Saturday, 10 March 2012

The Earth's Radiation Budget


The energy entering, reflected, absorbed, and emitted by the Earth system are the components of the Earth's radiation budget. Based on the physics principle of conservation of energy, this radiation budget represents the accounting of the balance between incoming radiation, which is almost entirely solar radiation, and outgoing radiation, which is partly reflected solar radiation and partly radiation emitted from the Earth system, including the atmosphere. A budget that's out of balance can cause the temperature of the atmosphere to increase or decrease and eventually affect our climate. The units of energy employed in measuring this incoming and outgoing radiation are watts per square meter (W/m2).
Full page spread showing the photographic view explaining the Earth's radiation budget, and the diagram showing radiation emitting from Earth's surface and atmosphere. Detailed views shown below.

INCOMING SOLAR RADIATION

Incoming ultraviolet, visible, and a limited portion of infrared energy (together sometimes called "shortwave radiation") from the Sun drive the Earth's climate system. Some of this incoming radiation is reflected off clouds, some is absorbed by the atmosphere, and some passes through to the Earth's surface. Larger aerosol particles in the atmosphere interact with and absorb some of the radiation, causing the atmosphere to warm. The heat generated by this absorption is emitted as longwave infrared radiation, some of which radiates out into space.

ABSORBED ENERGY

The solar radiation that passes through Earth's atmosphere is either reflected off snow, ice, or other surfaces or is absorbed by the Earth's surface.
A photographic view of mountains, clouds, rolling hills and water used as a backdrop to explain the Earth's radiation budget. Subsequent illustrations describe the overlying diagram of arrows explaining the budget.

Emitted LONGWAVE Radiation

Heat resulting from the absorption of incoming shortwave radiation is emitted as longwave radiation. Radiation from the warmed upper atmosphere, along with a small amount from the Earth's surface, radiates out to space. Most of the emitted longwave radiation warms the lower atmosphere, which in turn warms our planet's surface.
A diagram of arrows showing radiation emitting from the Earth's surface and atmosphere. This longwave radiation either escapes out to space or absorbed by the lower atmosphere. Much of what is absorbed by the atmosphere is emitted back to the surface of Earth.

GREENHOUSE EFFECT

Greenhouse gases in the atmosphere (such as water vapor and carbon dioxide) absorb most of the Earth's emitted longwave infrared radiation, which heats the lower atmosphere. In turn, the warmed atmosphere emits longwave radiation, some of which radiates toward the Earth's surface, keeping our planet warm and generally comfortable. Increasing concentrations of greenhouse gases such as carbon dioxide and methane increase the temperature of the lower atmosphere by restricting the outward passage of emitted radiation, resulting in "global warming," or, more broadly, global climate change.
Incoming shortwave radiation enters our atmosphere and is either reflected or absorbed by the atmosphere; reflected by light colored areas on the Earth's surface such as ice and snow; or the radiation is absorbed by the surface.
Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

RADIATION AND THE CLIMATE SYSTEM

For scientists to understand climate change, they must also determine what drives the changes within the Earth's radiation budget. The Clouds and the Earth's Radiant Energy System (CERES) instrument aboard NASA's Aqua and Terra satellites measures the shortwave radiation reflected and longwave radiation emitted into space accurately enough for scientists to determine the Earth's total radiation budget. Other NASA instruments monitor changes in other aspects of the Earth's climate system—such as clouds, aerosol particles, and surface reflectivity—and scientists are examining their many interactions with the radiation budget.

Reflected Near-Infrared Waves


An illustration of the Near-Infrared region of the spectrum by wavelength from 3.0 microns to 0.75 microns. Shortwave IR is roughly between 3.0 and 1.4 microns. Near IR is between 1.4 and 0.75 microns. The diameter of the E. coli bacteria is roughly the size of the length of these wavelengths.

NEAR INFRARED RADIATION

A portion of radiation that is just beyond the visible spectrum is referred to as near-infrared. Rather than studying an object's emission of infrared, scientists can study how objects reflect, transmit, and absorb the Sun's near-infrared radiation to observe health of vegetation and soil composition.

HEALTHY VEGETATION

Our eyes perceive a leaf as green because wavelengths in the green region of the spectrum are reflected by pigments in the leaf, while the other visible wavelengths are absorbed. In addition, the components in plants reflect, transmit, and absorb different portions of the near-infrared radiation that we cannot see.
Reflected near-infrared radiation can be sensed by satellites, allowing scientists to study vegetation from space. Healthy vegetation absorbs blue- and red-light energy to fuel photosynthesis and create chlorophyll. A plant with more chlorophyll will reflect more near-infrared energy than an unhealthy plant. Thus, analyzing a plants spectrum of both absorption and reflection in visible and in infrared wavelengths can provide information about the plants' health and productivity.
A cut-away illustration of the internal structure of a leaf. Shows indicating red, gree, blue, and infrared light energy interact with the leaf structure. The Red and Blue light is absorbed, green light is reflected by the top layer of the mesophyll and the infrared energy is reflected off the bottom layer.
Credit: Jeff Carns

