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Infrared, Gases and the Greenhouse Effect

by Ron Kurtus (revised 3 March 2010)

Water vapor and certain gases in the atmosphere absorb infrared radiation, causing what is called the Greenhouse Effect. This infrared radiation comes from materials on the Earth's surface that have been heated by sunlight. The absorption of infrared heats the atmosphere to the common temperatures we experience.

As the percentage of some greenhouse gases, such as carbon dioxide, increases in the atmosphere, the average temperature also increases.

The atmosphere, oceans and surface of the Earth are heated by the absorption of visible light from the Sun. Those items, in turn, emit infrared radiation that is primarily absorbed by gases and water vapor in the atmosphere, increasing its temperature. This is called the Greenhouse Effect.

Stability of this effect is important in keeping the Earth at reasonable temperatures. However, changes in the amount of absorbing gases can bring about instability in the Earth's climate.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



How the Earth is heated

The Earth is heated to relatively moderate temperatures by the absorption of some of the Sun's radiated energy.

About 30% of the visible light from the Sun is reflected back into space, while the other 70% is absorbed by the ground and oceans, resulting in heating the Earth. Since the atmosphere is transparent to visible light, it absorbs little energy from the sunlight and does not make much of a contribution in heating the Earth.

The soil, rocks and water that have been heated by the sunlight then give off longer wavelength infrared radiation, some of which is absorbed by the Greenhouse gases in the atmosphere. These gases include water vapor (H2O), carbon dioxide (CO2), methane (CH4) and ozone (O3). Absorption of the infrared radiation heats the atmosphere to acceptable temperatures, provided they are in the proper proportion.

Greenhouse Effect

The Greenhouse Effect is a misnomer and not a correct description of how the Earth is heated by the Sun. A greenhouse uses glass windows to trap the infrared in the building, thus heating its air. In the Greenhouse Effect, gases in the Earth's atmosphere absorb the radiation and thus heat the air. It is a much stronger effect than simply having glass windows trap infrared and warm air.

(For more information, see Heating a Greenhouse.)

Although this "Greenhouse Effect" has done a good job at keeping the temperatures on the Earth at a reasonable level over many years, the apparent excess of certain gases in the atmosphere has thrown that level off kilter and has resulted in what is called global warming.

Absorption of infrared

Water vapor (H2O) is the most active molecule in absorbing infrared radiation and thus in heating the atmosphere. It accounts for about 55% of the absorption of thermal radiation in the atmosphere.

Carbon dioxide (CO2) also absorbs infrared radiation and accounts for about 18%, methane (CH4) accounts for about 6% and ozone (O3) accounts for about 5% of heating the atmosphere.

Since some infrared radiation is lost into space, the recent percentages of carbon dioxide and methane in the atmosphere have allowed the temperature to remain stable.

CO2 has stayed at the proper percentage in the atmosphere until recently, when an increasing amount of the gas has been emitted due to the burning of fossil fuels. Automobile exhaust and industrial smoke contribute the most CO2.

Problem of methane

A potential problem is that the recent increase in temperature is melting the tundra in the Siberian area of Russia. This is causing the release of enormous quantities of methane gas (CH4) that has been locked in the frozen soil. This gas can cause a greater absorption of infrared radiation than CO2, thus causing an even greater heating of the atmosphere. Once the tundra starts to melt, even getting rid of automobiles will not stop the global warming trend.

Warming trend seen

Although the average temperature around the Earth has fluctuated over the eons, it seems that there has been a rapid increase in recent years as the world becomes more industrialized. This has been noticed in the melting of glaciers that have existed for thousands of years.

Note that there are some who claim that global warming is not a problem and is not caused by the greenhouse effect. But most scientists say that the gases are caused by human activity.

Other factors

There are other factors affecting the temperature of the Earth and its atmosphere. One is called global dimming, where the amount of sunlight reaching the ground has been decreasing in recent years due to the dust and exhaust particles in the air.

The amount of sunlight reaching the ground dropped 4% from 1960 to 1990, primarily due to air pollution. That trend was reversed since then, due to stricter pollution controls in industry and with automobiles. However, the increased pollution from emerging industrial countries like China and India, global dimming may be an issue again. It can also slow down global heating.

Problems caused by global warming

Runaway increases in the temperature of the Earth can cause numerous problems. The Arctic and Antarctic ice caps can start to melt, thus raising the level of the ocean and flooding many coastal cities.

Increased temperature will reduce the amount of snowfall in the mountains. Many countries depend on the runoff of that snow to help irrigate their farms and provide food for their people.

Increased temperatures mean more energy in the air, and thus more violent storms.

Existing plants and crops depend on certain climate conditions to grow. Increased average temperatures can disrupt the growing process in many areas.

Summary

The Greenhouse Effect is the process where infrared radiation from the Sun is absorbed by gases in the atmosphere, thus helping to heat the Earth. An excess of absorbing gases such as carbon dioxide in the atmosphere can drastically change the climate and cause numerous problems.


This is the only Earth we have


Resources and references

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