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 High School Earth Science/Energy in the Atmosphere - Wikibooks, open books for an open world High School Earth Science/Energy in the Atmosphere From Wikibooks, open books for an open world 10 Points to Consider Wind and precipitation, warming and cooling depend on how much energy is in the atmosphere and where that energy is located. Much more energy from the Sun reaches low latitudes (nearer the equator) than high latitudes (nearer the poles).

These energy differences cause the winds, affect climate, and even drive ocean currents. Heat is held in the atmosphere by greenhouse gases. Describe how energy is transmitted. Describe the Earth's heat budget and what happens to the Sun's energy. Discuss the importance of convection in the atmosphere. Describe how a planet's heat budget can be balanced. Describe the greenhouse effect and why it is so important for life on Earth. Energy, Temperature, and Heat[ edit ] Figure 15.9: Waves. The high points are the crests, the low point is the trough. The wavelength is the distance from crest to crest. Every material has energy: All the molecules within it vibrate. Gas molecules contain more energy than an equal number of liquid molecules (under the same temperature and pressure conditions) and move freely. Liquid molecules contain more energy than solids and move more freely than solids. Energy travels through space or material. You know this because you can stand near a fire and feel the warmth. In this situation, energy is being transferred as invisible waves that can travel through air, glass, and even the vacuum of outer space. These waves have electrical and magnetic properties, so they are called electromagnetic waves.

The transfer of energy from one object to another through electromagnetic waves is known as radiation. Different types of electromagnetic waves have different wavelengths. A wavelength is the horizontal distance from trough-to-trough or crest-to-crest of adjacent waves (Figure 15.9). Humans are able to see some wavelengths of light, the wavelengths known as 'visible light'. These wavelengths appear to us as the colors of the rainbow (Figure 15.10). The longest wavelengths of visible light appear red and the shortest wavelengths appear violet. Wavelengths that are longer than visible red are infrared. Snakes can see infrared energy. We can record this with special equipment. Wavelengths that are shorter than violet are ultraviolet. Infrared and ultraviolet wavelengths of energy are just as important as the wavelengths in visible light; we just can't see them. Figure 15.10: The electromagnetic spectrum showing the wavelengths of energy. Solar radiation that reaches the outer atmosphere includes radio waves, along with visible light plus the ultraviolet range nearest to violet and the near infrared. Other wavelengths of electromagnetic radiation are blocked by different parts of Earth’s atmosphere. Some objects radiate electromagnetic waves in the visible light spectrum. Two familiar sources are the Sun and a light bulb.

Some objects radiate electromagnetic waves at wavelengths that we cannot see. The glass of water sitting next to you does not radiate visible light, but it does radiate a tiny amount of heat. You should be aware that some objects appear to radiate visible light, but they actually do not. The moon and the planets, for example, do not emit light of their own. They reflect the light of the Sun. Reflection is when light bounces back from a surface. Albedo is a measure of how well a surface reflects light. A surface that reflects a high percentage of the light that strikes it has high albedo and one that reflects a small percentage has low albedo. One important fact to remember is that energy cannot be created or destroyed. It can only be changed from one form to another. In photosynthesis, for example, plants convert the Sun's energy into food energy. They do not create new energy. When energy is transformed, often some becomes heat. Heat transfers between materials easily, from warmer objects to cooler ones. If no more heat is added, eventually all of a material will reach the same temperature. Temperature is a measure of how fast the atoms in a material are vibrating. High temperature particles vibrate faster than low temperature particles.

Rapidly vibrating atoms smash together, which generates heat. As a material cools down, the atoms vibrate more slowly and collide less frequently. As a result, they emit less heat. What is the difference between heat and temperature? Temperature measures how fast a material's atoms are vibrating. Heat measures the material's total energy. Think of a candle flame and a bathtub full of hot water. Which has a higher heat and which has a higher temperature? Surprisingly, the flame has a higher temperature, but much less heat because the hot region is very small. The bathtub has lower temperature but contains much more heat because it has many more vibrating atoms. Even though it’s at a lower temperature, the bathtub has a greater total energy. Heat is taken in or released when an object changes state, or changes from a gas to a liquid or a liquid to a solid. This heat is called latent heat. When a substance changes state, latent heat is released or absorbed. A substance that is changing its state of matter does not change temperature. All of the energy that is released or absorbed goes toward changing the material's state. For example, imagine a pot of boiling water on the stove: that water is at 100°C (212°F).

If a cook increases the temperature of the burner beneath the pot, more heat enters the water. But still the water remains at its boiling temperature. The additional energy goes into changing the water from liquid to gas. This allows the water to evaporate more rapidly. When water changes from a liquid to a gas it takes in heat. Since evaporation takes in heat, this is called evaporative cooling. Evaporative cooling is an inexpensive way to cool homes in hot, dry areas. Substances also differ in their specific heat, the amount of energy needed to raise the temperature of one gram of the material by 1.0°C (1.8°F). Water has a very high specific heat, which means it takes a lot of energy to change the temperature of water. Let's compare a puddle and asphalt, for example. If you are walking barefoot on a sunny day, which would you rather walk across, the shallow puddle or an asphalt parking lot? Due to its high specific heat, the water stays cooler than the asphalt, even though it receives the same amount of solar radiation.