INFRARED FILM

Color Infrared film can record near-infrared energy and can help scientists study plant diseases where there is a change in pigment and cell structure. These two images show the difference between a color infrared photo and a natural color photo of trees in a park.
Two photographs of a park with grass and trees. The color film photograph shows the trees as green, like how our eyes perceive green leaves. The same trees appear in various shades of red in the Color Infrared photograph.
Credit: Ginger Butcher
Two photographs of a park with grass and trees. The color film photograph shows the trees as green, like how our eyes perceive green leaves. The same trees appear in various shades of red in the Color Infrared photograph.

SPECTRAL SIGNATURES OF VEGETATION

Data from scientific instruments can provide more precise measurements than analog film. Scientists can graph the measurements, examine the unique patterns of absorption and reflection of visible and infrared energy, and use this information to identify types of plants. The graph below shows the differences among the spectral signatures of corn, soybeans, and Tulip Poplar trees.
Spectral Signatures of Vegetation
Credit: Eric Brown de Colstoun

ASSESSING VEGETATION FROM SPACE

Data and imagery from the U.S. Geological Service (USGS) and NASA Landsat series of satellites are used by the U.S. Department of Agriculture to forecast agricultural productivity each growing season. Satellite data can help farmers pinpoint where crops are infested, stressed, or healthy.
Satellite image of crops appear as a mosaic of red and brown squares.
Near-infrared data collected by the Landsat 7 satellite, such as this image of Minnesota, can help farmers assess the health of their crops. Shades of red in this image indicate good crop health, and yellow colors reveal where crops are infested. Credit: Jesse Allen, using Landsat data provided by the United States Geological Survey

SOIL COMPOSITION

Near-infrared data can also help identify types of rock and soil. This image of the Saline Valley area in California was acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) onboard NASA's Terra satellite.
Data from ASTER's visible and near-infrared bands at 0.81 µm, 0.56 µm, and .66 µm are composited in red, green, and blue creating the false-color image below. Vegetation appears red, snow and dry salt lakes are white, and exposed rocks are brown, gray, yellow, and blue. Rock colors mainly reflect the presence of iron minerals and variations in albedo (solar energy reflected off the surface).
A false color satellite image revealing varied soil composition in the valley.
Credit: NASA, GSFC, MITI, ERSDAC, JAROS, and the U.S./Japan ASTER Science Team

PLANETS IN NEAR-INFRARED

This false-color composite of Jupiter combines near-infrared and visible-light data of sunlight reflected from Jupiter's clouds. Since methane gas in Jupiter's atmosphere limits the penetration of sunlight, the amount of reflected near-infrared energy varies depending on the clouds' altitude. The resulting composite image shows this altitude difference as different colors. Yellow colors indicate high clouds; red colors are lower clouds; and blue colors show even lower clouds in Jupiter's atmosphere. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) onboard NASA's Hubble Space Telescope captured this image at the time of a rare alignment of three of Jupiter's largest moons—Io, Ganymede, and Callisto—across the planet's face.
This false-color Hubble image of Jupiter shows pastel pinks, greens, yellows and blues stretched in lines across the gas planet.
Credit: NASA and E. Karkoschka (University of Arizona)

Infrared Waves


An illustration of the Infrared region of the spectrum by wavelength from 1000 microns to 0.7 microns. The diameter of human hair is about 25 microns in size. This spectrum id sub-divided into 4 areas: Far I.R. from about 1000 microns to 15 microns; Thermal I.R. from about 15 microns to 8 microns; Mid I.R. from about 8 microns to 3 microns; and Near I.R. from about 3 microns to 0.7 microns.

INFRARED ENERGY

A remote control uses light waves just beyond the visible spectrum of light—infrared light waves—to change channels on your TV. This region of the spectrum is divided into near-, mid-, and far-infrared. The region from 8 to 15 microns (µm) is referred to by Earth scientists as thermal infrared since these wavelengths are best for studying the longwave thermal energy radiating from our planet.
a television remote control
A typical television remote control uses infrared energy at a wavelength around 940 nanometers. While you cannot "see" the light emitting from a remote, some digital and cell phone cameras are sensitive to that wavelength of radiation. Try it out!
An infrared heat lamp
Infrared lamps heat lamps often emit both visible and infrared energy at wavelengths between 500nm to 3000nm in length. They can be used to heat bathrooms or keep food warm. Heat lamps can also keep small animals and reptiles warm or even to keep eggs warm so they can hatch.

