What is the Greenhouse Effect? | Global Warming What Is the Greenhouse Effect? By Marc Lallanila | April 12, 2016 03:05pm ET MORE While other planets in Earth's solar system are either scorching hot or bitterly cold, Earth's surface has relatively mild, stable temperatures. Earth enjoys these temperatures because of its atmosphere, which is the thin layer of gases that cloak and protect the planet. However, 97 percent of climate scientists agree that humans have changed Earth's atmosphere in dramatic ways over the past two centuries, resulting in global warming. To understand global warming, it's first necessary to become familiar with the greenhouse effect, though. Energy in, energy out There's a delicate balancing act occurring every day all across the Earth, involving the radiation the planet receives from space and the radiation that's reflected back out to space. Earth is constantly bombarded with enormous amounts of radiation, primarily from the sun. This solar radiation strikes the Earth's atmosphere in the form of visible light, plus ultraviolet (UV), infrared (IR) and other types of radiation that are invisible to the human eye.

UV radiation has a shorter wavelength and a higher energy level than visible light, while IR radiation has a longer wavelength and a weaker energy level. About 30 percent of the radiation striking Earth's atmosphere is immediately reflected back out to space by clouds, ice, snow, sand and other reflective surfaces, according to NASA . The remaining 70 percent of incoming solar radiation is absorbed by the oceans, the land and the atmosphere. As they heat up, the oceans, land and atmosphere release heat in the form of IR thermal radiation, which passes out of the atmosphere and into space. It's this equilibrium of incoming and outgoing radiation that makes the Earth habitable, with an average temperature of about 59 degrees Fahrenheit (15 degrees Celsius), according to NASA. Without this atmospheric equilibrium, Earth would be as cold and lifeless as its moon, or as blazing hot as Venus. The moon , which has almost no atmosphere, is about minus 243 F (minus 153 C) on its dark side. Venus, on the other hand, has a very dense atmosphere that traps solar radiation; the average temperature on Venus is about 864 F (462 C).

The greenhouse effect The exchange of incoming and outgoing radiation that warms the Earth is often referred to as the greenhouse effect because a greenhouse works in much the same way. Incoming UV radiation easily passes through the glass walls of a greenhouse and is absorbed by the plants and hard surfaces inside. Weaker IR radiation, however, has difficulty passing through the glass walls and is trapped inside, thus warming the greenhouse. This effect lets tropical plants thrive inside a greenhouse, even during a cold winter. A similar phenomenon takes place in a car parked outside on a cold, sunny day. Incoming solar radiation warms the car's interior, but outgoing thermal radiation is trapped inside the car's closed windows. Greenhouse gases and global warming "Gas molecules that absorb thermal infrared radiation, and are in significant enough quantity, can force the climate system. These type of gas molecules are called greenhouse gases," Michael Daley, an associate professor of Environmental Science at Lasell College told Live Science. Carbon dioxide (CO2) and other greenhouse gases act like a blanket, absorbing IR radiation and preventing it from escaping into outer space. The net effect is the gradual heating of Earth's atmosphere and surface, a process known as global warming .

These greenhouse gases include water vapor, CO2, methane, nitrous oxide (N2O) and other gases, according to the Environmental Protection Agency (EPA). Since the dawn of the Industrial Revolution in the early 1800s, the burning of fossil fuels like coal, oil and gasoline have greatly increased the concentration of greenhouse gases in the atmosphere, especially CO2, National Oceanic and Atmospheric Administration (NOAA). "Deforestation is the second largest anthropogenic source of carbon dioxide to the atmosphere ranging between 6 percent and 17 percent," said Daley. Atmospheric CO2 levels have increased by more than 40 percent since the beginning of the Industrial Revolution, from about 280 parts per million (ppm) in the 1800s to 400 ppm today. The last time Earth's atmospheric levels of CO2 reached 400 ppm was during the Pliocene Epoch, between 5 million and 3 million years ago, according to the University of California, San Diego's Scripps Institution of Oceanography .

The greenhouse effect, combined with increasing levels of greenhouse gases and the resulting global warming, is expected to have profound implications, according to the near-universal consensus of scientists. If global warming continues unchecked, it will cause significant climate change, a rise in sea levels, increasing ocean acidification, extreme weather events and other severe natural and societal impacts, according to NASA, the EPA and other scientific and governmental bodies. Can the greenhouse effect be reversed? Many scientists agree that the damage to the Earth's atmosphere and climate is past the point of no return or that the damage is near the point of no return. "I agree that we have passed the point of avoiding climate change," Josef Werne, an associate professor at the department of geology & planetary science at the University of Pittsburgh told Live Science. In Werne's opinion, there are three options from this point forward: Do nothing and live with the consequences. Adapt to the changing climate (which includes things like rising sea level and related flooding). Mitigate the impact of climate change by aggressively enacting policies that actually reduce the concentration of CO2 in the atmosphere.

