. Greenhouse gas emissions increased 70 percent between 1970 and 2004. Emissions of carbon dioxide, the most important greenhouse gas, rose by about 80 percent during that time. The amount of carbon dioxide in the atmosphere today far exceeds the natural range seen over the last 650,000 years.

    Most of the carbon dioxide that people put into the atmosphere comes from burning fossil fuels such as oil, coal, and natural gas. Cars, trucks, trains, and planes all burn fossil fuels. Many electric power plants also burn fossil fuels.


    Another way people release carbon dioxide into the atmosphere is by cutting down forests. This happens for two reasons. Decaying plant material, including trees, releases tons of carbon dioxide into the atmosphere. Living trees absorb carbon dioxide. By diminishing the number of trees to absorb carbon dioxide, the gas remains in the atmosphere.

    Most methane in the atmosphere comes from livestock farming, landfills, and fossil fuel production such as coal mining and natural gas processing. Nitrous oxide comes from agricultural technology and fossil fuel burning.

    Fluorinated gases include chlorofluorocarbons, hydrochlorofluorocarbons, and hydrofluorocarbons. These greenhouse gases are used in aerosol cans and refrigeration.

    All of these human activities add greenhouse gases to the atmosphere, trapping more heat than usual and contributing to global warming.

    Effects of Global Warming

    Even slight rises in average global temperatures can have huge effects. Perhaps the biggest, most obvious effect is that glaciers and ice caps melt faster than usual. The meltwater drains into the oceans, causing sea levels to rise and oceans to become less salty.

    Ice sheets and glaciers advance and retreat naturally. As Earth’s temperature has changed, the ice sheets have grown and shrunk, and sea levels have fallen and risen. Ancient corals found on land in Florida, Bermuda, and the Bahamas show that the sea level must have been 5 to 6 meters (16-20 feet) higher 130,000 years ago than it is today. Earth doesn’t need to become oven-hot to melt the glaciers. Northern summers were just 3 to 5 degrees Celsius (5-9 degrees Fahrenheit) warmer during the time of those ancient fossils than they are today.

    However, the speed at which global warming is taking place is unprecedented. The effects are unknown.

    Glaciers and ice caps cover about 10 percent of the world’s landmass today. They hold about 75 percent of the world’s fresh water. If all of this ice melted, sea levels would rise by about 70 meters (230 feet). The IPCC reported that the global sea level rose about 1.8 millimeters (0.07 inches) per year from 1961 to 1993, and 3.1 millimeters (0.12 inches) per year since 1993.

    Rising sea levels could flood coastal communities, displacing millions of people in areas such as Bangladesh, the Netherlands, and the U.S. state of Florida. Forced migration would impact not only those areas, but the regions to which the “climate refugees” flee. Millions more people in countries like Bolivia, Peru, and India depend on glacial meltwater for drinking, irrigation, and hydroelectric power. Rapid loss of these glaciers would devastate those countries.

    Glacial melt has already raised the global sea level slightly. However, scientists are discovering ways the sea level could increase even faster. For example, the melting of the Chacaltaya Glacier in Bolivia has exposed dark rocks beneath it. The rocks absorb heat from the sun, speeding up the melting process.

    Many scientists use the term “climate change” instead of “global warming.” This is because greenhouse gas emissions affect more than just temperature. Another effect involves changes in precipitation like rain and snow. Patterns in precipitation may change or become more extreme. Over the course of the 20th century, precipitation increased in eastern parts of North and South America, northern Europe, and northern and central Asia. However, it has decreased in parts of Africa, the Mediterranean, and parts of southern Asia.

    Future Changes

    Nobody can look into a crystal ball and predict the future with certainty. However, scientists can make estimates about future population growth, greenhouse gas emissions, and other factors that affect climate. They can enter those estimates into computer models to find out the most likely effects of global warming.

    The IPCC predicts that greenhouse gas emissions will continue to increase over the next few decades. As a result, they predict the average global temperature will increase by about 0.2 degrees Celsius (0.36 degrees Fahrenheit) per decade. Even if we reduce greenhouse gas and aerosol emissions to their 2000 levels, we can still expect a warming of about 0.1 degree Celsius (0.18 degrees Fahrenheit) per decade.

