FIRST STOP: EARTH’S ATMOSPHERE

Once the sun’s energy reaches earth, it is intercepted first by the atmosphere. A small part of the sun’s energy is directly absorbed, particularly by certain gases such as ozone and water vapor.

Some of the sun’s energy is reflected back to space by clouds and the earth’s surface.

Copyright 2000-2001 University Corporation for Atmospheric Research. All Rights Reserved. Used with permission.

Most of the radiation, however, is absorbed by the earth’s surface. When the radiation is absorbed by a substance, the atoms in the substance move faster and the substance becomes warm to the touch. The absorbed energy is transformed into heat energy. This heat energy plays an important role in regulating the temperature of the earth’s crust, surface waters, and lower atmosphere.

Every surface on earth absorbs and reflects energy at varying degrees, based on its color and texture. Dark-colored objects absorb more visible radiation; light-colored objects reflect more visible radiation. Shiny or smooth objects reflect more, while dull or rough objects absorb more. Differences in reflection impact temperature, weather, and climate.

REFLECT OR ABSORB?

Scientists use the term albedo to describe the percentage of solar radiation reflected back into space by an object or surface.

A perfectly black surface has an albedo of 0 (all radiation is absorbed). A perfectly white surface has an albedo of 1.0 (all radiation is reflected).

Different features of earth (such as snow, ice, tundra, ocean, and clouds) have different albedos. For example, land and ocean have low albedos (typically from 0.1 to 0.4) and absorb more energy than they reflect. Snow, ice, and clouds have high albedos (typically from 0.7 to 0.9) and reflect more energy than they absorb.

Earth’s average albedo is about 0.3. In other words, about 30 percent of incoming solar radiation is reflected back into space and 70 percent is absorbed.

A sensor aboard NASA’s Terra satellite is now collecting detailed measurements of how much sunlight the earth’s surface reflects back up into the atmosphere. By quantifying precisely our planet’s albedo, the Moderate Resolution Imaging Spectroradiometer (MODIS) is helping scientists understand and predict how various surface features influence both short-term weather patterns as well as longer-term climate trends.

Image courtesy of NASA Earth Observatory.

The colors in this image emphasize the albedo over the earth’s land surfaces, ranging from 0.0 to 0.4. Areas colored red show the brightest, most reflective regions; yellows and greens are intermediate values; and blues and violets show relatively dark surfaces. White indicates where no data were available, and no albedo data are provided over the oceans.

As shown in the image, the snow- and ice-covered Arctic has a high albedo. (Though no data were available, Antarctica would also have a high albedo.) Desert areas, such as the Sahara in Northern Africa, also reflect a great deal of radiation. Forested areas or areas with dark soil absorb more radiation and have lower albedos.

Human and natural processes have changed the albedo of earth’s land surfaces. For example, earth’s average albedo was much higher during the last ice age than it is today. Human impacts such as deforestation, air pollution, and the decrease in Arctic sea ice have also affected albedo values. These changes alter the net amounts of energy absorbed and radiated back to space.

EARTH’S RADIATION BUDGET

NASA Atmospheric Science Data Center. Used with permission.

Earth’s radiation budget is a concept that helps us understand how much energy Earth receives from the Sun, and how much energy Earth radiates back to outer space.

Changes in the earth’s crust such as glaciation, deforestation, and polar ice melting alter the quantity and wavelength of electromagnetic absorption and reflection at the earth’s surface.

ICE, CLIMATE CHANGE, AND THE EARTH’S ENERGY BUDGET

Ice affects the entire earth system in a variety of ways. In the ocean and at the land-sea boundary, ice prevents relatively warm ocean water from evaporating, transferring heat to the colder atmosphere and thereby increasing global air temperature.

Image courtesy of Hugo Ahlenius, UNEP/GRID-Arendal Maps and Graphics Library.

Ice also reflects sunlight, thus preventing additional heat from being absorbed by water or land. The ice-covered polar regions are colder than other places on earth, due in part to the high albedo of the snow and ice cover.

