Sunlight

• The greenhouse effect begins with that part of the Sun's radiation that reaches the Earth's surface, and is not deflected either by the planet's magnetic sphere or it's cloud cover. Most of this radiation takes the form of visible light, which is absorbed by the planet's surface, causing it to heat up and emit infra-red radiation. That infra-red radiation, or heat energy, is then absorbed by the atmosphere, which also heats up and re-emits heat. Some of this is emitted upwards and outwards, back into space. The rest is sent downwards, back towards the Earth's surface.
  
  

The Atmosphere

• Earth's atmosphere is full of gases that serve as absorbers and/or reflectors for heat energy. According to Real Climate, the four most important are water vapor, carbon dioxide, methane and ozone, in that order. Exactly how much each contributes to keeping the Earth's surface warm is not very well established. Water vapor (including cloud cover) is believed to contribute between 66 and 85 percent, a very wide margin of error. Also, it is important to understand that not all of these gases have the same effect. Methane, for example, has 25 to 30 times the warming potential of carbon dioxide.
  
  

Global Warming

• Human industrial activity is largely based on the burning of fossil fuels for energy, and the Earth's deposits of coal and oil are essentially giant deposits of carbon. By burning them, we release that stored carbon into the atmosphere, mostly in the form of carbon dioxide. Scientific data collected from ice core samples taken in the Antarctic and other places suggest that while the level of carbon dioxide in the atmosphere has changed over time, both rising and falling, the gas is currently at its highest known level of concentration. In other words, there might be more carbon dioxide around now than there has ever been before. Other greenhouse gas levels, such as methane, have been rising as well. More greenhouse gases means that more heat is absorbed by the atmosphere, part of which is reflected back towards the Earth's surface again, causing higher temperatures.
  
  

Feedback

• A potentially dangerous result of rising temperatures is the creation of feedback, or a self-reinforcing cycle where warmer temperatures beget results that create still warmer temperatures. To take one very simple example, rising temperatures around the world will create more evaporation from the world's oceans, leading to more water vapor in the atmosphere. More water vapor means more global warming. Another example of feedback is the warming of the Siberian and Canadian permafrost. The permafrost is currently storing a vast amount of frozen methane, so as it thaws out, that methane is released. As previously noted, methane is a dramatically more powerful greenhouse gas than carbon dioxide, and a massive release of it is thought to have been what brought the last Ice Age to an end. In these ways, the greenhouse effect could become a chain reaction, leading to the release of more and more greenhouse gas.