Courtesy of SOHO consortium. SOHO (Solar and Heliospheric Observatory) is a project of international cooperation between the European Space Agency and NASA.
Did you know that the sun blasts more than a billion tons of matter out into space at millions of kilometers per hour? Ultimately, energy from the sun is the driving force behind weather and climate, and life on earth. But what kinds of energy come from the sun? How does that energy travel through space? And what happens when it reaches earth?
Copyright 2000-2001 University Corporation for Atmospheric Research. All Rights Reserved. Used with permission.
The sun emits many forms of electromagnetic radiation in varying quantities. As shown in the diagram (opposite), about 43 percent of the total radiant energy emitted from the sun is in the visible parts of the spectrum. The bulk of the remainder lies in the
near-infrared (49 percent) and ultraviolet section (7 percent). Less than 1 percent of solar radiation is emitted as x-rays, gamma waves, and radio waves.
The transfer of energy from the sun across nearly empty space (remember that space is a vacuum) is accomplished primarily by radiation. Radiation is the transfer of energy by electromagnetic wave motion.
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.
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.
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.
This article was written by Kimberly Lightle. For more information, see the Contributors page. Email Kimberly at firstname.lastname@example.org.
Copyright October 2008 – The Ohio State University. This material is based upon work supported by the National Science Foundation under Grant No. 0733024. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.This work is licensed under an Attribution-ShareAlike 3.0 Unported Creative Commons license.
Click here for a print quality PDF version of the radiation parameters diagram
The MY NASA DATA Live Access Server contains a number of parameters of the Earth’s Radiation Budget. The schematic at the right shows where these parameters fit into the bigger picture of the Earth system. The information below further explains these various parameters, which are marked in bold in the text below.
See a NASA Fact Sheet on this topic.
View Earth’s Energy Budget (graphic courtesy Loeb et al., 2009).
There are two main locations where we measure the radiation budget:
- at the surface (SFC; using surface instrumentation or satellite techniques)
- at the top of the atmosphere (TOA; using satellites).
In theory one could also measure the radiation flow at any level between those two, but it’s not so easy to do in practice. Also, the meaning of TOA depends on what you are measuring. For Earth radiation, the best value for the TOA is 20 km, which corresponds to the absorption cross-section of the Earth-atmosphere system (i.e., sunlight that is captured by the Earth and its atmosphere).
There are two important directions when dealing with the Earth’s radiation budget. Energy can either be
- incoming (Down)
- or outgoing (Up).
Of course energy goes sideways as well, but that has no effect on the Earth’s radiation budget since it does not change the amount of energy in the Earth system.
The NET energy added to the Earth system is simply: NET = incoming – outgoing = Down – Up.
Everything emits electromagnetic radiation, according to its temperature. The graph shows examples for three objects with widely different temperatures (given in degrees Kelvin [K = C + 273.15]). Note that hotter objects emit more energy than cooler objects at every wavelength, and that the hotter objects emit most of their energy at shorter wavelengths than do cooler objects. The peak of the Sun’s energy emission is in the wavelengths of visible light (denoted by the rainbow colors in the graph), although it emits other types of energy (like ultraviolet [UV] and infrared [IR]) also. In comparison, the Earth emits orders of magnitude less radiation, and that radiation peaks in the infrared.
A Bit About Units
The units for this energy emission start with the familiar unit Watts, which is used to identify the brightness of light bulbs. However, the units here are more complicated, as they identify Watts PER square meter, PER micron, PER steradian (sr). Decoding this, it means the number of Watts reaching a square meter of area (m2), which depends on the wavelength (measured in microns), and is spread over a solid angle (measured in steradians). A solid angle is an angle measured in 3D space. The Sun emits in all directions, which means its energy is spread over 4π steradians. Only a small amount of the Sun’s energy emission reaches the Earth. Most of it goes off into space in other directions.
The Sun is also very far from the Earth (about 150 million km). So, by the time the Sun’s emitted energy reaches the Earth, it is spread over a very large area (a sphere with a radius 216 times larger than the radius of the Sun itself), thus reducing the intensity of energy that Earth receives.
Energy at the Earth
The figure at right compares the emission from the Earth with the amount of solar energy that reaches the top of the Earth’s atmosphere. (The amount of energy from the Sun that actually reaches the Earth’s surface is smaller still, and has some very definite spectral features due to absorption and scattering that occur at specific wavelengths in the atmosphere. See a picture. In particular, most of the UV radiation is absorbed in the ozone layer, protecting living things from these harmful rays.) Notice that now, the emission from the Earth is larger than the energy received from the Sun, for wavelengths longer than about 5 microns (a micron is 1.e-6 meters, or one thousandth of a meter). The exact cross-over point depends on the Earth’s emission temperature. The average surface temperature is a bit warmer than the emission temperature shown in this graph, so the cross-over point for surface emission occurs at a shorter wavelength. This slightly fuzzy cross-over point is used to define two principal wavelength ranges:
- shortwave (SW) radiation and
- longwave (LW) radiation.
Photosynthetically active radiation (PAR) (wavelengths between 0.4 and 0.7 microns) is a subset of the solar radiation that is important for photosynthetic activity in plants.
The combination of SW and LW radiation covers the whole spectrum and is sometimes called the total (TOT) radiation.
The components of the radiation budget change with time. It is not really practical to keep an infinitely detailed, second by second, record. We typically have information averaged over time periods such as
- annual averages or
- climatology: an average over several years.
The numbers in the radiation budget diagram at the top of the page are climatological values.
Most of the parameters in the LAS are for monthly or longer time periods, with a few parameters available at a daily time scale. Given monthly information, you can create seasonal, annual or longer averages.
The Earth’s radiation budget is determined by the amount of energy that actually enters and leaves the Earth, under the sky conditions that actually exist. Clouds have the most influence on what the upward and downward energy actually is. In the shortwave, clouds reflect sunlight. In the longwave, clouds effectively trap infrared energy. To help study and understand how clouds affect the radiation budget, we therefore look at radiation budget parameters under two different sky conditions:
- All-sky is the term for the actual observed conditions, including clear or cloudy skies wherever they occur.
- Clear-sky parameters are obtained by taking all observations for a month, discarding any where clouds were present, and computing average conditions for cases of clear sky.
Clear-sky parameters therefore allow you to see more clearly the effects of the Sun’s location, and of the Earth’s surface (i.e., land vs ocean).
Important parameters of the Earth’s radiation budget include:
- Flux: can be SW, LW, or total (TOT=SW+LW); can be Up or Down.
- Albedo: Albedo is defined for SW radiation. It tells what fraction of the incoming (Down) radiation is reflected (Up). The albedo of the surface (SFC) is NOT the same as the albedo at the TOA, due to the effects of clouds, the atmosphere, and aerosols.
- Net radiation: This gives the amount of energy actually added to the system. It is easy to calculate:
NET = Energy in – Energy out
- But there are multiple kinds of net radiation, both at TOA and at the surface:
- SW_NET = SW_Down – SW_Up.
- LW_NET = LW_Down – LW_Up.
- TOT_NET = SW_NET + LW_NET = SW_Down – SW_Up + LW_Down – LW_Up