The greenhouse effect is when heat from the sun enters Earth's atmosphere and is reflected off the Earth's surface and then it reaches the atmosphere again, it cannot escape because the atmosphere is to thick for it to pass through. Earth’s natural greenhouse effect makes life as we know it possible. However, human activities, primarily the burning of fossil fuels and clearing of forests, have intensified the natural greenhouse effect, causing global warming. Normally the climate would rise then fall naturally, but since we are putting so much green house gasses in the atmosphere it is only increasing. Earth is not the only planet with the greenhouse effect. Mars, Venus, Pluto and one moon Titan also have atmosphere's that capture heat and increase the planet/moon's temp. naturally. A runaway greenhouse effect involving carbon dioxide and water vapor is thought to have occurred on Venus.