DISCOVERY OF INFRARED

An illustration showing a line of thermometers placed along a rainbow. The thermometers show cooler temperature at the blue end of the rainbow and higher temperatures at the red end of the spectrum.
Credit: Troy Benesch
In 1800, William Herschel conducted an experiment measuring the difference in temperature between the colors in the visible spectrum. He placed thermometers within each color of the visible spectrum. The results showed an increase in temperature from blue to red. When he noticed an even warmer temperature measurement just beyond the red end of the visible spectrum, Herschel had discovered infrared light!

THERMAL IMAGING

We can sense some infrared energy as heat. Some objects are so hot they also emit visible light—such as a fire does. Other objects, such as humans, are not as hot and only emit only infrared waves. Our eyes cannot see these infrared waves but instruments that can sense infrared energy—such as night-vision goggles or infrared cameras–allow us to "see" the infrared waves emitting from warm objects such as humans and animals. The temperatures for the images below are in degrees Fahrenheit.
A true-color image of a small dog along with images of the same dog in thermal infrared. The color -coded infrared images reveal areas of higher temperature around the eyes and mouth, with cooler temperatures on the nose and snout.
Credit: NASA/JPL-Caltech

COOL ASTRONOMY

Many objects in the universe are too cool and faint to be detected in visible light but can be detected in the infrared. Scientists are beginning to unlock the mysteries of cooler objects across the universe such as planets, cool stars, nebulae, and many more, by studying the infrared waves they emit.
The Cassini spacecraft captured this image of Saturn's aurora using infrared waves. The aurora is shown in blue, and the underlying clouds are shown in red. These aurorae are unique because they can cover the entire pole, whereas aurorae around Earth and Jupiter are typically confined by magnetic fields to rings surrounding the magnetic poles. The large and variable nature of these aurorae indicates that charged particles streaming in from the Sun are experiencing some type of magnetism above Saturn that was previously unexpected.
Saturn's Aurora in IR

SEEING THROUGH DUST

Infrared waves have longer wavelengths than visible light and can pass through dense regions of gas and dust in space with less scattering and absorption. Thus, infrared energy can also reveal objects in the universe that cannot be seen in visible light using optical telescopes. The James Webb Space Telescope (JWST) has three infrared instruments to help study the origins of the universe and the formation of galaxies, stars, and planets.
Spitzer image of Orion in Infrared & James Web Telescope
When we look up at the constellation Orion, we see only the visible light. But NASA's Spitzer space telescope was able to detect nearly 2,300 planet-forming disks in the Orion nebula by sensing the infrared glow of their warm dust. Each disk has the potential to form planets and its own solar system. Credit: Thomas Megeath (Univ. Toledo) et al., JPL, Caltech, NASA
A pillar composed of gas and dust in the Carina Nebula is illuminated by the glow from nearby massive stars shown below in the visible light image from the Hubble Space Telescope. Intense radiation and fast streams of charged particles from these stars are causing new stars to form within the pillar. Most of the new stars cannot be seen in the visible-light image (left) because dense gas clouds block their light. However, when the pillar is viewed using the infrared portion of the spectrum (right), it practically disappears, revealing the baby stars behind the column of gas and dust.
Two images showing the Carina Nebula in different wavelengths. The Visible Light image reveals a brilliant display of yellow and gold dust lit up by stars. The Infrared image only shows the bright stars that were behind the dust.
Credit: NASA, ESA, and the Hubble SM4 ERO Team

MONITORING THE EARTH

To astrophysicists studying the universe, infrared sources such as planets are relatively cool compared to the energy emitted from hot stars and other celestial objects. Earth scientists study infrared as the thermal emission (or heat) from our planet. As incident solar radiation hits Earth, some of this energy is absorbed by the atmosphere and the surface, thereby warming the planet. This heat is emitted from Earth in the form of infrared radiation. Instruments onboard Earth observing satellites can sense this emitted infrared radiation and use the resulting measurements to study changes in land and sea surface temperatures.
There are other sources of heat on the Earth's surface, such as lava flows and forest fires. The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Aqua and Terra satellites uses infrared data to monitor smoke and pinpoint sources of forest fires. This information can be essential to firefighting efforts when fire reconnaissance planes are unable to fly through the thick smoke. Infrared data can also enable scientists to distinguish flaming fires from still-smoldering burn scars.
A satellite image showing smoke from forest fires wisping across the forest covered landscape of Northern California. Bright red areas at the base of these smoke plumes indicates the size of the actual area on fire.
Credit: Jeff Schmaltz, MODIS Rapid Response Team
an infrared image of the Earth taken by the GOES 6 satellite
Credit: Space Science and Engineering Center, University of Wisconsin-Madison, Richard Kohrs, designer
The global image on the right is an infrared image of the Earth taken by the GOES 6 satellite in 1986. A scientist used temperatures to determine which parts of the image were from clouds and which were land and sea. Based on these temperature differences, he colored each separately using 256 colors, giving the image a realistic appearance.
Why use the infrared to image the Earth? While it is easier to distinguish clouds from land in the visible range, there is more detail in the clouds in the infrared. This is great for studying cloud structure. For instance, note that darker clouds are warmer, while lighter clouds are cooler. Southeast of the Galapagos, just west of the coast of South America, there is a place where you can distinctly see multiple layers of clouds, with the warmer clouds at lower altitudes, closer to the ocean that's warming them.
We know, from looking at an infrared image of a cat, that many things emit infrared light. But many things also reflect infrared light, particularly near infrared light. Learn more about REFLECTED Near-infrared radiation.