Keith Peterman, a professor of chemistry at York College of Pennsylvania, and Gregory Foy, an associate professor of chemistry at York College of Pennsylvania believes that the damage isn't to that point yet, and that international agreements and action can save the planet's atmosphere. Additional reporting by Alina Bradford, Live Science Contributor For the latest information on the greenhouse effect, visit: The Greenhouse Effect The Radiation Balance The Spectrum of Electromagnetic Radiation An idealized spectrum of electromagnetic radiation from the Sun, with a temperature of 6000 °C (~11,000 °F) , and the much cooler Earth, with a surface temperature of 15 °C (~60 °F). Wavelength along the horizontal axis (measured in microns where 1000 microns = 1mm) plotted against the amount of radiation emitted on the vertical axis. Note how the Sun emits the most radiation in the visible range (with some overlap in the UV and IR) while the Earth emits mostly infrared radiation . The incoming energy from the Sun to Earth is mainly visible sunlight, called the �visible portion of the spectrum of electromagnetic radiation .� We perceive visible sunlight as colors from violet (short-wave radiation) to red (long-wave radiation).

The sequence of colors seen in the rainbow represents the � spectrum � of this light, ordered according to wavelength. A relatively minor amount of energy leaves the sun as radiation with shorter wavelength (� ultraviolet �) and as radiation with longer wavelength (�infrared� or � heat radiation �). Visible light (the colors of the rainbow) occupies the narrow part of the spectrum between the dashed lines in the first figure. The (invisible) light with wavelengths just shorter than violet is called �ultraviolet,� meaning �beyond violet.� It is largely absorbed in the atmosphere and only a modest amount of this light arrives at the surface of Earth. This is fortunate, because ultraviolet light , abbreviated �UV,� can cause damage to skin and to vertebrate retinas and interferes with photosynthesis in algae and plants. Protection from UV light is provided (among other things) through the ozone layer in the lower stratosphere (The ozone layer will be discussed in more detail later Section 2.6). The (invisible) light with wavelengths just longer than red is called � infrared ,� meaning �below red� and often simply referred to as �IR.� IR is heat radiation coming from the Sun.

Interestingly, some organisms, especially some insects, can see UV and navigate by it, while some snakes have adapted to see in IR. (See the Glossary for more reading on electromagnetic radiation under Spectrum.) The Sun�s Radiation All objects (unless they have a temperature of zero degrees Kelvin ) radiate energy. The temperature of an object determines the type of radiation it emits. Hence, every star radiates energy with wavelengths corresponding to its surface temperature: a cooler star would radiate a more reddish light, a hotter one a more bluish one. Reddish and bluish stars can be seen readily in the night sky. Most of the light emitted by our star, the Sun, is yellowish. By measuring the light received from the Sun we know that its radiation corresponds to a surface temperature of about 6000°C (or 6300 K where �K� is the symbol for units of Kelvin � see the Units Glossary for more about the Kelvin scale). The organisms on Earth have long adapted to the nature of this sunlight. Blue-green light penetrates most deeply into the sea, so visual acuity of deep-living fishes is greatest in the blue-green part of the spectrum.

Our own eyes are specialized for yellow, green and red (the colors of traffic lights). Plants use mostly red light to grow and reflect the rest, making them appear green. The nuclear fusion generator that powers the Sun is deep in the Sun�s center (called the �core� of the Sun), hidden by a thick layer of hot hydrogen and helium . This is fortunate for us, because no one could look at the Sun's power plant and survive the experience: the temperature is near 15 million degrees of Kelvin. The reason the power plant does not blow the sun apart is that the enormous pressure of the solar matter surrounding it prevents it from doing so. Conversely, the Sun does not collapse because of the counter-pressure generated in the core because in the Sun the gravitational and radiative pressures are in balance. It takes about a million years for the energy to made in the core to reach the surface of the Sun. From there it takes less than 10 minutes at the speed of light to reach Earth. The energy the core generates comes from the fusion of hydrogen nuclei to make helium nuclei.

Through this process some of the mass is �lost� and reappears as energy (according to the famous equation of Albert Einstein E=mc2), resulting in the loss of 4.5 million tons of mass from the Sun each second. Not to worry however: there is still plenty of hydrogen to burn �about two-thirds of the mass of the sun consists of hydrogen � and the process has been going on for some 5 billion years and will do so in the future for about as long (Also see the Glossary entry on Solar constant .). The Earth�s Radiation Just as the temperature of the Sun's surface determines the kind of electromagnetic radiation it delivers, so the temperature of the Earth determines what kind of radiation it puts out to space, which turns out to be infrared , or �heat� radiation. As mentioned, the amount of heat Earth has to get rid of is entirely determined by the amount it receives from the Sun in the first place minus the portion it immediately reflects back into space. (The reflected portion cannot be included in the portion that is re-radiated because it does not actually heat the Earth.

However, this reflected portion is visible from a spacecraft or when standing on the Moon: the Earth is reasonably bright as planets go, mainly because of its clouds and its ice caps and reflects 30% of the light it receives back to space. This proportion, called the �albedo� of Earth, is less than that reflected by Venus, but more than that of Mars - see the Glossary for more on albedo .). The kind of infrared radiation given off by the various areas of Earth's surface depends on their temperature, which in turn depends on a number of factors such as the amount of sunlight absorbed and the heat spent in evaporating water. In the desert, after sundown, one can readily sense the high-energy infrared given off by rocks recently warmed by the Sun's rays, but all surfaces radiate heat, whether recently warmed by the Sun or not. Typically, temperatures on the surface of Earth vary somewhere between freezing and 90°F, which roughly defines the broad "spectrum" of infrared radiation emitted upward into the atmosphere. Graph showing the mostly visible radiation spectrum emitted from the Sun as seen from the surface of the Earth (left curve) and the IR radiation emitted Earth as seen from space (right curve).

