I’m going to attempt to offer a simple answer to this question and how greenhouse gases contribute to the overall problem.
First, lets start with the four most important greenhouses gases: carbon dioxide, methane, nitrous oxide, and fluorinated gases. There have always been greenhouse gases on Earth, and without them life would not be possible. They trap in the heat from the Sun’s radiation, and Earth would be 60% cooler without them.
The problem? Emissions from transportation, agriculture, manufacturing, and other activities have greatly increased the amount of these greenhouse gases in the atmosphere. This means that more heat is being trapped on Earth, instead of escaping back into space in the form of infrared radiation. Since 1750, the concentration of carbon dioxide in the atmosphere has increased over 36 percent, methane has increased over 148 percent and nitrous oxide has increased over 18 percent. If we continue to produce greenhouse gases at even the current rate, by 2100 the average global temperature will likely increase 3 to 7 degrees Fahrenheit (Information comes from National Geographic, link below).
This is where the term “global warming” comes from, though it may be better to call it climate change. Even if there is a particularly cold winter, the planet is still getting warmer than it would without humans. Much warmer. This warming leads to sea level rise as Arctic ice melts, which is a big problem given so many major cities are located virtually at sea level. For example, Venice, Italy is flooded on a near constant basis now. Another effect of this warming are more severe storms, such as Superstorm Sandy last October that rocked the eastern seaboard of the United States. New York City, one of the most densely populated cities on Earth, was underwater.
If we do not work to reduce our emissions of greenhouse gases, we will find more of our major cities underwater, permanently.
“The higher ocean levels would cover cities (and states) that are home to millions of people. Some of these places include Long Island, New York City, most of the giant sandbar known as Florida, New Orleans, Los Angeles and more.”
Map and story from: http://suite101.com/article/how-the-world-will-look-if-the-ice-caps-melt-a172728
For more information about global warming: http://greenliving.nationalgeographic.com/brief-explanation-global-warming-2170.html