What is greenhouse effect?

The theory of the greenhouse effect is that certain gases concentrate in the atmosphere, where they function as an insulating barrier, absorbing infrared radiation that would otherwise be reflected back into the upper atmosphere. In other words, like glass in a greenhouse, the gases let in incoming solar radiation but retard its reradiation back into space. The greenhouse effect could cause a gradual warming of the earth's surface and the lower atmosphere. Higher temperatures would have significant impacts on the earth's ecosystems.

What gases are we taking about?
Carbon dioxide. created mostly by burning fossil fuels, is the most plentiful of the gases and is thought to be responsible for about half of the warming. Each year, we send about 5.6 billion tons of CO2 into the atmosphere, only half of which is absorbed by the oceans and forests. Burning the tropical rain forests not only adds to the emissions but also means losing trees that naturally absorb CO2.