Compared it to Figure 1 this graph shows the wavelengths that are absorbed by atmospheric gases, causing gaps in the spectrum. For the left curve, it is obvious how ozone acts to absorb UV light before it reaches the Earth�s surface with some visible light also being absorbed by water vapor. For the right curve, the impact that the major greenhouse gases have on the amount of IR that Earth emits to space is clear: carbon dioxide, ozone , and water vapor act together to absorb and re-emit radiation that stays trapped in the lower atmosphere (that is, the outgoing radiation from Earth is much less intense than expected from the temperature one would measure in space due to the effect of greenhouse gases). Carbon dioxide absorbs at wavelengths centered on 15 microns, ozone at wavelengths of 10 microns, and water vapor over broad ranges of wavelengths. The Absorption of IR by Greenhouse Gases Now that we have explained the relationship between the radiation emitted by an object and its temperature, we can explain how greenhouse gases warm the Earth. Certain �lines� within the electromagnetic spectrum, specifically certain wavelengths of infrared radiation, have precisely the right energy to interact with certain molecules present within Earth�s atmosphere.

When such a special packet of light (called a � photon �) interacts with the appropriate molecule, the molecule absorbs the energy, and increases its temperature accordingly. It then re-radiates heat to its surroundings. When measured with an instrument, this absorbed heat forms absorption lines or even �absorption bands� that are broader than lines and may include several lines. The absorption bands of different greenhouse gases may or may not overlap with each other. When a greenhouse gas is very abundant the absorption lines for which it is active are said to �become saturated,� that is, most of the available IR will have been absorbed by the molecules of that gas. Adding more of that gas will not absorb more IR in the proportion of the addition. For example, many of carbon dioxide�s absorption lines are fairly well saturated. This is the fundamental reason that the 30 percent increase in carbon dioxide since the industrial revolution has not increased the background greenhouse effect by 30 percent. Only a doubling of CO2 will have a substantial effect, through the amplification caused by water vapor (resulting in a 4 to 6°F increase, according to the best estimates).

Another doubling on top of this presumably will have a similar effect, in part through a broadening of the absorption lines affected. Other greenhouse gases that were once rare or even nonexistent are now being introduced vigorously by human activities, and they can take up new absorption lines that have not been previously occupied. Thus, their effect on interception of IR and the associated greenhouse effect is accordingly enhanced. Methane is such a gas, as are the chlorofluorocarbon compounds (abbreviated CFC�s) formerly used in refrigeration. Greenhouse Effect: Background Material The Greenhouse Effect This section provides an overview of the earth's atmospheric "greenhouse effect" by briefly exploring the atmospheres of nearby planets and discussing our atmosphere's greenhouse gases. The general concepts found in this section include the following: The earth's "greenhouse effect" is what makes this planet suitable for life as we know it. The earth's atmosphere contains trace gases, some of which absorb heat. These gases (water vapor, carbon dioxide, methane, ozone, and nitrous oxide) are referred to as "greenhouse gases." Albedo has an important influence on the earth's temperature. Greenhouses are structures designed to retain heat.

The heat-trapping ability of a greenhouse is influenced by a number of factors including the transparency of the greenhouse cover, color of the surfaces inside the greenhouse, and type of surfaces inside. This section includes two classroom activities. Introduction The Goldilocks Principle can be summed up neatly as "Venus is too hot, Mars is too cold, and Earth is just right." The fact that Earth has an average surface temperature comfortably between the boiling point and freezing point of water, and thus is suitable for our sort of life, cannot be explained by simply suggesting that our planet orbits at just the right distance from the sun to absorb just the right amount of solar radiation. Our moderate temperatures are also the result of having just the right kind of atmosphere. A Venus-type atmosphere would produce hellish, Venus-like conditions on our planet; a Mars atmosphere would leave us shivering in a Martian-type deep freeze. Instead, parts of our atmosphere act as an insulating blanket of just the right thickness, trapping sufficient solar energy to keep the global average temperature in a pleasant range. The Martian blanket is too thin, and the Venusian blanket is way too thick!

The 'blanket' here is a collection of atmospheric gases called 'greenhouse gases' based on the idea that the gases also 'trap' heat like the glass walls of a greenhouse do. These gases, mainly water vapor ( ), carbon dioxide ( ), methane ( ), and nitrous oxide ( ), all act as effective global insulators. To understand why, it's important to understand a few basic facts about solar radiation and the structure of atmospheric gases. Solar Radiation The sun radiates vast quantities of energy into space, across a wide spectrum of wavelengths. Most of the radiant energy from the sun is concentrated in the visible and near-visible parts of the spectrum. The narrow band of visible light, between 400 and 700 nm, represents 43% of the total radiant energy emitted. Wavelengths shorter than the visible account for 7 to 8% of the total, but are extremely important because of their high energy per photon. The shorter the wavelength of light, the more energy it contains. Thus, ultraviolet light is very energetic (capable of breaking apart stable biological molecules and causing sunburn and skin cancers).

The remaining 49 - 50% of the radiant energy is spread over the wavelengths longer than those of visible light. These lie in the near infrared range from 700 to 1000 nm; the thermal infrared, between 5 and 20 microns; and the far infrared regions. Various components of earth's atmosphere absorb ultraviolet and infrared solar radiation before it penetrates to the surface, but the atmosphere is quite transparent to visible light. Absorbed by land, oceans, and vegetation at the surface, the visible light is transformed into heat and re-radiates in the form of invisible infrared radiation. If that was all there was to the story, then during the day earth would heat up, but at night, all the accumulated energy would radiate back into space and the planet's surface temperature would fall far below zero very rapidly. The reason this doesn't happen is that earth's atmosphere contains molecules that absorb the heat and re-radiate the heat in all directions. This reduces the heat radiated out to space.