Energy from the Sun[ edit ] File:Blue sun.jpg Figure 15.11: An image of the sun taken with a sensor picking up only the 17.1 nm wavelength, in the ultraviolet wavelengths. Most of the energy that reaches the Earth's surface comes from the Sun. The Sun emits energy in a continuous stream of wavelengths (Figure 15.11). These wavelengths include visible light, infrared, ultraviolet radiation, and others. About 44% of solar radiation is in the visible light wavelengths. When viewed together, all of the wavelengths of visible light appear white. But a prism or water droplets, for example, can break the white light into different wavelengths so that you can see separate colors (Figure 15.12). Figure 15.12: A prism breaks apart white light by wavelength so that you can see all the colors of the rainbow. Figure 15.13: An image of a dog taken by an infrared sensor. The image shows the different amounts of heat radiating from the dog. Only about 7% of solar radiation is in the ultraviolet (UV) wavelengths. Of the solar energy that reaches the outer atmosphere, UV wavelengths have the greatest energy.

There are three types of UV energy: UVC has the shortest wavelengths and is the most energetic; UVA is the longest wavelengths and is the least energetic; and UBV is in the middle of the two. UV radiation will tan or burn human skin. The remaining solar radiation is the longest wavelength, infrared. Most objects radiate infrared energy, which we feel as heat (Figure 15.13). Some of the wavelengths of solar radiation traveling through the atmosphere may be lost because they are absorbed by various gases (Figure 15.14). Ozone, for example, completely removes UVC, most UVB and some UVA from incoming sunlight. O2, CO2 and H2O also filter out other wavelengths from solar energy. Figure 15.14: In the atmosphere, gases filter some wavelengths from incoming solar energy. The yellow field shows the wavelengths of energy that reach the top of the atmosphere. The red field shows the wavelengths that reach sea level. The amount of radiation is reduced overall as different gases filter out different wavelengths. Ozone filters out the shortest wavelength ultraviolet and oxygen filters out most infrared, at about 750 nm. Different parts of the Earth receive different amounts of solar radiation.

This is because the Sun's rays strike the Earth's surface most directly at the equator. As you move away from the equator, you will notice that different areas also receive different amounts of sunlight in different seasons. But what causes the seasons? The Earth revolves around the Sun once each year and spins on its axis of rotation once each day. This axis of rotation is tilted 23.5° relative to its plane of orbit around the Sun. The axis of rotation happens to be pointed to the star Polaris, or the North star. As the Earth orbits the Sun, the tilt of Earth's axis stays lined up with the North star. This means that the North Pole is tilted towards the Sun and the Sun's rays strike the Northern Hemisphere more directly in summer. At the summer solstice, June 21 or 22 of each year, the Sun's rays are hit the Earth most directly along the Tropic of Cancer. This is a circle of latitude exactly 23.5° north of the equator. When it is summer solstice in the Northern Hemisphere, it is winter solstice in the Southern Hemisphere. Winter solstice for the Northern Hemisphere happens on December 21 or 22.

The tilt of Earth's axis points away from the sun in the winter and the Sun's rays strike most directly at the Tropic of Capricorn (Figure 15.15). The Tropic of Capricorn is a line of latitude exactly 23.5° south of the equator. The light from the Sun gets spread out over a larger area, so that area isn't heated as much. There are also fewer daylight hours in winter, so there is also less time for the Sun to warm that place. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere. Figure 15.15: Arctic winter solstice. The sun's rays are directly overhead at the Tropic of Capricorn. Sunlight is striking the south pole, but it is spread out. No sunlight is getting to the North pole. Halfway between the two solstices, the Sun's rays shine most directly at the equator. We call these times an 'equinox' (Figure 15.16). The daylight and nighttime hours are exactly equal on an equinox.

The autumnal equinox happens on September 22 or 23 and the vernal or spring equinox happens March 21 or 22 in the Northern Hemisphere. Thus the seasons are caused by the direction Earth's axis is pointing relative to the Sun. Figure 15.16: Where sunlight reaches on spring equinox, summer solstice, vernal equinox, and winter solstice. The time is 9:00 pm Universal Time, at Greenwich, England. Heat Transfer in the Atmosphere[ edit ] Heat can move in three different ways. We've already examined radiation, in which electromagnetic waves transfer heat between two objects. Conduction is a type of heat transfer that occurs when heat moves from areas of more heat to areas of less heat by direct contact. Warmer molecules vibrate more rapidly than cooler molecules. They collide directly with other nearby molecules, giving them some of their energy, which transfers heat. When all the molecules are moving at the same rate, the substance is the same temperature throughout. Heat in the atmosphere is transferred by conduction. This is more effective at lower altitudes where air molecules are packed more densely together. Conduction can transfer heat upward to where the molecules are spread further apart.

It can also transfer heat laterally from a warmer to a cooler spot, where the molecules are moving less vigorously. The most important way heat is transferred in the atmosphere is by convection currents. Convection is the transfer of heat by movement of heated materials. The radiation of heat from the ground warms the air above it. This warmer air is less dense than the air above it and so it rises. As the heated air rises it begins to cool, since it is further from the heat source. As it cools, it contracts, becomes denser and sinks. Air moves horizontally between warm, rising air and cooler, sinking air. This entire structure is a convection cell. Heat at Earth's Surface[ edit ] Not all energy coming in from the Sun makes it to the Earth's surface. About half is filtered out by the atmosphere. Besides being absorbed by gases, energy is reflected by clouds or is scattered. Scattering occurs when a light wave strikes a particle and bounces off in some other direction. Of the energy that strikes the ground, about 3% is reflected back into the atmosphere. The rest warms the soil, rock or water that it reaches. Some of the absorbed energy radiates back into the air as heat.