Visualization: From Energy to Image


HOW DO WE VISUALIZE LIGHT WE CAN'T SEE?

False color, or representative color, is used to help scientists visualize data from wavelengths beyond the visible spectrum. Scientific instruments onboard NASA spacecraft sense regions within the electromagnetic spectrum—spectral bands. The instruments direct the electromagnetic energy onto a detector, where individual photons yield electrons related to the amount of incoming energy. The energy is now in the form of "data," which can be transmitted to Earth and processed into images.

DIGITAL CAMERA

Digital cameras operate similarly to some scientific instruments. A sensor in the camera captures the brightness of red, green, and blue light and records these brightness values as numbers. The three sets of data are then combined in the red, green, and blue channels of a computer monitor to create a color image.
Three small grayscale images showing each channel of a digital photo of a hot air balloon. The blue channel shows a light gray area along the blue strip of the balloon. The composite shows a full color image with bright yellow, blue, orange and red stripes.

NATURAL COLOR IMAGES

Instruments onboard satellites can also capture visible light data to create natural color, or true color, satellite images. Data from visible light bands are composited in their respective red, green, and blue channels on screen. The image simulates a color image that our eyes would see from the vantage point of the spacecraft.
Three small grayscale images showing each channel of an image of Saturn. The forth image shows a full color image of Saturn with light browns and warm grays.
Credit: NASA and The Hubble Heritage Team

FALSE COLOR IMAGES

Sensors can also record brightness values in regions beyond visible light. This Hubble image of Saturn was taken at longer infrared wavelengths and composited in the red, green, and blue channels respectively. The resulting false-color composite image reveals compositional variations and patterns that would otherwise be invisible.
Three small grayscale images showing each channel of an image of Saturn in false color. The forth image show Saturn with brilliant colors of purple, blue, green and orange.
Credit: NASA/JPL/STScI
false-color infrared image from the Thermal Emission Imaging System (THEMIS) camera onboard the Mars Odyssey spacecraft
Martian Soil
This false-color infrared image from the Thermal Emission Imaging System (THEMIS) camera onboard the Mars Odyssey spacecraft reveals the differences in the mineralogy, chemical composition, and structure of the Martian surface. Large deposits of the mineral olivine appear in this image as magenta to purple-blue.

DATA FROM MULTIPLE SENSORS

This composite image of the spiral galaxy Messier 101 combines views from Spitzer, Hubble, and Chandra space telescopes. The red color shows Spitzer's view in infrared light. It highlights the heat emitted by dust lanes in the galaxy where stars can form. The yellow color is Hubble's view in visible light. Most of this light comes from stars, and they trace the same spiral structure as the dust lanes. The blue color shows Chandra's view in x-ray light. Sources of x-rays include million-degree gas, exploded stars, and material colliding around black holes.
The three small images used for the composite show a galaxy in red, yellow, and blue. The composite shows all three colors together revealing a multi-colored galaxy.
Credit: NASA, ESA, CXC, JPL, Caltech and STScI
Such composite images allow astronomers to compare how features are seen in multiple wavelengths. It's like "seeing" with a camera, night-vision goggles, and x-ray vision all at once.

COLOR MAPS

Often a data set, such as elevation or temperature data, is best represented as a range of values. To help scientists visualize the data, the values are mapped to a color scale. The color code is arbitrary and thus can be chosen according to how the data can best be visualized. The sea surface temperature map below uses a scale from dark blue for cold temperatures to red for warm temperatures.
An image of the Earth with red color around the equator representing ocean temperatures of 30 degrees centigrade. The colors get cooler, from yellow to green to blue, the closer the ocean is to the poles.
Credit: NASA/Goddard Space Flight Center
Evaporation at the ocean's surface leaves minerals and salts behind. For this and other reasons, the salinity of the ocean varies from place to place. This map shows the long-term averages of sea surface salinity using practical salinity units—units used to describe the concentration of dissolved salts in water. The white regions have the highest salinity and the dark regions have the lowest.
An image of the Earth with the ocean colored a variety of shades from white to dark blue. The white indicates high levels of salinity and are prevalent in the Atlantic Ocean, Mediterranean Sea and bodies of water around the Middle East.
Credit: NASA/Goddard Space Flight Center