Called 'greenhouse gases' because they serve to hold heat in like the glass walls of a greenhouse, these molecules are responsible for the fact that the earth enjoys temperatures suitable for our active and complex biosphere. Greenhouse Gases Carbon dioxide ( ) is one of the greenhouse gases. It consists of one carbon atom with an oxygen atom bonded to each side. When its atoms are bonded tightly together, the carbon dioxide molecule can absorb infrared radiation and the molecule starts to vibrate. Eventually, the vibrating molecule will emit the radiation again, and it will likely be absorbed by yet another greenhouse gas molecule. This absorption-emission-absorption cycle serves to keep the heat near the surface, effectively insulating the surface from the cold of space. Carbon dioxide, water vapor ( ), methane ( ), nitorus oxide ( ), and a few other gases are greenhouse gases. They all are molecules composed of more than two component atoms, bound loosely enough together to be able to vibrate with the absorption of heat. The major components of the atmosphere ( and ) are two-atom molecules too tightly bound together to vibrate and thus they do not absorb heat and contribute to the greenhouse effect.

Greenhouse Effect Atmospheric scientists first used the term 'greenhouse effect' in the early 1800s. At that time, it was used to describe the naturally occurring functions of trace gases in the atmosphere and did not have any negative connotations. It was not until the mid-1950s that the term greenhouse effect was coupled with concern over climate change. And in recent decades, we often hear about the greenhouse effect in somewhat negative terms. The negative concerns are related to the possible impacts of an enhanced greenhouse effect. This is covered in more detail in the Global Climate Change section of this Web site. It is important to remember that without the greenhouse effect, life on earth as we know it would not be possible. While the earth's temperature is dependent upon the greenhouse-like action of the atmosphere, the amount of heating and cooling are strongly influenced by several factors just as greenhouses are affected by various factors. In the atmospheric greenhouse effect, the type of surface that sunlight first encounters is the most important factor. Forests, grasslands, ocean surfaces, ice caps, deserts, and cities all absorb, reflect, and radiate radiation differently.

Sunlight falling on a white glacier surface strongly reflects back into space, resulting in minimal heating of the surface and lower atmosphere. Sunlight falling on a dark desert soil is strongly absorbed, on the other hand, and contributes to significant heating of the surface and lower atmosphere. Cloud cover also affects greenhouse warming by both reducing the amount of solar radiation reaching the earth's surface and by reducing the amount of radiation energy emitted into space. Scientists use the term albedo to define the percentage of solar energy reflected back by a surface. Understanding local, regional, and global albedo effects is critical to predicting global climate change. Concluding Thoughts The ability of certain trace gases to be relatively transparent to incoming visible light from the sun, yet opaque to the energy radiated from the earth is one of the best understood processes in the atmospheric sciences. This phenomenon, the greenhouse effect, is what makes the earth habitable for life. For students to truly understand the nature and importance of the greenhouse effect, they should understand the answers to these questions: What is a greenhouse and how does it work? How is the earth's atmosphere similar to a greenhouse? What factors influence the function of a greenhouse? What is albedo and how is it related to understanding global climate change?

What are the limitations in comparing the earth's atmosphere to a greenhouse? Activities Untitled Document Intergovernmental Panel on Climate Change (IPCC) Introduction The current concern about the greenhouse effect and climate stems from the amounts of greenhouse gases that are being released into the atmosphere from the burning of fossil fuels, deforestation, agricultural and industrial practices, release of synthetic chlorofluorocarbons, and other humankind activities. Accumulation of these heat-absorbing greenhouse gases in the atmosphere can result in an enhanced greenhouse effect and consequent global warming amplified by human activities. The concern is that an enhanced greenhouse effect may elevate global temperatures above levels that have not occurred for hundreds of thousands of years. The degree to which the accelerating rate of increase of greenhouse gases in the atmosphere will impact our climate is a topic of much debate and uncertainty because of the many variables that are involved in the climate system and their feedbacks. Atmospheric Absorption of Energy The sun primarily provides the energy that drives the Earth’s climate and weather by influencing atmospheric and surface processes ( click here to see a related discussion of the Electromagnetic Spectrum - Box 1 ).

The temperature of the surface of the sun is approximately 5480 oC and the radiation energy from the sun travels through space and impacts the Earth’s atmosphere. This radiation energy is primarily in the form of visible, short wave (ultraviolet), and long wave (infrared) radiation. Some of the sun’s energy is reflected back to space by the outer atmosphere, but the remainder of energy passes through the space-atmosphere boundary. Figure 4. Illustration of relationship between radiation intensity and incoming solar radiation at the top of Earth’s atmosphere and outgoing Earth radiation. (Note: Much of the incoming shortwave UV solar radiation is absorbed by oxygen (O2 and O3) in the upper atmosphere. The outgoing long wavelength radiation emitted by the Earth is partially to totally absorbed by the greenhouse gases of water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and tropospheric ozone (O3). The atoms and molecules that make up the atmosphere absorb the different types of radiation (visible, ultraviolet, and infrared) to varying degrees (see Figure 4).