These infrared wavelengths can only be seen by infrared sensors. It might occur to you that if solar energy continually enters the Earth's atmosphere and ground surface, then the planet must always be getting hotter. This is not true, because energy from the Earth escapes into space through the top of the atmosphere, just as energy from the Sun enters through the top of the atmosphere. If the amount that exits is equal to the amount that comes in, then there is no increase or decrease in average global temperature. This means that the planet's heat budget is in balance. If more energy comes in than goes out, the planet warms. If more energy goes out than comes in, the planet cools. To say that the Earth's heat budget is balanced ignores an important point. The amount of incoming solar energy varies at different latitudes (Figure 15.17). This is partly due to the seasons. At the equator, days are about the same length all year and the Sun is high in the sky. More sunlight hits the regions around the equator and air temperatures are warmer. At the poles, the Sun does not rise for months each year.

Even when the Sun is out all day and night in the summer, it is at a very low angle in the sky. This means that not much solar radiation reaches the ground near the poles. Because of this, during a large part of the year, the polar areas are covered with ice and snow. These brilliant white substances have a high albedo and reflect solar energy back into the atmosphere. For all of these reasons, the region around the equator is much warmer than the areas at the poles. Figure 15.17: The average annual temperature of the Earth, showing that the equatorial region is much warmer than the polar regions. There is a roughly gradual temperature gradient from the low to the high latitudes. The difference in the amount of solar energy that the planet receives at different latitudes drives much of the activity that takes place at the Earth's surface. This includes the wind, water cycle, and ocean currents. The differences in solar energy around the globe drive the way the atmosphere circulates. The Greenhouse Effect[ edit ] The remaining factor in the Earth's heat budget s the role of greenhouse gases. Greenhouse gases warm the atmosphere by trapping heat.

Sunlight strikes the ground, is converted to heat, and radiates back into the lower atmosphere. Some of the heat is trapped by greenhouse gases in the troposphere, and cannot exit into space. Like a blanket on a sleeping person, greenhouse gases act as insulation for the planet. The warming of the atmosphere due to insulation by greenhouse gases is called the greenhouse effect (Figure 15.18). Figure 15.18: The Earth's heat budget, showing the amount of energy coming into and going out of the Earth system and the importance of the greenhouse effect. The numbers are the amount of energy that is found in one square meter of that location. The greenhouse effect is very important, since without it the average temperature of the atmosphere would be about -18°C (0°F). With the greenhouse effect, the average temperature of the atmosphere is a pleasant 15°C (59°F). Without insulation, daytime temperatures would be very high and nighttime temperatures would be extremely low. This is the situation on all of the planets and moons that have no atmosphere. If the Earth did not have insulation, temperatures would likely be too cold and too variable for complex life forms.

There are many important greenhouse gases in the atmosphere including CO2, H2O, methane, O3, nitrous oxides (NO and NO2), and chlorofluorocarbons (CFCs). All of these gases are a normal part of the atmosphere except CFCs, which are human-made. However, human activity has significantly raised the levels of many of these gases; for example, methane levels are about 2 1/2 times higher as a result of human activity. Table 15.2 shows how each greenhouse gas naturally enters the atmosphere. Table 15.2: Sources of Greenhouse Gases Greenhouse Gas Chlorofluorocarbons Not naturally occurring; made by humans Different greenhouse gases have different abilities to trap heat. For example, one methane molecule can trap 23 times as much heat as one CO2 molecule. One CFC-12 molecule (a type of CFC) can trap 10,600 times as much heat as one CO2. Still, CO2 is a very important greenhouse gas because it is much more abundant in the atmosphere than the others. The greenhouse effect is very important for another reason.

If greenhouse gases in the atmosphere increase, they trap more heat and warm the atmosphere. If greenhouse gases in the atmosphere decrease, less heat is trapped and the atmosphere cools. The increase or decrease of greenhouse gases in the atmosphere affect climate and weather the world over. Lesson Summary[ edit ] All materials contain energy, which can radiate through space as electromagnetic waves. The wavelengths of energy that come from the Sun include visible light, which appears white but can be broken up into many colors. Ultraviolet waves are very high energy. The highest energy UV, UVC and some UVB, gets filtered out of incoming sunlight by ozone. More solar energy reaches the low latitudes and the redistribution of heat by convection drives the planet's air currents. Review Questions[ edit ] What is the difference between temperature and heat? Give a complete description of these three categories of energy relative to each other in terms of their wavelengths and energy: infrared, visible light, and ultraviolet. Why do the polar regions have high albedo? Give an example of the saying "energy can't be created or destroyed". Describe what happens to the temperature of a pot of water and to the state of the water as the dial on the stove is changed from no heat to the highest heat.

Describe where the Sun is relative to the Earth on summer solstice, autumnal equinox, winter solstice and spring equinox. How much sunlight is the North pole getting on June 21? How much is the South pole getting on that same day? What is the difference between conduction and convection? What is a planet's heat budget? Is Earth's heat budget balanced or not? On a map of average annual temperature, why are the lower latitudes so much warmer than the higher latitudes? Why is carbon dioxide the most important greenhouse gas? How does the amount of greenhouse gases in the atmosphere affect the atmosphere's temperature? Vocabulary[ edit ] albedo Greenhouse Gas Emissions: Causes & Sources Greenhouse Gas Emissions: Causes & Sources By Marc Lallanilla | February 10, 2015 07:36pm ET MORE Behind the struggle to address global warming and climate change lies the increase in greenhouse gases in our atmosphere. A greenhouse gas is any gaseous compound in the atmosphere that is capable of absorbing infrared radiation, thereby trapping and holding heat in the atmosphere. By increasing the heat in the atmosphere, greenhouse gases are responsible for the greenhouse effect, which ultimately leads to global warming .

