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The sun is the Earth’s primary energy source, a burning star so hot that we can feel its heat from over 150 million kilometers away.

About one third of this solar energy is reflected back into the universe by shimmering glaciers, water and other bright surfaces. Two thirds, however, is absorbed by the Earth, thus warming land, oceans, and the atmosphere.

Much of this heat radiates back out into space, but some of it is stored in the atmosphere. This process is called the greenhouse effect. Without it, the Earth’s average temperature would be a chilly -18 degrees Celsius, despite the sun’s constant energy supply.

In a world like this, life on Earth would probably have never emerged from the sea. Thanks to the greenhouse effect, however, we enjoy a comfortable average temperature of 14 degrees Celsius.

So, how does it work?

Only about half of all solar energy that reaches the Earth is infrared radiation, which causes warming. The other half only translates into heat once it hits Earth and is reflected back into space as infrared radiation.

This transformation of solar radiation into infrared radiation is crucial, because infrared radiation can be absorbed by the atmosphere. Like a radiator in the middle of a room, our atmosphere radiates this heat in all directions.How is this radiation absorbed?

Sunrays enter a greenhouse, heat up the ground, which, in turn, heats up the air inside the greenhouse. The glass roof and walls trap that warm air and temperatures increase.

Earth has no glass walls, but it does have the atmosphere, and within the atmosphere it has greenhouse gases.

Ninety-eight percent of the Earth’s atmosphere is made up of nitrogen, oxygen, and argon. But they do not absorb much infrared radiation, and don't contribute to the greenhouse effect.

Greenhouse gases like water vapour, carbon dioxide, ozone, methane, nitrous oxide, and chlorofluorocarbons do absorb heat, and thus increase atmospheric temperatures.

Studies indicate that until some 2.7 billion years ago, there was so much carbon dioxide (CO2) and methane in our atmosphere that average temperatures on Earth were as high as 70 degrees Celsius.

But bacteria and plants slowly turned CO2 into oxygen and the concentration of CO2 in our current atmosphere dropped to just about 0.038 percent or 383 parts per million (ppm). 

Slight changes in the amounts of carbon dioxide in the atmosphere are already changing the way our planetary heating system works.

Measurements of carbon dioxide amounts from Mauna Loa Observatory in Hawaii show that CO2 has increased from about 313ppm in 1960 to as high as 400ppm in 2013.

That means for every million particles in our atmosphere, there are now 62 CO2 particles more than in 1960. Even if this does not seem like much, scientists say this increase—most probably caused by human activities—is mainly responsible for rising global temperatures throughout the last decades.

Even if the term “greenhouse effect” is somewhat of a misnomer, it still might be a useful handle from which the public can grasp an otherwise intricate natural process.

Most people can relate to how hot and stuffy a greenhouse can get. Now that the Earth has started to heat up, we realize that our own global greenhouse has no window that we can open to catch some fresh air. 

Thilo Kunzemann

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