Oxygen, in the form of O2 (diatomic oxygen) and O3 (triatomic oxygen, ozone), is the most important absorber of incoming radiation in the atmosphere. High in the atmosphere, diatomic oxygen (O2) absorbs radiation with wavelength less than 240 nanometers (240 x 10-9 meters) and at lower altitude ozone (O3) absorbs radiation within the globally encircling stratospheric ozone layer with wavelengths mainly between 200 to 300 nanometers (200 to 300 x 10-9 meters). This incoming solar radiation is actually strong enough to break bonds holding the diatomic oxygen (O2) and ozone (O3) molecules together causing the molecule to split. Here are some equations representing those reactions. O2 + solar radiation = O + O (1) and O3 + solar radiation = O2 + O (2). Most ozone in the atmosphere occurs in the stratosphere. The absorption of solar radiation by ozone in the stratosphere is the source for heat in the stratosphere and mesosphere (see Figure 3). The ozone also absorbs and blocks most ultraviolet radiation below 300 nanometers.

As solar radiation continues to penetrate the atmosphere and gets closer to the Earth’s surface, it is scattered, reflected, and absorbed by air molecules, clouds, and various types of particles. Of the incoming solar radiation that hits the boundary between the Earth's atmosphere and outer space, about 30% is reflected back to space by atmospheric clouds and the Earth’s surface, 25% is absorbed by the atmosphere and reradiated back to space, and 45% is absorbed by the surface of land and ocean (Figure 5). The temperature of the Earth’s surface and lower atmosphere is higher than would be expected for a planet the distance of the Earth from the sun. This is because of the insulating qualities of the greenhouse gases in the Earth's atmosphere. When short wavelength radiation from the sun is not intercepted by the outer atmosphere or the ozone layer, it penetrates to the surface of the planet, is absorbed by the Earth's surface, and it is reradiated back as energy of a longer wavelength (infrared radiation) because the Earth is much cooler than the sun. Water vapor, carbon dioxide and other greenhouse gases absorb and trap this longer wavelength radiation leading to a natural warming of Earth’s surface and the lower atmosphere (see Figure 6).

So it important to realize that the greenhouse gases don't trap incoming short wave radiation, but rather the long wave radiation that is emitted by the Earth's surface due to absorbing the short wave radiation. Figure 5. Earth’s radiation budget. Incoming solar radiation is shortwave, ultraviolet, and visible radiation; outgoing Earth radiation is long wave infrared radiation. The quantity of carbon dioxide residing in the atmosphere affects the amount of heat retained in the atmosphere and this in turn impacts climate. The more carbon dioxide (CO2) in the atmosphere, everything else being equal, the warmer the climate will be. Nitrous oxide (N2O), water vapor (H2O, the most important greenhouse gas), methane (CH4) and other gases have effects similar to those of carbon dioxide in controlling the amount of heat retained by the atmosphere. This overall process is the natural greenhouse effect. Without the naturally occuring greenhouse gases in the Earth’s atmosphere, the planetary surface temperature would be –18 oC. This is 33 oC cooler than its present average of 15 oC ( see Box 2: Solar and Earth Radiation and the Greenhouse Effect ).

If there were no naturally occuring greenhouse gases, the Earth would likely be entirely covered in ice and so life as we know it would not exist. Climate Science Investigations South Florida - Energy: The Driver of Climate Energy: The Driver of Climate The Greenhouse Effect Joseph Fourier. Image Credit: Wikipedia In 1827, Joseph Fourier, a French mathematician and physicist, wondered why Earth's average temperature is approximately 15°C (59°F). He reasoned that there must be some type of balance between the incoming energy and the outgoing energy to maintain this fairly constant temperature. His calculations indicated that Earth should actually be much colder (-18°C or 0°F). Image Credit: Microsoft Clip Art To have an average temperature of 15°C (59°F), Fourier knew that there had to be another process occurring in the atmosphere –– something similar to the way a greenhouse retains heat. A greenhouse's glass enclosure allows visible light to enter and be absorbed by the plants and soil. The plants and soil then emit the absorbed heat energy as infrared radiation. The glass of the greenhouse then absorbs that infrared radiation, emitting some of it back into the greenhouse and thus keeping the greenhouse warm even when the temperature outside is lower.

Because the two processes are similar, the name “greenhouse effect” was coined to describe Fourier's explanation. However, part of a greenhouse's warmth results from the physical barrier of the glass, which prevents the warmer air from flowing outward. So despite the fact that the atmospheric greenhouse effect has some processes in common with an actual greenhouse, the overall mechanisms driving the greenhouse effect are different and more complex. Greenhouse Gases Greenhouse Gases You have already learned that Earth's atmosphere is composed primarily of nitrogen and oxygen. These gases are transparent to incoming solar radiation. They are also transparent to outgoing infrared radiation, which means that they do not absorb or emit solar or infrared radiation. However, there are other gases in Earth's atmosphere that do absorb infrared radiation. These gases are known as greenhouse gases. Below are the most important greenhouse gases that influence Earth's climate system. Water vapor (H2O) is the strongest greenhouse gas, and the concentration of this gas is largely controlled by the temperature of the atmosphere. As air becomes warmer, it can hold more moisture or water vapor. When the air becomes saturated (or holds as much moisture as the air can at that temperature), the excess moisture will condense into cloud droplets.

