The greenhouse effect is the heating of the surface of a planet or moon due to the presence of an atmosphere containing gases that absorb and emit infrared radiation.[1] Thus, greenhouse gases trap heat within the surface-troposphere system.[2] This mechanism is fundamentally different from that of an actual greenhouse, which works by isolating warm air inside the structure so that heat is not lost by convection. The greenhouse effect was discovered by Joseph Fourier in 1824, first reliably experimented on by John in 1858, and first reported quantitatively by Svante Arrhenius in 1896.[3]
The black body temperature of the Earth is 5.5 °C.[4][5] Since the Earth's surface reflects about 28% of incoming sunlight[6], the planet's mean temperature would be far lower, about -18 or -19 °C.[7] [8] Along with the added contribution of the greenhouse effect, it is instead much higher, roughly 14 °C.[9]
Global warming, a recent warming of the Earth's surface and lower atmosphere,[10] is believed to be the result of an "enhanced greenhouse effect" mostly due to human-produced increases in atmospheric greenhouse gases.[11] This human induced part is referred to as anthropogenic global warming (AGW).
The Earth receives energy from the Sun mostly in the form of visible light and nearby wavelengths. About 50% of the sun's energy is absorbed at the Earth's surface. Like all bodies with a temperature above absolute zero the Earth's surface radiates energy in the infrared range. Greenhouse gases in the atmosphere absorb most of the infrared radiation emitted by the surface and pass the absorbed heat to other atmospheric gases through molecular collisions. The greenhouse gases also radiate in the infrared range. Radiation is emitted both upward, with part escaping to space, and downward toward Earth's surface. The surface and lower atmosphere are warmed by the part of the energy that is radiated downward, making our life on earth possible.
The term "greenhouse effect" is an analogy to greenhouses, and as all such things is not exact, and can and has been abused.
There is considerable confusion on the matter and a more thorough discussion is useful for understanding how greenhouses and the greenhouse effect work. Many sources make the "heat trapping" analogy of how a greenhouse limits convection to how the atmosphere performs a similar function through a different mechanism involving absorption and emission of infrared absorbing gases.
A greenhouse is built of any material that passes sunlight, usually glass, or plastic. It mainly heats up because the sun warms the ground inside, which then warms the air in the greenhouse. The air continues to heat because it is confined within the greenhouse, unlike the environment outside the greenhouse where warm air near the surface rises and mixes with cooler air aloft. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. It has also been demonstrated experimentally (R. W. Wood, 1909) that a "greenhouse" with a cover of rock salt heats up an enclosure similarly to one with a glass cover.[23] Thus greenhouses work by a different mechanism, primarily by preventing convective cooling.
On the other hand, if one limits infrared radiation from the greenhouse, especially at night, one can substantially increase the temperature of the greenhouse, or limit the amount of heating that is needed. Aluminized screens which reflect the infrared thermal radiation so that it cannot heat the greenhouse windows are used for this purpose [26] As energy prices rise, similar screens are coming into increasing use in greenhouses.
This is an important hint for understanding the Woods experiment. In that case when a glass window was used at the top of the tube, it absorbed thermal energy transferred by radiation and radiatively emitted that energy to the outside from the top of the tube. When a rock salt window which transmits infrared was used, essentially the same amount of thermal infrared radiation was passed to the outside by radiation from the bottom and walls of the cylinder.
In the atmosphere, as in the greenhouse, sunlight heats the surface. A radiative balance is achieved when the thermal energy radiated to space matches the energy absorbed from the sun. Without greenhouse gases all of the energy radiated from the surface would reach space at the speed of light. As a practical matter none of the energy radiated from the surface at wavelengths that can be absorbed by the principal greenhouse gases reaches space directly. Rather due to cycles of absorption and emission, radiation at those frequencies is trapped within the atmosphere and can only be emitted by greenhouse gases high in the troposphere where the lapse rate ensures that it is significantly colder. Since the rate of emission by the colder molecules is much slower, the entire earth system must heat up in order to restore a radiative balance. In this way, the greenhouse effect limits the emission from the earth by radiation.
Both the greenhouse effect and the greenhouse limit the rate of thermal energy flowing out of the system. In that way they are similar. On the other hand in the case of a greenhouse, cutting off convection is the principal limit on the flow of energy. In the case of the greenhouse effect the rate of radiation from the Earth to space is limited by the greenhouse gases.
The greenhouse effect is the rise in temperature that the Earth experiences because certain gases in the atmosphere(water vapor, carbon dioxide, nitrous oxide, and methane, for example) trap energy from the sun. Without these gases, heat would escape back into space and Earth's average temperature would be about 60°F colder. Because of how they warm our world, these gases are referred to as greenhouse gases.
What Causes the Greenhouse Effect?
Life on earth depends on energy from the sun. About 30 percent of the sunlight that beams toward Earth is deflected by the outer atmosphere and scattered back into space. The rest reaches the planet's surface and is reflected upward again as a type of slow-moving energy called infrared radiation. The heat caused by infrared radiation is absorbed by "greenhouse gases" such as water vapour, carbon dioxide, ozone and methane, which slows its escape from the atmosphere.
Although greenhouse gases make up only about 1 percent of the Earth's atmosphere, they regulate our climate by trapping heat and holding it in a kind of warm-air blanket that surrounds the planet.
This phenomenon is what scientists call the "greenhouse effect." Without it, scientists estimate that the average temperature on Earth would be colder by approximately 30 degrees Celsius (54 degrees Fahrenheit), far too cold to sustain our current ecosystem.
The Average Global Temperature is Increasing Quickly
Today, the increase in the Earth's temperature is increasing with unprecedented speed. To understand just how quickly global warming is accelerating, consider this:
During the entire 20th century, the average global temperature increased by about 0.6 degrees Celsius (slightly more than 1 degree Fahrenheit).
Using computer climate models, scientists estimate that by the year 2100 the average global temperature will increase by 1.4 degrees to 5.8 degrees Celsius (approximately 2.5 degrees to 10.5 degrees Fahrenheit).
Not All Scientists Agree
While the majority of mainstream scientists agree that global warming is a serious problem that is growing steadily worse, there are some who disagree. John Christy, a professor and director of the Earth System Science Centre at the University of Alabama in Huntsville is a respected climatologist who argues that global warming isn't worth worrying about.
Christy reached that opinion after analyzing millions of measurements from weather satellites in an effort to find a global temperature trend. He found no sign of global warming in the satellite data, and now believes that predictions of global warming by as much as 10 degrees Fahrenheit by the end of the 21st century are incorrect.
Carbon Dioxide Emissions are the Biggest Problem
Currently, carbon dioxide accounts for more than 60 percent of the enhanced greenhouse effect caused by the increase of greenhouse gases, and the level of carbon dioxide in the atmosphere is increasing by more than 10 percent every 20 years.
If emissions of carbon dioxide continue to grow at current rates, then the level of the gas in the atmosphere will likely double, or possibly even triple, from pre-industrial levels during the 21st century.
Climate Changes are Inevitable
According to the United Nations, some climate change is already inevitable because of emissions that have occurred since the dawn of the Industrial Age.
While the Earth's climate does not respond quickly to external changes, many scientists believe that global warming already has significant momentum due to 150 years of industrialization in many countries around the world. As a result, global warming will continue to affect life on Earth for hundreds of years, even if greenhouse gas emissions are reduced and the increase in atmospheric levels halted.
