Global Warming or Climate Change

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Global Warming




Greenhouse Effect

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Global Warming or Climate Change, measurable increases in the average temperature of Earth’s atmosphere, oceans, and landmasses. Scientists believe Earth is currently facing a period of rapid warming brought on by rising levels of heat-trapping gases, known as greenhouse gases, in the atmosphere.

Greenhouse gases retain the radiant energy (heat) provided to Earth by the Sun in a process known as the greenhouse effect. Greenhouse gases occur naturally, and without them the planet would be too cold to sustain life as we know it. Since the beginning of the Industrial Revolution in the mid-1700s, however, human activities have added more and more of these gases into the atmosphere. For example, levels of carbon dioxide, a powerful greenhouse gas, have risen by 35 percent since 1750, largely from the burning of fossil fuels such as coal, oil, and natural gas. With more greenhouse gases in the mix, the atmosphere acts like a thickening blanket and traps more heat.




Milankovitch Cycles

Milankovitch cycles are three periodic variations in Earth’s orientation toward the Sun. Scientists believe the three variations combine to produce cyclical changes in climate about once every 100,000 years.

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Earth has warmed and cooled many times since its formation about 4.6 billion years ago. Global climate changes were due to many factors, including massive volcanic eruptions, which increased carbon dioxide in the atmosphere; changes in the intensity of energy emitted by the Sun; and variations in Earth’s position relative to the Sun, both in its orbit and in the inclination of its spin axis.

Variations in Earth’s position, known as Milankovitch cycles, combine to produce cyclical changes in the global climate. These cycles are believed to be responsible for the repeated advance and retreat of glaciers and ice sheets during the Pleistocene Epoch (1.8 million to 11,500 years before present), when Earth went through fairly regular cycles of colder “glacial” periods (also known as ice ages) and warmer “interglacial” periods. Glacial periods occurred at roughly 100,000-year intervals.

An interglacial period began about 10,000 years ago, when the last ice age came to an end. Prior to that ice age, an interglacial period occurred about 125,000 years ago. During interglacial periods, greenhouse gases such as carbon dioxide and methane naturally increase in the atmosphere from increased plant and animal life. But since 1750 greenhouse gases have increased dramatically to levels not seen in hundreds of thousands of years, due to the rapid growth of the human population combined with developments in technology and agriculture. Human activities now are a powerful factor influencing Earth’s dynamic climate.

The ice of the polar regions furnishes clues to the makeup of Earth’s ancient atmosphere. Ice cores that scientists have bored from the ice sheets of Greenland and Antarctica provide natural records of both temperature and atmospheric greenhouse gases going back hundreds of thousands of years. Layers in these ice cores created by seasonal snowfall patterns allow scientists to determine the age of the ice in each core. By measuring tiny air bubbles trapped in the ice and properties of the ice itself, scientists can estimate the temperature and amount of greenhouse gases in Earth’s past atmosphere at the time each layer formed. Based on this data, scientists know that greenhouse gases have now risen to levels higher than at any time in the last 650,000 years.

Greenhouse gases are rising, and temperatures are following. Before the late 1800s, the average surface temperature of Earth was almost 15°C (59°F). Over the past 100 years, the average surface temperature has risen by about 0.7 Celsius degrees (1.3 Fahrenheit degrees), with most of the increase occurring since the 1970s. Scientists have linked even this amount of warming to numerous changes taking place around the world, including melting mountain glaciers and polar ice, rising sea level, more intense and longer droughts, more intense storms, more frequent heat waves, and changes in the life cycles of many plants and animals. Warming has been most dramatic in the Arctic, where temperatures have risen almost twice as much as the global average.




Scientists project global warming to continue at a rate that is unprecedented in hundreds of thousands or even millions of years of Earth’s history. They predict considerably more warming in the 21st century, depending on the level of future greenhouse gas emissions. For a scenario (possible situation) assuming higher emissions—in which emissions continue to increase significantly during the century—scientists project further warming of 2.4 to 6.4 Celsius degrees (4.3 to 11.5 Fahrenheit degrees) by the year 2100. For a scenario assuming lower emissions—in which emissions grow slowly, peak around the year 2050, and then fall—scientists project further warming of 1.1 to 2.9 Celsius degrees (1.9 to 5.2 Fahrenheit degrees) by the year 2100.

Melting polar ice and glaciers, as well as warming of the oceans, expands ocean volume and raises sea level, which will eventually flood some coastal regions and even entire islands. Patterns of rainfall are expected to change, with higher latitudes (closer to the poles) projected to receive more rainfall, and subtropical areas (such as the Mediterranean and southern Africa) projected to receive considerably less. Changes in temperature and precipitation patterns may damage food crops, disrupting food production in some parts of the world. Plant and animal species will shift their ranges toward the poles or to higher elevations seeking cooler temperatures, and species that cannot do so may become extinct. Increasing levels of carbon dioxide in the atmosphere also leads to increased ocean acidity, damaging ocean ecosystems.

Human beings face global warming with a huge population at risk. The potential consequences are so great that many of the world’s leading scientists—and increasingly, politicians, business leaders, and other citizens—are calling for international cooperation and immediate action to counteract the problem.




Industrial Air Pollution

Contaminants pouring from industrial smokestacks contribute to the world’s atmospheric pollution. Some of these contaminants, such as carbon dioxide and nitrous oxide, are greenhouse gases. Once in the atmosphere, these gases act to retain the long-wave radiation (heat) emitted by Earth in a process known as the greenhouse effect.