And if these droplets are large enough, they will fall as precipitation. Carbon dioxide (CO2) is also an important greenhouse gas. It has a long lifetime in Earth's atmosphere. Carbon dioxide strongly absorbs energy with a wavelength of 15 μm (micrometers). This makes carbon dioxide a good absorber of wavelengths falling in the infrared radiation region of the spectrum. Carbon dioxide constantly moves into and out of the atmosphere through four major processes: photosynthesis, respiration, organic decomposition or decay, and combustion or the burning of organic material. You will learn more about carbon dioxide and the carbon cycle in Module 4. Methane (CH4) is 30 times stronger than carbon dioxide as an absorber of infrared radiation. Methane, however, is present in smaller concentrations than carbon dioxide, so its net contribution to the greenhouse effect is not as large. Methane is also relatively short-lived (lasting approximately 8 years) in the atmosphere. Methane is produced when bacteria decompose organic plant and animal matter in such places as wetlands (e.g., marshes, mudflats, flooded rice fields), sewage treatment plants, landfills, and the guts of cattle and termites.

Scientists are concerned about the concentration of methane increasing in regions where the Arctic and alpine permafrost is thawing and releasing methane as it warms. Halocarbons are composed of carbon, chlorine, fluorine, and hydrogen. They include chlorofluorocarbons (CFCs), which are man-made gases commonly used in refrigerators and air conditioners. Concentrations of CFC gases in the atmosphere are the highest of any of the halocarbons, and they can absorb more infrared radiation than any other greenhouse gas. The impact of 1 molecule of a CFC gas is equivalent to 10,000 molecules of carbon dioxide. Nitrous oxide (N2O), a relatively long-lived gas, has increased in atmospheric concentration due mainly to agriculture. Nitrate (NO3-) and ammonia (NH4+) are used as fertilizers. Bacteria convert a small amount of this nitrate and ammonia into the form of nitrous oxide. Internal combustion engines also produce nitrous oxide. Ozone (O3) is also a relatively minor greenhouse gas because it is found in relatively low concentrations in the troposphere (the lowest layer of the atmosphere).

In the troposphere, it is produced by a combination of pollutants — mostly hydrocarbons and nitrogen oxide compounds. John Tyndall. - Image Credit: Wikipedia In the 1860s, John Tyndall, an Irish scientist who was fascinated by the growth and formation of glaciers, wanted to test his ideas explaining how Earth maintained a fairly constant temperature. He began a series of experiments to measure the amount of radiant heat (infrared radiation) that certain gases could absorb and transmit. Tyndall found that water vapor and carbon dioxide were good absorbers and emitters of infrared radiation. The relative importance of a greenhouse gas depends on its abundance in Earth's atmosphere and how much the gas can absorb specific wavelengths of energy. An effective absorber of infrared radiation has a broader absorption profile, which means that it can absorb a wider spectrum of wavelengths. Water vapor and carbon dioxide can absorb radiation wavelengths in the range of 4 μm to 80 μm, except those between 8 μm and 12 μm. Ozone can absorb wavelengths between 9 μm and 10 μm, but as you have learned, it is found in low concentrations.

The sun's ultraviolet wavelengths are strongly absorbed by ozone in the stratosphere. Greenhouse Gas Absorption Spectrum How the Greenhouse Effect Works The sun's visible wavelengths of radiation pass easily through the atmosphere and reach Earth. Approximately 51% of this sunlight is absorbed at Earth's surface by the land, water, and vegetation. Some of this energy is emitted back from the Earth's surface in the form of infrared radiation. Water vapor, carbon dioxide, methane, and other trace gases in Earth's atmosphere absorb the longer wavelengths of outgoing infrared radiation from Earth's surface. These gases then emit the infrared radiation in all directions, both outward toward space and downward toward Earth. This process creates a second source of radiation to warm to surface – visible radiation from the sun and infrared radiation from the atmosphere – which causes Earth to be warmer than it otherwise would be. This process is known as the natural greenhouse effect and keeps Earth's average global temperature at approximately 15°C (59°F). The Natural Greenhouse Effect Use the buttons below to see an animation of the natural Greenhouse Effect I. The sun's visible wavelengths of radiation pass easily through the atmosphere and reach Earth.

Approximately 51% of this sunlight is absorbed at Earth's surface by the land, water, and vegetation. II. Some of this energy is emitted from Earth's surface back into space in the form of infrared radiation. III. Much of this infrared radiation does not reach space, however, because it is absorbed by greenhouse gases in atmosphere, and is then emitted as infrared radiation back toward the Earth's surface. This process is known as the greenhouse effect. GLOBAL WARMING | EFFECT OF GREENHOUSE GASES 3.0 EFFECT OF GREENHOUSE GASES (GHG) It was perceived that the global warming is caused by the increase of GHG, such as Carbon Dioxide, Ozone, water vapour, methane, nitrous oxide, Chlorofluorocarbon (CFC) and etcetera. Figure 3.1 and 3.2 below shows the absorption spectrum for varies GHG.[6],[7] Figure 3.1 - gives the amount of energy absorbed by green house gases in various wavelength regions, from ultraviolet radiation on the left, to visible light in the middle, to infrared radiation on the right.