Solar radiation and the greenhouse effect Global warming isn't a new study in science. The basics of the phenomenon were worked out by Svante Arrhenius in 1896 . His paper, published in the Philosophical Magazine and Journal of Science, was the first to quantify the contribution of carbon dioxide to the greenhouse effect. The sun bombards Earth with enormous amounts of radiation, which strike 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. About 30 percent of the radiation striking the Earth is reflected back out to space by clouds, ice and other reflective surfaces. The remaining 70 percent is absorbed by the oceans, the land and the atmosphere, according to NASA . As they absorb radiation and heat up, the oceans, land and atmosphere release heat in the form of IR thermal radiation, which passes out of the atmosphere into space. The balance between incoming and outgoing radiation keeps Earth's overall average temperature at about 59 degrees Fahrenheit (15 degrees Celsius), according to NASA. This exchange of incoming and outgoing radiation that warms 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 out through the glass walls and is trapped inside, warming the greenhouse. How greenhouse gases affect global warming The gases in the atmosphere that absorb radiation are known as "greenhouse gases" (sometimes abbreviated as GHG) because they are largely responsible for the greenhouse effect. The greenhouse effect, in turn, is one of the leading causes of global warming. The most significant greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), according to the Environmental Protection Agency (EPA). "While oxygen (O2) is the second most abundant gas in our atmosphere, O2 does not absorb thermal infrared radiation," Michael Daley, an associate professor of environmental science at Lasell College, told Live Science. While some say that global warming is a natural process and that there have always been greenhouse gasses, the amount of gasses in the atmosphere has skyrocketed in recent history. The Industrial Revolution had a big part to play in the amount of atmospheric CO2 being released.

Before, CO2 fluctuated between about 180 ppm during ice ages and 280 ppm during interglacial warm periods. Since the Industrial Revolution, though, the amount of CO2 has dramatically increased to 100 times faster than the increase when the last ice age ended, according to the National Oceanic and Atmospheric Administration (NOAA). Fluorinated gases — that is, gases to which the element fluorine was added — including hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride, are created during industrial processes and are also considered greenhouse gases. Though they are present in very small concentrations, they trap heat very effectively, making them high "global-warming potential" (GWP) gases. Chlorofluorocarbons (CFCs), once used as refrigerants and aerosol propellants until they were phased out by international agreement, are also greenhouse gases. Three factors affect the degree to which any greenhouse gas will influence global warming: Its abundance in the atmosphere How long it stays in the atmosphere Its global-warming potential Carbon dioxide has a significant impact on global warming partly because of its abundance in the atmosphere.

According to the EPA , in 2012, U.S. greenhouse gas emissions totaled 6,526 million metric tons of carbon dioxide equivalents, which equaled 82 percent of all human caused greenhouse gasses. Additionally, CO2 stays in the atmosphere for thousands of years. However, methane is about 21 times more efficient at absorbing radiation than CO2, giving it a high GWP rating, even though it stays in the atmosphere only about 10 years, according to the EPA. Sources of greenhouse gases Some greenhouse gases, like methane, are produced through agricultural practices including livestock manure management. Others, like CO2, largely result from natural processes like respiration and from the burning of fossil fuels like coal, oil and gas. The production of electricity is the source of 70 percent of the United States' sulfur dioxide emissions, 13 percent of nitrogen oxide emissions, and 40 percent of carbon dioxide emissions, according to the EPA . The second cause of CO2 release is deforestation, according to research published by Duke University . When trees are killed to produce goods or heat, they release the carbon that is normally stored for photosynthesis.

This process releases nearly a billion tons of carbon into the atmosphere per year, according to the 2010 Global Forest Resources Assessment . It's worth noting that forestry and other land-use practices offset some of these greenhouse gas emissions, according to the EPA. "Replanting helps to reduce the buildup of carbon dioxide in the atmosphere as growing trees sequester carbon dioxide through photosynthesis. Atmospheric carbon dioxide is converted and stored in the vegetation and soils of the forest. However, forests cannot sequester all of the carbon dioxide we are emitting to the atmosphere through the burning of fossil fuels and a reduction in fossil fuel emissions is still necessary to avoid build up in the atmosphere," said Daley. Worldwide, the output of greenhouse gases is a source of grave concern: From the time the Industrial Revolution began to the year 2009, atmospheric CO2 levels have increased almost 38 percent and methane levels have increased a whopping 148 percent, according to NASA, and most of that increase has been in the past 50 years. Because of global warming, 2014 was the warmest year on record and 10 of the hottest years have all come after 1998.

"The warming we observe affects atmospheric circulation, which impacts rainfall patterns globally. This will lead to big environmental changes, and challenges, for people all across the globe," Josef Werne, an associate professor in the department of geology and planetary science at the University of Pittsburgh, told Live Science. If these trends continue, scientists, government officials and a growing number of citizens fear that the worst effects of global warming — extreme weather, rising sea levels, plant and animal extinctions, ocean acidification, major shifts in climate and unprecedented social upheaval — will be inevitable. In answer to the problems caused by global warming by greenhouse gasses, the government created a climate action plan in 2013. Additional reporting by Alina Bradford, Live Science Contributor For the latest information on greenhouse gases, visit: National Geographic Magazine - NGM.com Burn natural gas and it warms your house. But let it leak, from fracked wells or the melting Arctic, and it warms the whole planet. By Marianne Lavelle Photograph by Mark Thiessen The last rays of sun filter through the snow-covered spruces along the shore of Goldstream Lake, just outside Fairbanks, Alaska.