James L. Stanfield/National Geographic Society

The energy that lights and warms Earth comes from the Sun. Short-wave radiation from the Sun, including visible light, penetrates the atmosphere and is absorbed by the surface, warming Earth. Earth’s surface, in turn, releases some of this heat as long-wave infrared radiation.

Much of this long-wave infrared radiation makes it back out to space, but a portion remains trapped in Earth’s atmosphere, held in by certain atmospheric gases, including water vapor, carbon dioxide, and methane. Absorbing and reflecting heat radiated by Earth, these gases act somewhat like the glass in a greenhouse, and are thus known as greenhouse gases.

Only greenhouse gases, which make up less than 1 percent of the atmosphere, offer the Earth any insulation. All life on Earth relies on the greenhouse effect—without it, the average surface temperature of the planet would be about -18°C (0°F) and ice would cover Earth from pole to pole.



Types of Greenhouse Gases

Greenhouse gases occur naturally in the environment and also result from human activities. By far the most abundant greenhouse gas is water vapor, which reaches the atmosphere through evaporation from oceans, lakes, and rivers. The amount of water vapor in the atmosphere is not directly affected by human activities. Carbon dioxide, methane, nitrous oxide, and ozone all occur naturally in the environment, but they are being produced at record levels by human activities. Other greenhouse gases do not occur naturally at all and are produced only through industrial processes. Human activities also produce airborne particles called aerosols, which offset some of the warming influence of increasing greenhouse gases.



Carbon Dioxide

Carbon Cycle

Carbon, used by all living organisms, continuously circulates in Earth’s ecosystem. In the atmosphere it exists as carbon dioxide, a colorless, odorless gas. Plants absorb carbon dioxide in the process of photosynthesis. Animals acquire the carbon stored in plant tissue when they eat, and they exhale carbon dioxide as a by-product of metabolism. Surface bodies of water, especially oceans, absorb vast amounts of atmospheric carbon dioxide. Some carbon is removed from circulation as deposits of coal, oil, natural gas, and limestone. The burning of fossil fuels adds additional carbon dioxide to the atmosphere.

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Carbon dioxide is the second most abundant greenhouse gas, after water vapor. Carbon dioxide constantly circulates in the environment through a variety of natural processes known as the carbon cycle. It is released into the atmosphere from natural processes such as eruptions of volcanoes; the respiration of animals, which breathe in oxygen and exhale carbon dioxide; and the burning or decay of plants and other organic matter. Carbon dioxide leaves the atmosphere when it is absorbed into water, especially the oceans, and by plants, especially trees. Through a process called photosynthesis, plants use the energy of light to convert carbon dioxide and water into simple sugars, which they use as food. In the process, plants store carbon in new tissue and release oxygen as a byproduct.


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Humans are significantly increasing the amount of carbon dioxide released to the atmosphere through the burning of fossil fuels (such as coal, oil, and natural gas), solid wastes, and wood and wood products to heat buildings, drive vehicles, and generate electricity. At the same time, the number of trees available to absorb carbon dioxide through photosynthesis has been greatly reduced by deforestation, the widespread cutting of trees for lumber or to clear land for agriculture.

Coal-Burning Power Plant

This power plant burns coal to produce electricity. The burning of coal and other fossil fuels releases carbon dioxide into the atmosphere. Carbon dioxide is the second most abundant greenhouse gas, after water vapor. Human activities have significantly increased the amount of greenhouse gases in the atmosphere, and scientists have linked this increase to global warming.

Lester Lefkowitz/Corbis

Human activities are causing carbon dioxide to be released to the atmosphere much faster than Earth’s natural processes can remove it. In addition, carbon dioxide can remain in the atmosphere a century or more before nature can dispose of it. Before the Industrial Revolution began in the mid-1700s, there were about 280 molecules of carbon dioxide per million molecules of air (abbreviated as parts per million, or ppm). Concentrations of carbon dioxide have risen since then as industrial production and fossil fuel-based transportation and electricity generation have spread around the world, accelerating in the last 50 years. In 2007 the Intergovernmental Panel on Climate Change (IPCC), a major scientific organization, reported that levels of carbon dioxide had risen to a record high of 379 ppm and are increasing an average of 1.9 ppm per year.

To stabilize atmospheric concentrations of carbon dioxide, global emissions would need to be cut significantly—on the order of 70 to 80 percent. If efforts are not made to reduce greenhouse gas emissions, carbon dioxide is projected to reach concentrations more than double or even triple the level prior to the Industrial Revolution by 2100. In a higher-emissions scenario carbon dioxide is projected to reach 970 ppm by 2100, more than tripling preindustrial concentrations. In a lower-emissions scenario, carbon dioxide is projected to reach 540 ppm by 2100, still almost doubling preindustrial concentrations. (For a description of these two emissions scenarios, see the Introduction: Global Warming in the Future section of this article.)




Alaskan Tundra

Global warming has been most dramatic in the Arctic, where temperatures have risen almost twice as much as the global average. The vast tundra (Arctic plains) of Alaska, Siberia, and other subpolar regions contains a layer of frozen subsoil called permafrost. The area of frozen ground has decreased due to rising temperatures, and scientists are concerned that as the permafrost melts it will release large amounts of methane, a potent greenhouse gas, into the atmosphere. This would accelerate global warming.

Pat O'Hara Photography