    The panel also predicts global warming will contribute to some serious changes in water supplies around the world. By the middle of the 21st century, the IPCC predicts, river runoff and water availability will most likely increase at high latitudes and in some tropical areas. However, many dry regions in the mid-latitudes and tropics will experience a decrease in water resources.

    As a result, millions of people may be exposed to water shortages. Water shortages decrease the amount of water available for drinking, electricity, and hygiene. Shortages also reduce water used for irrigation. Agricultural output would slow and food prices would climb. Consistent years of drought in the Great Plains of the United States and Canada would have this effect.

    IPCC data also suggest that the frequency of heat waves and extreme precipitation will increase. Weather patterns such as storms and tropical cyclones will become more intense. Storms themselves may be stronger, more frequent, and longer-lasting. They would be followed by stronger storm surges, the immediate rise in sea level following storms. Storm surges are particularly damaging to coastal areas because their effects (flooding, erosion, damage to buildings and crops) are lasting.

    What We Can Do

    Reducing our greenhouse gas emissions is a critical step in slowing the global warming trend. Many governments around the world are working toward this goal.

    The biggest effort so far has been the Kyoto Protocol, which was adopted in 1997 and went into effect in 2005. By the end of 2009, 187 countries had signed and ratified the agreement. Under the protocol, 37 industrialized countries and the European Union have committed to reducing their greenhouse gas emissions.

    There are several ways that governments, industries, and individuals can reduce greenhouse gases. We can improve energy efficiency in homes and businesses. We can improve the fuel efficiency of cars and other vehicles. We can also support development of alternative energy sources, such as solar power and biofuels, that don’t involve burning fossil fuels.

    Some scientists are working to capture carbon dioxide and store it underground, rather than let it go into the atmosphere. This process is called carbon sequestration.

    Trees and other plants absorb carbon dioxide as they grow. Protecting existing forests and planting new ones can help balance greenhouse gases in the atmosphere.

    Changes in farming practices could also reduce greenhouse gas emissions. For example, farms use large amounts of nitrogen-based fertilizers, which increase nitrogen oxide emissions from the soil. Reducing the use of these fertilizers would reduce the amount of this greenhouse gas in the atmosphere.

    The way farmers handle animal manure can also have an effect on global warming. When manure is stored as liquid or slurry in ponds or tanks, it releases methane. When it dries as a solid, however, it does not.

    Reducing greenhouse gas emissions is vitally important. However, the global temperature has already changed and will most likely continue to change for years to come. The IPCC suggests that people explore ways to adapt to global warming as well as try to slow or stop it. Some of the suggestions for adapting include:

    The Greenhouse Effect and our Planet
    The lonely polar bear—the unofficial mascot of global warming.

    Disappearing Penguins
    Emperor penguins made a showbiz splash in the 2005 film March of the Penguins. Sadly, their encore might include a disappearing act. In the 1970s, an abnormally long warm spell caused these Antarctic birds' population to drop by 50 percent. Some scientists worry that continued global warming will push the creatures to extinction by changing their habitat and food supply.

    Barking Up the Wrong Tree
    Spruce bark beetles in Alaska have had a population boom thanks to 20 years of warmer-than-average summers. The insects have managed to chew their way through 1.6 million hectares (4 million acres) of spruce trees.

    Shell Shock
    A sudden increase in the amount of carbon dioxide in the atmosphere does more than change Earth's temperature. A lot of the carbon dioxide in the air dissolves into seawater. There, it forms carbonic acid in a process called ocean acidification. Ocean acidification is making it hard for some sea creatures to build shells and skeletal structures. This could alter the ecological balance in the oceans and cause problems for fishing and tourism industries.

  • Term Part of Speech Definition Encyclopedic Entry
    abnormal Adjective


    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 technology Noun

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

    ancient Adjective

    very old.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    biofuel Noun

    energy source derived directly from organic matter, such as plants.

    buffer Noun

    a cushion or shield.