Out on the lake Katey Walter Anthony stares at the black ice beneath her feet and at the white bubbles trapped inside it. Large and small, in layer upon layer, they spread out in every direction, like stars in the night sky. Walter Anthony, an ecologist at the University of Alaska Fairbanks, grabs a heavy ice pick and wraps the rope handle around her wrist. A graduate student holds a lighted match above a large bubble; Walter Anthony plunges the pick into it. Gas rushing from the hole ignites with a whoomp that staggers her. “My job’s the worst, because usually you catch on fire,” she says, smiling. In the gathering twilight she and her team ignite one bubble after another. The flames confirm that the bubbles are methane, the main component of natural gas. By counting and measuring them, Walter Anthony is trying to gauge how much methane is rising from Goldstream Lake—and from the millions of similar lakes that now occupy nearly a third of the Arctic region. The Arctic has warmed much faster than the rest of the planet in recent decades, and as the permafrost has melted, old lakes have grown and new ones have formed.

Methane bubbles from their muddy depths in a way that is hard to quantify—until the first clear ice of fall captures a snapshot of the emissions from an entire lake. Sometimes as Walter Anthony walks that ice, in Alaska, Greenland, or Siberia, a stamp of her boot is enough to release an audible sigh. Some lakes, she says, have “hot spots” where the methane bubbling is so strong that ice never forms, leaving open holes big enough to spot from an airplane. “It could be 10 or 30 liters of methane per day from one little hole, and it does that all year,” she says. “And then you realize there are hundreds of spots like that and millions of lakes.” By venting methane into the atmosphere, the lakes are amplifying the global warming that created them: Methane is a potent greenhouse gas. Carbon dioxide is the main one, because the atmosphere holds 200 times as much of it. But a given amount of methane traps at least 25 times as much heat—unless you burn it first. Then it enters the atmosphere as CO₂. That’s the other side of this Jekyll-and-Hyde story: A lot of methane is being burned these days.

In the past decade the technology called hydraulic fracturing, “fracking” for short, has enabled drillers in the United States to extract natural gas from deeply buried shales they couldn’t tap before. Natural gas supplies have surged; prices have plummeted. Fracking is now spreading around the world, and it’s controversial. The gas boom has degraded landscapes and polluted water. But it has also had environmental benefits. Natural gas burns much cleaner than coal. In part because American power plants have been switching from coal to cheap gas, U.S. emissions of CO₂ from fossil fuels fell last year, even as the world set another record. The catch is, methane emissions are rising. What’s coming out of Arctic lakes is troubling, Walter Anthony says, because some of it seems to be coming not from bottom mud but from deeper geologic reservoirs that had hitherto been securely capped by permafrost—and that contain hundreds of times more methane than is in the atmosphere now. Still, most methane emissions today come from lower latitudes, and most are related more directly to human activities. A growing amount seems to be leaking, for instance, from gas wells and pipelines.

Just how warm Earth gets this century will hinge in part on how we balance the good and bad of methane—on how much of it we capture and burn, and how much we inadvertently let loose. Methane is the simplest hydrocarbon—a single carbon atom surrounded by four hydrogen atoms. It usually forms when larger organic molecules are broken down, either by microbes or by heat. The microbes produce it when they eat dead plant matter in wet, oxygen-poor environments. They’re the source of the methane bubbling up from Goldstream Lake; from swamps and marshes all over; from human-made rice fields, landfills, and manure lagoons; and from the stomachs of cows and other ruminants. Termites emit a lot of methane too. Most of the natural gas we tap for fuel, however, was formed not by microbes but by heat and pressure deep underground—as oil and coal were, and often in the same places. In coal mines methane is an explosion hazard; in oil fields it was long considered a nuisance to be burned off or, worse, vented directly into the atmosphere. Liquid oil was more valuable as fuel and much easier to transport to markets.

Then pipelines built during the post–World War II construction boom made gas more transportable. The energy industry began to exploit massive natural gas reservoirs in places like Russia, Qatar, and Iran. The United States produces the bulk of its own gas, but U.S. production peaked in 1973. By 2005 the country seemed to be running short, and the industry was building expensive new tanker terminals to import liquefied natural gas. The fracking boom changed that. Since 2005 gas production from deep shales has increased more than tenfold; it now accounts for more than a third of total production, which last year surpassed the 1973 record. Within a decade, according to a Department of Energy (DOE) forecast, the U.S. will become a net exporter of gas. Estimates of how much gas is locked up in shales and how long the boom can last have varied widely. In 2011 DOE put the amount of “unproved resources” of shale gas at 827 trillion cubic feet; in 2012 it cut that estimate by more than 40 percent. Production from fracked wells has declined faster than DOE analysts had expected.

So some critics believe the boom is a bubble that will soon burst. But DOE still projects that U.S. gas production will rise rapidly and that shale gas will make up half the total by 2035. And deep shales are not the last methane source. DOE and the industry are trying to figure out how to tap the largest one of all—the methane hydrates that lie frozen under vast areas of seafloor and Arctic permafrost. Worldwide, hydrates may contain more energy than all other fossil fuels combined. They’re usually snow-white and look like ice, but they’re strange stuff, and extracting the methane is tricky. Each molecule is trapped in a cage of water molecules that’s stable only at high pressure and low temperatures; change either just a bit, and the cage crumbles. The escaping methane balloons in volume by a factor of 164. Oil companies working on continental margins have to take care that extracting oil through an overlying hydrate layer does not disrupt it and perhaps damage the well. Climate scientists worry that global warming could destabilize hydrate layers, on land or at sea, triggering a massive methane release that would amplify the warming.

