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What are greenhouse gases?
A Green House gas is any gas that absorbs infra-red radiation in the atmosphere. Greenhouse gases include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halogenated fluorocarbons (HCFCs) , ozone (O3), perfluorinated carbons (PFCs), and hydrofluorocarbons (HFCs).

How does the greenhouse effect work?
The "greenhouse effect" is the warming that happens when certain gases (like water vapor, carbon dioxide, and methane) in Earth's atmosphere trap heat. These gases let in light but keep heat from escaping, like the glass walls of a greenhouse.

First, sunlight shines onto the Earth's surface, where it is absorbed and then radiates back into the atmosphere as heat. In the atmosphere, “greenhouse” gases trap some of this heat, and the rest escapes into space. The more greenhouse gases are in the atmosphere, the more heat gets trapped.

What is causing global warming?

Scientists have been trying to figure out what is causing global warming. They looked at the natural cycles and events that are known to influence climate but found that the amount and pattern of warming that's been measured can't be explained by these factors alone. The only way to explain the pattern is to include the effect of greenhouse gases (GHGs) emitted by humans.
The United Nations formed a group of scientists called the International Panel on Climate Change, or IPCC. The IPCC meets every few years to review the latest scientific findings. It then writes a report that summarizes all that is known about global warming. Each report represents a consensus, or agreement, among hundreds of leading scientists.