    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.

    carbonic acid Noun

    chemical produced as carbon dioxide dissolves in water.

    carbon sequestration Noun

    process of capturing carbon emissions and storing them underground.

    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
    climate refugee Noun

    person forced to leave his or her home and community because of climate change.

    Encyclopedic Entry: environmental refugee
    climatologist Noun

    person who studies long-term patterns in weather.

    coal Noun

    dark, solid fossil fuel mined from the earth.

    coast Noun

    edge of land along the sea or other large body of water.

    Encyclopedic Entry: coast
    committee Noun

    group of people elected or appointed to perform a task.

    conservation Noun

    management of a natural resource to prevent exploitation, destruction, or neglect.

    Encyclopedic Entry: conservation
    consistent Adjective

    maintaining a steady, reliable quality.

    cope Verb

    to handle or deal with problems.

    coral Noun

    tiny ocean animal, some of which secrete calcium carbonate to form reefs.

    coral reef Noun

    rocky ocean features made up of millions of coral skeletons.

    critical Adjective

    very important.

    cyclone Noun

    weather system that rotates around a center of low pressure and includes thunderstorms and rain. Usually, hurricanes refer to cyclones that form over the Atlantic Ocean.

    data Plural Noun

    (singular: datum) information collected during a scientific study.

    decade Noun

    10 years.

    decay Verb

    to rot or decompose.

    desalination Noun

    process of converting seawater to fresh water by removing salt and minerals.

    devastate Verb

    to destroy.

    diminish Verb

    to become smaller or less important.

    dissolve Verb

    to break up or disintegrate.

    diverse Adjective

    varied or having many different types.

    drought Noun

    period of greatly reduced precipitation.

    Encyclopedic Entry: drought
    electricity Noun

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

    emission Noun

    discharge or release.

    encore Noun

    reappearance by a performer after the end of the performance.

    energy efficiency Noun

    use of a relatively small amount of energy for a given task, purpose, or service; achieving a specific output with less energy input.

    erosion Noun

    act in which earth is worn away, often by water, wind, or ice.

    Encyclopedic Entry: erosion
    establish Verb

    to form or officially organize.

    European Union Noun

    association of European nations promoting free trade, ease of transportation, and cultural and political links.

    evaluate Verb

    to decide something's worth.

    farming Noun

    the art, science, and business of cultivating the land for growing crops.

    fertilizer Noun

    nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.

    flee Verb

    to run away.

    fluorinate Verb

    to add or combine with the element fluorine (F).

    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.

    freeze Noun

    weather pattern of temperatures below 0 degrees Celsius (32 degrees Fahrenheit).

    fuel efficiency Noun

    ability to produce as much power with as little fuel consumed as possible.

    geographer Noun

    person who studies places and the relationships between people and their environments.

    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
    Great Plains Noun

    grassland region of North America, between the Rocky Mountains and the Mississippi River.

    greenhouse Noun

    building, often made of glass or other clear material, used to help plants grow.

    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.

    heat-health action plan Noun

    public system for preventing or reducing death and disease due to extreme heat waves.

    heat wave Noun

    period of unusually hot weather.

    hydrochlorofluorocarbon Noun

    greenhouse gas often used as an industrial cooling material.

    hydroelectric power Noun

    usable energy generated by moving water converted to electricity.

    hygiene Noun

    science and methods of keeping clean and healthy.

    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 cap Noun

    area of fewer than 50,000 square kilometers (19,000 square miles) covered by ice.

    Encyclopedic Entry: ice cap
    ice sheet Noun

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

    Encyclopedic Entry: ice sheet
    industrialization Noun

    growth of machine production and factories.

    Industrial Revolution Noun

    change in economic and social activities, beginning in the 18th century, brought by the replacement of hand tools with machinery and mass production.

    infrastructure Noun

    structures and facilities necessary for the functioning of a society, such as roads.

    insect Noun

    type of animal that breathes air and has a body divided into three segments, with six legs and usually wings.