A few scientists take seriously a catastrophic scenario in which the release happens rapidly, within a human lifetime, and the planet’s temperature spikes. The atmospheric methane concentration has risen nearly 160 percent since preindustrial times, to 1.8 parts per million. For a few years, from 1999 to about 2006, it seemed to level off. Some researchers credit Asian rice farmers, who began draining their paddies during the growing season to conserve water—which reduced methane emissions as well. Another theory credits the oil industry, which started capturing and selling methane it used to simply vent. Since 2006, though, atmospheric methane has been rising again. Many observers believe it’s no coincidence that the number of wells punched into deep shales has been soaring too. The largest U.S. shale formation, the Marcellus, lies about a mile under the Appalachian Mountains, in an arc that runs from West Virginia to New York through Ohio and Pennsylvania. The Pennsylvania stretch is pretty country: rolling hills and pastures and, in the northwest, the forests of the Pennsylvania Wilds, which boast some 2,000 trout streams and one of the darkest night skies in the East.

These days tank trucks, sand haulers, flatbeds stacked with pipe, and cement mixers rumble continually over the winding two-lane roads. Here and there in patches cut from forest or farm are flattened, four-acre mounds of fresh dirt. For a few weeks at a time tall derricks rise from these drill pads, and the trucks and trailers congregate around them. Contaminated water from the new wells pours into tank trucks or into lagoons lined with dark plastic. The derricks soon disappear, but the wells stay, connected by clusters of green pipes and valves to permanent new pipelines, condensate tanks, and compressor stations. Much of Pennsylvania has been transformed since 2008. The boom’s roots go back to the 1980s and to Texas, where a wildcatter named George Mitchell, facing dwindling reserves, began probing the Barnett Shale near Dallas. Black shales, the compressed mud of ancient seas, were known as petroleum source rocks. But over geologic time much of the oil and gas had migrated out of the shales into porous sandstone traps—and that’s where the industry sank its wells.

Wells ending in shale never yielded much; the shales were too dense and impermeable to allow gas to flow. Mitchell Energy’s workaround, developed over 20 years with support from DOE, became the recipe for the fracking boom. It has two parts. First, drill down to the shale, then continue drilling horizontally for a mile or so inside it; that puts more gas close to the well. Second, inject millions of gallons of water, chemical lubricants, and sand at high pressure to shatter the shale, allowing methane to rush into the well. The gas from fracked wells has benefited consumers; 55 percent of the homes in the U.S. have gas heat, and prices last winter reached a ten-year low. In Pennsylvania the boom has revived businesses; created some 18,000 jobs, by the state’s reckoning; and paid millions of dollars in lease-signing bonuses and royalties. However, some landowners who leased their land to gas companies have since had second thoughts. Sherry Vargson is one. In 2008 Chesapeake Energy began drilling on her family’s 197-acre dairy farm in Granville Summit, in northeastern Pennsylvania.

In June 2010, after a crew had been working on the well, Vargson turned on her kitchen tap to find it backed up with what she thought was air. “It was like drawing a glass of Alka-Seltzer, very sizzly and bubbly,” she recalls. Testing showed the water contained more than twice the methane that’s considered an explosion threat. Chesapeake has been supplying her with bottled water ever since, while arguing that the contamination is natural. Meanwhile Vargson’s monthly royalty checks have shrunk from more than $1,000 to less than $100, as production from the gas well has plummeted. The industry’s main argument in attempting to reassure a worried public in Pennsylvania and elsewhere has been that shales typically lie thousands of feet below drinking-water aquifers. So contamination, whether by shale gas or fracking wastewater—which contains fracking chemicals, salt, heavy metals, and radioactive elements leached from the rock—should be physically impossible. The argument makes intuitive sense, but the jury is still out. Duke University scientists have recently reported evidence that fluids—albeit not fracking fluids—have migrated upward from the Marcellus Shale through natural fissures.

In an earlier study the Duke researchers sampled 60 private water wells in northeastern Pennsylvania and found no sign of fracking fluids. But they did find that methane levels were on average 17 times higher in wells near drilling sites and that some of the methane had the chemical signature of shale gas. It may have leaked into the shallow aquifers, they said, through faulty casings around the gas wells. The Pennsylvania Department of Environmental Protection (DEP) also blamed faulty casings in 2009 when it fined Cabot Oil & Gas for contaminating the drinking supplies of 19 homes in Dimock Township, 60 miles east of the Vargson farm. In that case the methane came not from the shale but from shallow deposits traversed by the gas wells. DEP has also fined gas companies for mishandling fracking wastewater and allowing spills that polluted creeks and rivers. In Pennsylvania and elsewhere, shale-gas drilling has raced far ahead of efforts to understand and limit its impact. So far, however, its impact seems much smaller than that of coal mining—which in Pennsylvania has caused far worse river pollution, in West Virginia has lopped the tops off numerous mountains, and in the U.S.

still kills hundreds of miners a year, mostly through black lung disease. The comparison is relevant because cheap natural gas is reducing coal burning. As recently as 2007, coal generated nearly half of U.S. electricity. Last March its share fell to 34 percent. John Hanger, a Pennsylvania lawyer who helped author the state’s renewable-energy standards, ran the DEP from 2008 to early 2011. Though he tightened regulations on the gas industry and handed out substantial fines, he was attacked by opponents who wanted a complete halt to fracking. Hanger believes such critics are missing the big picture. “The massive switching from coal to gas has done more to clean Pennsylvania’s air, and America’s air, than probably any other single thing we’ve ever done,” he says. Unlike coal, natural gas burns without spewing sulfur dioxide, mercury, or particulates into the air or leaving ash behind. And it emits only half as much carbon dioxide. The greenhouse gas inventory compiled by the U.S.

