In general, the greenhouse effect refers to any situation where short wavelengths of light pass through some medium (it could be glass or the atmosphere) and get absorbed whereas longer wavelengths of infrared radiation pass through, are re-radiated from objects and then unable to pass through the medium. This results in the trapping of longer wavelengths and a higher temperature inside the medium.
When referring to the Earth's climate, greenhouse effect is the heating of the planet's surface due to the absorption of outgoing infrared or thermal radiation due to atmospheric greenhouse gases such as methane, carbon dioxide, and water vapour. This occurs naturally without any human emissions; the existence of the greenhouse effect is a vital component of an inhabitable Earth, as it keeps the surface at a livable temperature - without it the Earth would be much colder, with an average temperature of about -18°C (see Earth Temperature without GHGs). Figure 1 shows a diagram that illustrates how the natural greenhouse effect works on Earth to maintain a comfortable temperature.
Although the greenhouse effect is a natural phenomenon, there are concerns with something known as the enhanced greenhouse effect. The enhanced greenhouse effect is generally what is being talked about when people refer to the greenhouse effect and climate change. This effect refers to the increased heating of the Earth's surface as a result of a higher amount of greenhouse gases being released into the atmosphere from human activities. These greenhouse gases trap more of the outgoing radiation from the surface of the Earth, meaning that less escapes to space and the planet heats up.
The natural atmosphere is composed of 78% nitrogen, 21% oxygen, 0.9% argon, and only about 0.1% natural greenhouse gases. Although a small amount, these greenhouse gases make a big difference - they are the gases that allow the greenhouse effect to exist by trapping in some heat that would otherwise escape to space.
When present in the upper atmosphere in larger concentrations, however, these greenhouse gases contribute to global climate change. The cause of this contribution is due to the absorption and re-emission of radiation within the infrared range. Humans input greenhouse gases into the atmosphere that would otherwise not find their way there, affecting the natural balance; see anthropogenic carbon emissions for more information.
The level of harm that greenhouse gases can do is measured by their global warming potential.
Although the greenhouse effect tends to be associated with the negative effects of global warming and climate change, the natural greenhouse effect is actually necessary for life on Earth. The comfortable temperature of the Earth is determined by how much energy the greenhouse effect traps on the surface of the planet and how much it allows to escape into space. As well, the temperature of other planets - which can vary drastically - is determined by how their respective greenhouse effects work. The temperature of a planet depends strongly on the composition of the atmosphere. This is due to the greenhouse effect having such significant effects.
On Earth, the temperature is kept at a comfortable level since the atmosphere traps some of the radiant heat from the Sun, warming the surface and sustaining life. This trapping is done by greenhouse gases in our atmosphere, which absorbs some infrared heat radiation and re-radiates some to the surface of the Earth, warming it. This process, as explained above, is the natural greenhouse effect and is completely necessary to our life on this planet. NASA has reported that the average temperature of the Earth as a result of the warming from the greenhouse effect is 15°C. This average temperature increase is starting to harm various environments.
Without the influence of the greenhouse effect on our planet, the average surface temperature would be 255 Kelvin - which can also be expressed as -18°C or 0°F. If this were the case, water on Earth would freeze and life as we know it would not exist. The average temperature of the Earth is actually approximately 15°C, a significant difference!
The greenhouse effect is not the same on all planets, and differs dramatically based on the thickness and composition of the atmosphere. Three planets that show how dramatically the conditions of a planet can change with the different levels of the greenhouse effect are Venus, Earth, and Mars. These planets illustrate a kind of "Goldilocks effect", meaning that the influence of the greenhouse effect on Venus is too great, making the planet too hot to live on. Conversely, the greenhouse effect on Mars is too small, making it far too cold. Earth exists as the "just right" planet, with the greenhouse effect having just enough influence to make the planet livable.
The rapid increase in human activities in recent history has led to the continuing emission of large amounts of greenhouse gases. Although necessary in the atmosphere in smaller concentrations, the increased amount of carbon dioxide, methane, and other gases in the atmosphere is leading to enhanced global warming. Never before has the Earth seen such a large increase in the amount of greenhouse gases in the atmosphere in such a short time, and this is leading to significant changes in the climate of the Earth.
The enhanced greenhouse effect disrupts the Earth's climate equilibrium and has led to an increase in the global average surface temperatures. This increase in the Earth's temperature is predicted to have serious ongoing effects, such as changes in rainfall, ocean circulation, increasing numbers of extreme weather events, and a rising sea level. These changes can have further consequences for agriculture, biodiversity, and human health.