Figure 3.2 shows the absorption of ultraviolet, visible, and infrared radiation by various gases in the atmosphere for longer wavelength. Most of the ultraviolet light (below 0.3 microns) is absorbed by ozone (O3) and oxygen (O2). Carbon dioxide has three large absorption bands in the infrared region at about 2.7, 4.3, and 15 microns. Water has several absorption bands in the infrared, and even has some absorption well into the microwave region. We take the plot for oxygen and ozone as an example; the absorption is very high in the ultraviolet region (below 0.3 microns or 300 nanometers) but essentially zero in the visible and infrared regions, except for isolated peaks. It is interpreted that this gas absorbs essentially all radiation in the ultraviolet but transparent in the visible and mostly transparent in infrared portions of the spectrum. Oxygen and ozone is responsible for shielding earth-based biological systems from ultraviolet radiation, but allows visible light and infrared radiation to pass through without much absorption. Other gases have much different absorption properties.

Methane (CH4) has a couple of very small wavelength regions in which it absorbs strongly and these occur at about 3.5 and 8 microns, which are in the infrared region. Nitrous oxide, N2O, having peaks at about 5 and 8 microns, absorbs in fairly narrow wavelength ranges. Carbon dioxide has a more complex absorption spectrum with isolated peaks at about 2.7, 4.3 and 15 microns. The plot for water vapour shows an absorption spectrum more complex even than carbon dioxide, with numerous broad peaks in the infrared region between 0.8 to 10 microns, absorption well into the microwave region after 11 microns and fully shielded after 19 microns. The total spectrum of all atmospheric gases is given in the bottom plot of figure 3.1 and top plot of figure 3.2. The total spectrum shows a ï¿½holeï¿½ between 0.3 and 0.8 microns (the visible region), which allows solar radiation (without UV component) to reach the earth's surface.

The second ï¿½holeï¿½ at 8 and 10 microns are corresponding earth surface temperature, where heat from the earth is released to outer-space via radiation. Figure 3.3 shows the solar radiation spectrum. The spectrum of the Sun 's solar radiation is close to that of a black body with a temperature of about 5,800 K , which is approximately the sunï¿½s surface temperature. About half that lies in the visible short-wave part of the electromagnetic spectrum and the other half mostly in the near- infrared part. Some also lies in the ultraviolet part of the spectrum. [17] When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause damage to the skin known as sunburn or trigger an adaptive change in human skin pigmentation . Figure 3.3 - The solar radiation spectrum for direct light at both the top of the Earth's atmosphere and at sea level . These curves are based on the American Society for Testing and Materials (ASTM) Terrestrial Reference Spectra , which are standards adopted by the photovoltaic industry to ensure consistent test conditions and are similar to the light that could be expected in North America .

Regions for ultraviolet , visible and infrared light are indicated. Figure 3.4 shows the summary of the solar absorption spectrum of GHG.[18] The radiation maximum for the Sun and Earth occurs at two distinct wavelengths of radiation. Both the Earth and the Sun emit electromagnetic radiation that closely follows a blackbody spectrum . The temperature of the Earth is much less that of the Sun. For the sun, these emissions peak in the visible region and correspond to a temperature of ≈5500 K , it is about 0.5 micrometers. Emissions from the Earth vary following variations in temperature across different locations and altitudes, but always peak in the infrared , about 288K, approximately 10 micrometers. The position and number of absorption bands are determined by the chemical properties of the gases present. In the present atmosphere, water vapour is the most significant of these GHG, followed by carbon dioxide and various other minor GHG. As light passes through the atmosphere, some is absorbed by gases with specific absorption bands . Additional light is redistributed by Raleigh scattering , which is responsible for the atmosphere's blue colour.

Collectively these processes capture and redistribute 25-30% of the energy in direct sunlight passing through the atmosphere. By contrast, the greenhouse gases capture 70-85% of the energy in upgoing thermal radiation emitted from the Earth surface. Figure 3.4 ï¿½ The effect for both solar radiation and upgoing Earth thermal radiation (top panel), the absorption bands in the Earth's atmosphere (middle panel) and. Individual absorption spectrum for major greenhouse gases plus Rayleigh scattering are shown in the lower panel.[18] Figure 3.1 to 3.4 show that the effect of carbon dioxide in blocking the heat rejection from the earth is only on certain specific wavelength and minimal. Similar to other GHG except water vapour, the heat blocking by GHG is minimal. Water vapour plays major role in absorbing heat from solar incident before reaches the Earth surface, which heats up the atmosphere and insulating the earth from the outer space is the water vapour. What Is the Greenhouse Effect? - American Chemical Society American Chemical Society What Is the Greenhouse Effect What Is the Greenhouse Effect?

ACS Climate Science Toolkit | Narratives Remark: “The Earth is warming and climate science shows that the warming has something to do with the greenhouse effect.” Reply: “I know. I wish I knew more about it, but the greenhouse effect is too difficult to understand.” Not really. There are only a few things you need to know, some of which are probably already familiar, to understand the basic idea of the greenhouse effect and why it is so important for the Earth. You know that when you stand in sunlight, you feel warmer than when you are in shadow, so you can feel that the light (radiant energy) the sun gives off carries energy that can warm an object—you. Although you usually cannot see it, all objects give off radiant energy and you can sometimes feel this energy. For example, if there is a pot of hot water on your stove, you can feel the radiant energy it gives off without touching it. You usually call what you feel “heat,” but it is more accurate to think of it as a kind of invisible light called “infrared radiation” that warms your skin, just like the sunlight. The amount of infrared radiation energy a warmed object gives off depends on its temperature—the higher the temperature, the more energy is given off.