Environmental Protection Agency (EPA) shows that the nation’s CO₂ emissions in 2010 were lower than in 2005 by just over 400 million metric tons, or 7 percent. (Preliminary data for 2011 indicate a further decrease.) Reduced emissions from power plants, mostly because many have switched from coal to gas, accounted for a bit over a third of that. Some environmentalists who once welcomed shale gas with precisely that expectation changed their minds after watching the boom in Pennsylvania. But Hanger hopes it spreads around the world, as it seems likely to. “In China they’re sitting on potentially huge supplies of shale gas,” he says. “It would be an enormous climate benefit if China were to substitute gas for some of its coal burning. And it’s an immediate benefit—you don’t have to wait until 2040 or 2050.” Unless too much methane leaks into the atmosphere. As U.S. CO₂ emissions fell between 2005 and 2010, methane emissions rose.

By 2010, EPA says, the rise was equivalent in global warming potential to around 40 million metric tons of CO₂ annually, which means it offset 10 percent of the CO₂ decline. More than half of that methane increase, says EPA, came from the natural gas industry—the country’s biggest emitter. Judging by EPA’s numbers, fracking still seems like a clear win for the climate. But some scientists, notably Robert Howarth and his coworkers at Cornell University, believe EPA has underestimated methane emissions and, more important, the global warming potential of each methane molecule. They argue that methane leaking from wells, pipes, compressors, and storage tanks actually makes shale gas worse for the climate than coal. Other researchers question Howarth’s approach. The debate persists in part because methane numbers are so uncertain. New rules issued by EPA this year will require the gas industry to measure its emissions and also to reduce them. One of the biggest leaks occurs when a fracked well is completed and high-pressure fracking fluids surge back up the well, bringing methane with them. The new rules will require gas companies to start capturing that methane by 2015, using technology that’s already required in Wyoming, Colorado, and parts of Texas.

Some experts consider methane capture a great opportunity: an easier way than controlling CO₂ to slow global warming, at least in the short term, because small amounts of methane make a big difference and because it’s a valuable fuel. China, for instance, the world’s largest coal producer, vents huge amounts of methane from its mines to prevent explosions. In the 1990s, when Egyptian geologist Mohamed El-Ashry headed the Global Environment Facility, an agency created by the United Nations and the World Bank, it devoted ten million dollars to projects that siphoned methane from several Chinese mines and delivered it as fuel to thousands of nearby households. Hundreds of such projects await funding worldwide, El-Ashry says. Drew Shindell, a climate scientist at NASA’s Goddard Institute for Space Studies, recently led a global team of scientists in analyzing seven methane-reduction strategies, from draining rice fields to capturing the gas that escapes from landfills and gas wells. Unlike CO₂, methane affects human health, because it’s a precursor of smog.

When health impacts are included, Shindell’s group found, the benefits of methane controls outweigh the costs by at least 3 to 1, and in some cases by as much as 20 to 1. “There are some sources that are difficult, if not impossible, to control,” says Shindell. “The Arctic emissions—I’d probably vote those as being near impossible. But then you have long-distance gas pipelines, and we know exactly how to control leaks from those: put in and maintain high-quality seals. And there are other places, especially in oil, gas, and coal production. It’s really straightforward to get a substantial fraction of methane emissions under control.” Last spring, as the annual thaw began in Alaska, Katey Walter Anthony heard from her friend Bill Wetzen, who owns Goldstream Lake and sometimes brings her coffee out on the ice. When Wetzen bought the property 20 years ago, he built his bungalow about 20 yards from the lake; by last year it was nearly at the water’s edge. Now, Wetzen said, with the permafrost thawing beneath it, the walls and floors were tearing apart. He was going to have to move.

Also last spring, DOE-funded researchers on Alaska’s North Slope successfully tested a method of extracting methane from buried hydrates. Though the process “may take years” to become economically viable, said the DOE press release, “the same could be said of the early shale gas research ... that the Department backed in the 1970s and 1980s.” If even a small fraction of methane hydrates becomes recoverable, DOE estimates, that could double U.S. gas resources. Some of the methane bubbling from Arctic lakes, Walter Anthony says, might come from hydrates. Around 56 million years ago, in the Paleocene, a long planetary warming culminated in a sudden temperature spike of 9°F; many scientists suspect a massive destabilization of methane hydrates. Most, including Walter Anthony, do not think such a catastrophe is likely now. But Arctic methane could add a lot to global warming over the next few centuries. “If we could only capture it, it would make a great energy source,” Walter Anthony says. Marianne Lavelle is the energy editor at nationalgeographic.com . Staff photographer Mark Thiessen has a professional fascination with fire.

| The Greenhouse Effect General Overview: The Greenhouse Effect Introduction The " greenhouse effect " is widely discussed in the media, and although its details are complicated, its principles are not difficult to understand. Without a greenhouse effect, radiation from the Sun (mostly in the form of visible light ) would travel to Earth and be changed into heat, only to be lost to space. This scenario can be sketched as follows: Sun�s radiation → absorbed by Earth → Re-radiated to space as heat The greenhouse effect is a process where energy from the sun readily penetrates into the lower atmosphere and onto the surface of Earth and is converted to heat, but then cannot freely leave the planet . This can be sketched as follows: Sun�s Radiation → absorbed by Earth → some re-radiated to space as heat → some trapped by the atmosphere Due to the presence of certain �greenhouse gases� that trap heat, like carbon dioxide, methane , water vapor, and CFC�s, the atmosphere retains the sun�s radiation and warms up the planet.