    Intergovernmental Panel on Climate Change (IPCC) Noun

    group of scientists who review the most up-to-date research available related to global warming and climate change.

    irrigation Noun

    watering land, usually for agriculture, by artificial means.

    Encyclopedic Entry: irrigation
    Kyoto Protocol Noun

    (1997) international agreement to reduce greenhouse gas emissions.

    landfill Noun

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

    landmass Noun

    large area of land.

    latitude Noun

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

    Encyclopedic Entry: latitude
    livestock noun, plural noun

    animals raised for sale and profit.

    manure Noun

    animal excrement or waste used to fertilize soil.

    marsh Noun

    wetland area usually covered by a shallow layer of seawater or freshwater.

    Encyclopedic Entry: marsh
    meltwater Noun

    freshwater that comes from melting snow or ice.

    meteorologist Noun

    person who studies patterns and changes in Earth's atmosphere.

    methane Noun

    chemical compound that is the basic ingredient of natural gas.

    migration Noun

    movement of a group of people or animals from one place to another.

    model Noun

    image or impression of an object used to represent the object or system.

    multiple Adjective


    natural gas Noun

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

    Encyclopedic Entry: natural gas
    nitrogen Noun

    chemical element with the symbol N, whose gas form is 78% of the Earth's atmosphere.

    nitrous oxide Noun

    greenhouse gas used in medicine and the manufacture of rockets. Also known as laughing gas or happy gas.

    oil Noun

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

    penguin Noun

    bird native to the Antarctic.

    power plant Noun

    industrial facility for the generation of electric energy.

    precipitation Noun

    all forms in which water falls to Earth from the atmosphere.

    Encyclopedic Entry: precipitation
    protocol Noun

    series of rules.

    rain catchment Noun

    system to collect and store rainwater.

    ratify Verb

    to formally approve or confirm.

    refrigerant Noun

    substance used to keep materials cool.

    research Noun

    scientific observations and investigation into a subject, usually following the scientific method: observation, hypothesis, prediction, experimentation, analysis, and conclusion.

    runoff Noun

    overflow of fluid from a farm or industrial factory.

    Encyclopedic Entry: runoff
    sea level Noun

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

    Encyclopedic Entry: sea level
    seawall Noun

    barrier built to protect a beach or shoreline from erosion. Also called a bulkhead.

    ski resort Noun

    facility where people can ski for recreation or sport.

    skyrocket Verb

    to increase rapidly.

    slurry Noun

    liquid waste, such as that from the coal mining and cleaning process, also called sludge.

    snow Noun

    precipitation made of ice crystals.

    solar power Noun

    rate of producing, transferring, or using solar energy.

    spruce Noun

    coniferous, or cone-bearing, tree.

    storm Noun

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

    storm surge Noun

    abnormal rise in sea level accompanying a hurricane or other intense storm. Also called a storm tide.

    Encyclopedic Entry: storm surge
    storm surge barrier Noun

    walls or obstacles built to prevent ocean or river water from flowing into an area after a storm. Also called levee or floodgate.

    surveillance Noun

    observation of a person, community, or situation.

    temperature Noun

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

    Encyclopedic Entry: temperature
    tourism Noun

    the industry (including food, hotels, and entertainment) of traveling for pleasure.

    tropical Adjective

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

    United Nations Environment Programme Noun

    organization whose mission is to provide international leadership and encourage partnerships in caring for the environment.

    unprecedented Adjective

    never before known or experienced.

    vital Adjective

    necessary or very important.

    water shortage Noun

    reduction in the amount of fresh water available for drinking, hygiene, and industrial and agricultural use.

    water supply Noun

    amount of available fresh water for drinking, hygiene, and industrial and agricultural use.

    weather pattern Noun

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

    wetland Noun

    area of land covered by shallow water or saturated by water.

    Encyclopedic Entry: wetland
    World Meteorological Organization Noun

    United Nations agency that studies the Earth's atmosphere, its interaction with the oceans, the climate, and the distribution of water resources.