As you know, you can easily distinguish between a warm object and a hot object by holding your hand near the objects and feeling the difference in heating effect on your skin. These ideas are basic to understanding the energy balance between the sun and the Earth. Just as sunlight warms you, it warms the surface of the Earth as well. The Earth does not continue to get hotter and hotter as it absorbs energy from the sun, because it gives off energy to space as invisible infrared radiation. In order to come into energy balance, the amount of infrared radiation energy given off by the Earth has to be equal to the amount of energy absorbed from the sunlight. The amount of infrared radiation energy the Earth gives off depends on its temperature. The average Earth temperature required for energy balance with the sun would be a frigid –18 °C (0 °F), if there were no atmospheric greenhouse effect. The greenhouse effect has kept the Earth’s average temperature a good deal higher for billions of years, making it possible for life as we know it to evolve. Over the past several millennia the average Earth temperature has been about 15 °C (59 °F). The figure below illustrates how greenhouse gases keep the Earth warmer than it would be without them.

Energy from the sun is shown on the left where you see that part of the radiant energy from the sun passes through the atmosphere, is absorbed, and warms the Earth’s surface. The rest is reflected, largely by clouds in the atmosphere and ice and snow on the surface, and not absorbed. Energy lost by the Earth is shown at the right where the fates of infrared radiation emitted (given off) by the Earth are shown. The straight red arrow passing from the surface through the atmosphere represents the fraction of the emitted infrared radiation that passes into space through the atmosphere without change. The rest of the infrared radiation, the thick red arrow, is absorbed by the greenhouse gases and clouds in the atmosphere and then re-emitted in all directions as shown by the collection of orange arrows. This ability to absorb and re-emit infrared radiation is the critical requirement for greenhouse gases. All gases whose molecules have three or more atoms are greenhouse gases—carbon dioxide (CO2), water vapor (H2O), and methane (CH4) are important greenhouse gases that have maintained Earth’s warm temperature for billions of years.

Source: Intergovernmental Panel on Climate Change Some of the re-emitted energy remains within the atmosphere or returns to the surface and warms the lower atmosphere and surface. The rest of the re-emitted energy leaves the atmosphere and goes into space. The outcome of this absorption-emission process by the greenhouse gases is that less energy leaves the atmosphere than is emitted by the Earth below. But to be in energy balance with the sun, the amount of energy going into space has to be equivalent to the amount that would be emitted by a surface at –18 °C. In order for this amount of energy to leave the Earth’s greenhouse atmosphere, the surface must be warmer than –18 °C. For several millennia—until the past two centuries—an average surface temperature of about 15 °C kept the sun and Earth in energy balance. That is, the same amount of infrared radiant energy was leaving the atmosphere as was being absorbed at the surface from the sun’s radiant energy. The increased amounts of greenhouse gases human activities are adding to the atmosphere have upset the balance that has been in place since the end of the last ice age. Adding more greenhouse gases decreases the amount of infrared radiation energy leaving the atmosphere.

To get the energy back in balance, the surface of the Earth has to warm up, so that it will emit more infrared energy, some of which will leave the atmosphere and compensate for the effect of the added greenhouse gases. Thus, the greenhouse effect, which is essential for creating the climate for life on Earth, is also responsible for the Earth getting warmer than it was before we started burning large amounts of fossil fuels. The Greenhouse Effect Greenhouse Effect The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed, but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium. The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature. Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed, transparent windows, the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide in the atmosphere. The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space. Go Back Greenhouse Effect Example Bright sunlight will effectively warm your car on a cold, clear day by the greenhouse effect .

The longer infrared wavelengths radiated by sun-warmed objects do not pass readily through the glass. The entrapment of this energy warms the interior of the vehicle. The trapping of the hot air so that it cannot rise and lose the energy by convection also plays a major role. Short wavelengths of visible light are readily transmitted through the transparent windshield. (Otherwise you wouldn't be able to see through it!) Shorter wavelengths of ultraviolet light are largely blocked by glass since they have greater quantum energies which have absorption mechanisms in the glass. Even though you may be uncomfortably warm with bright sunlight streaming through, you will not be sunburned . Go Back Increase in Greenhouse Gases The increase in the concentration of carbon dioxide, one of the three major atmospheric contributers to the greenhouse effect has been carefully documented at the Mauna Loa Observatory in Hawaii. The 1990 rate of increase was about 0.4% per year. The interesting cyclic variations represent the reduction in carbon dioxide by photosynthesis during the growing season in the northern hemisphere.

Current analysis suggests that the combustion of fossil fuels is a major contributer to the increase in the carbon dioxide concentration, such contributions being 2 to 5 times the effect of deforestation (Kraushaar & Ristinen). Increase in Atmospheric Carbon Dioxide The Mauna Loa monitoring station reports the carbon dioxide level in the atmosphere today as about 380 parts per million compared to 315 ppm in 1958 when modern measurements were initiated. Measurements of air bubbles trapped in the Greenland ice sheet indicate concentrations of 270 ppm in preindustrial times. These are sketches of the graphs produced in the IPCC 2007 report of the increase in key greenhouse gases. They make clear that most of the increase of the last thousand years has occurred in the past 200 years. The radiative forcing of these gases is related to their concentration .

Question: In the greeenhouse effect ultraviolet light is reradiated and absorbed as what invisible energy?
Answer: Infrared