By increasing the abundance of these gases in the atmosphere, humankind is increasing the overall warming of the Earth�s surface and lower atmosphere, a process called " global warming ." The figure below illustrates the radiation balance and the role of greenhouse effect. The Radiation Balance Illustration of the Earth�s radiative balance. (Adapted from: NOAA) Another way to think about the greenhouse effect is to consider that according to physics the radiation we receive from the Sun must be equally balanced by the heat Earth radiates out to space. If we were to give back less energy than we receive, our planet would soon be too hot for life. Likewise, if we were to give back more energy that we receive, our planet would soon be too cold for life. This can be written as a balanced equation of radiation: Solar radiation output of re-radiated heat If we were to measure the temperature of the Earth from space, the Earth's "surface" would show a temperature appropriate for this requirement of energy balance: a measurement of roughly -18 degrees Celsius (about 0 °F). At this temperature, our planet radiates a quantity of heat into space that is equivalent to the amount of energy received from the Sun.

At this point you may be asking how we can speak of � global warming � when we have just stated that the Earth (as seen from space) MUST stay at the same temperature? And how is it that the temperature of the Earth�s surface is only a chilly 0°F? The key to understanding this apparent contradiction is to remember that we live at the bottom of the atmosphere. As far as the radiation balance is concerned, the lower atmosphere and the surface of Earth form part of a �warm interior� of the planet. Figure demonstrating the importance of greenhouse gases in regulating the temperature of the lower atmosphere. The top diagram shows a greenhouse Earth where the apparent temperature �surface� lies 5000m up in the atmosphere from the land surface. In the past 100 years this apparent temperature �surface� has been rising. By contrast, without a greenhouse effect, the Earth would look like the lower diagram. The apparent temperature "surface" that we would see from space is located well above the real surface of the Earth where we live. This apparent temperature "surface" is about 5000 meters up (17,000 feet) within the atmosphere.

To get a better handle on this concept consider the following: the difference in elevation between 0 meters and 5,000 meters corresponds to a difference in temperature of about 60°F. In other words, at sea level it is 60°F warmer than it would be without the atmosphere. For the last 100 years or so this apparent temperature �surface� has been moving upward in the atmosphere as a result of global warming. As the apparent "surface" rises, the bottom of the atmosphere gets warmer, a fact that can be seen in the positions of the snow line (the elevation where snow begins to form) and tree line (the elevation where it becomes to cold for trees to grow). However, despite all these changes happening in the lower atmosphere, the overall temperature of the planet as seen from space stays the same. How is it possible that the Earth exactly balances the incoming sunlight with the outgoing heat radiation ? The answer is simple: the amount of heat radiation from Earth is precisely tied to the temperature of the atmosphere. If the temperature of the apparent �surface� is too low and Earth radiates too little heat to keep the balance, Earth will warm up and radiate more heat into space.

If the temperature of the apparent �surface� is too high and Earth radiates more heat than it receives, the planet will become colder and radiate less energy back to space. Overall, this �negative feedback� stabilizes the radiation balance despite all the variations of temperature from one place to another and within the vertical column of the atmosphere. It sets the temperature so that the incoming and outgoing energy is balanced. Average Temperatures on the Moon We can get another idea about what the temperature on Earth would be like without a greenhouse atmosphere by contemplating the Moon. The Earth�s satellite has no atmosphere because its gravitational force is not strong enough to retain gas for long. It has the same distance from the Sun as the Earth, but its temperature varies enormously: where the Sun is shining, the Moon�s temperature rises to 230°F and where it is dark falls to negative 290°F. The average surface temperature of the moon, about the same distance as the Earth from the Sun, is also near 0°F, but of course, the moon has no atmosphere. By contrast, the average surface temperature of the Earth is 60°F at sea level.

On Earth, the contrast between maximum and minimum temperatures would not be as great as on the Moon, even without an atmosphere, because the Earth rotates once in a day, while the Moon only rotates once in a month. However, without an atmosphere the Earth�s contrast between day and night and the contrast between summer and winter would be very large indeed. Not all the gases in the atmosphere are equally active in keeping Earth warm. In fact, the atmosphere�s most abundant gas, molecular nitrogen, does very little in this regard, and the same is true for the second most abundant gas, molecular oxygen. The most important ingredient of the air for producing the greenhouse effect is water vapor. However, its abundance depends on the air's temperature. The warmer the air, the more water vapor it can hold. (As air cools, the vapor condenses into rain or snow.) It is carbon dioxide that moves the air toward higher temperature, so that water vapor can take over and warm it some more. Carbon dioxide molecules intercept infrared radiation, warming the air and increasing water vapor through evaporation from the sea surface and from plants and soil moisture. Water vapor then increases the temperature even more. The process is checked by a rise in infrared radiation to space and by formation of clouds .

Unfortunately, the role of clouds in the radiation balance is as yet poorly understood. Different types of clouds have different effects, and this makes the calculations complicated and the results uncertain. Solar Variations One last point to consider when discussing the greenhouse effect is the amount of sunlight coming in to Earth. The quantity of sunlight we receive depends on the size and the brightness of the Sun and the distance between it and the Earth. As far as we know, the size of the Sun does not change much over the time spans we are considering; we can assume it is constant. The sun�s brightness varies only a little, about one-thousandth over an eleven-year sunspot cycle , but perhaps more over longer time spans. We can take that as constant too, calling the incoming energy flux "the solar constant ." One of the contentious issues in the discussions about the global warming of the last 100 years that has not been fully resolved is the question of whether a brighter Sun may have contributed to the recently observed temperature rise.

Question: What is the name for a type of atmospheric gas that lets in more energy than it lets out?
Answer: Greenhouse Gas