While the greenhouse effect is one of the reasons we are even alive in the first place, it can also be twisted into a negative force as well. It works much the same way the inside of your car heats up on a sunny day. The car's interior surfaces, like the seats and dashboard, absorb some of the heat. Some of it reflects back upward. The heat from these objects is a different wavelength than the heat of the sun, so the windows do not let as much of it out as in, and the car heats up. Now, let's zoom outward and apply this analogy on a planetary level. About 70 percent of the sun's energy stays on Earth, taken in by oceans, land, plants and other items. The other 30 percent is reflected away from Earth by clouds, snow and other surfaces [source: NASA]. The items that take in the sun's radiation eventually emit it at a different wavelength, just like the surfaces in your car, and others (such as clouds) reflect it back to Earth to keep the radiation and climate relatively stable.
CO2 is a chemical that absorbs sunlight and warmth from space, keeping it tethered to the Earth instead of radiating back out after it hits the surface. However, as human sources pump excess CO2 into the atmosphere, more heat gets trapped on Earth, leading to a slow, subtle rise in global temperatures. In essence, the greenhouse effect, combined with the vast amounts of extra CO2 humans pour into the atmosphere, is responsible for global warming.
Human influence on greenhouse gasses has generated concern over the past few decades. The first large public discourse on the issue started when holes in the ozone layer were discovered over the Arctic and Antarctic regions. Ozone blocks a portion of the sun's ultraviolet rays from reaching the surface of the planet. After many years of study into the causes of ozone depletion and a worldwide effort to curb the use of chemicals that deplete the Earth's protective layer, the United Nations reports that global depletion has leveled off, and may return to normal by midcentury [source: UN].
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