As earth’s climate warms, ice in the form of glaciers and sea ice has decreased dramatically. Data generated from satellites that monitor the formation of polar sea ice indicate that both coverage and thickness have decreased over the past three decades. Recent studies show that the world’s highest glaciers (in the Himalayas) are receding at an average rate of 10 to 15 meters (33 to 49 feet) per year. A study released in June 2008 indicates that Arctic sea ice extent shrank to a record low in the summer of 2007.

The decreasing extent of ice in the polar regions (in particular, the sea ice of the Arctic) is part of a positive feedback loop that can accelerate climate change. Warmer temperatures melt snow and ice, which decreases earth’s albedo, causing further warming and more melting.

Human activities that create pollution also influence the energy balance. For example, when we burn coal, oil, wood, and other fuels, the carbon byproduct, soot, is released into the atmosphere and eventually deposited back on earth. The dark particles land on snow and ice, and decrease albedo. The darkened snow and ice absorb more radiation than pure snow and ice. In addition, as the snow and ice melt, the soot embedded in the snow is left behind and becomes more concentrated on the surface, further accelerating warming.

IN CONCLUSION

There’s no doubt about it – without the sun’s radiant energy, life on earth would not exist. But as the earth warms and polar ice declines, the balance of absorbed and reflected energy shifts – leading to further change.

Resources

Earth’s Albedo and Global Warming
This interactive activity adapted from NASA and the U.S. Geological Survey illustrates the concept of albedo – the measure of how much solar radiation is reflected from Earth’s surface.

Earth’s Cryosphere: The Arctic
This four-minute video segment adapted from NASA uses satellite imagery to provide an overview of the cryosphere (the frozen parts of the earth’s surface) in the Northern Hemisphere, including the Arctic.

Earth’s Cryosphere: Antarctica
This video segment adapted from NASA uses satellite imagery to provide an overview of the cryosphere in the Antarctic.

Arctic Sea Ice News & Analysis
The National Snow and Ice Data Center (NSIDC) provides the latest news, research, and analysis of Arctic sea ice.

Sea Level: Ice Volume Changes
This resource provides a simulation of icebergs and glaciers melting and the impact melting has on sea level.

NATIONAL SCIENCE EDUCATION STANDARDS: SCIENCE CONTENT STANDARDS

The entire National Science Education Standards document can be read online or downloaded for free from the National Academies Press web site. The following excerpt was taken from Chapter 6.

A study of energy, the sun, and albedo aligns with the Physical Science, Earth and Space Science, and the Science in Personal and Social Perspectives content standards of the National Science Education Standards:

Physical Science (Content Standard B): Grades K-4

As a result of their activities in grades K-4, all students should develop an understanding of properties of objects and materials including light, heat, electricity, and magnetism.

• Objects have many observable properties, including size, weight, shape, color, temperature, and the ability to react with other substances. Those properties can be measured using tools, such as rulers, balances, and thermometers.
• Light travels in a straight line until it strikes an object. Light can be reflected by a mirror, refracted by a lens, or absorbed by the object.
• Heat can be produced in many ways, such as burning, rubbing, or mixing one substance with another. Heat can move from one object to another by conduction.

Physical Science (Content Standard B): Grades 5-8

As a result of their activities in grades 5-8, all students should develop an understanding of earth in the solar system.

• The sun is the major source of energy for phenomena on the earth’s surface, such as growth of plants, winds, ocean currents, and the water cycle.
• Seasons result from variations in the amount of the sun’s energy hitting the surface, due to the tilt of the earth’s rotation on its axis and the length of the day.

Science in Personal and Social Perspectives (Content Standard F): Grades K-4

As a result of their activities in grades K-4, all students should develop an understanding of changes in environments.

• Environments are the space, conditions, and factors that affect an individuals’ and a populations’ ability to survive and their quality of life.
• Changes in environments can be natural or influenced by humans. Some changes are good, some are bad, and some are neither good nor bad.
• Some environmental changes occur slowly, and others occur rapidly.

Science in Personal and Social Perspectives (Content Standard F): Grades 5-8

As a result of their activities in grades 5-8, all students should develop an understanding of natural hazards.

• Human activities can induce hazards through resource acquisition, urban growth, land-use decisions, and waste disposal. Such activities can accelerate many natural changes.