The Greenhouse
Effect
Although many people think that the greenhouse effect is solely a negative thing, in truth, without the "greenhouse" aspect of our planet, people would not survive. Without the warmth people get from the greenhouse effect, our planet would be about -100*F. The term "greenhouse effect" describes how the gasses in Earth's atmosphere retain the radiant energy from the sun instead of letting it fly back out into space. The glass of the greenhouse works the same way.
In short, the gasses in the atmosphere let energy in and don't let all the energy back out. the energy that is trapped within the atmosphere heats the planet and keeps us alive. The human race is lucky that it lives on a planet that lets in and our the perfect amount of heat and light energy.
BUT WHAT IF IT DIDN'T??
Because most of the machines of the 21st century and the modern industries' widespread combustion of fossil fuels (coal, oil, and natural gas) much carbon dioxide has been given off as a by-product or waste, it has been accumulating in the atmosphere and this has made it hard for the heat to return to space. This means that heat is getting in but not getting back out. This is bad because if it gets too hot, our polar ice caps will melt and all the land on earth will be covered with water. And the amount of carbon dioxide is rising. Venus has such a runaway greenhouse effect and its temperature is 500*C (932*F).