Scientists estimate that without an atmosphere, the surface of the Earth would be about 54 degrees Fahrenheit (30 degrees Celsius) cooler than it is now [source: NASA]. That would bring it to about 5 degrees F (-15 degrees C); for comparison, the surface of Mars comes in around -9.4 degrees F (-23 degrees C) [source: Elert].
In other words, without Earth's atmosphere and the greenhouse effect it provides, we'd be up a creek. But it turns out that an overactive greenhouse effect can result a similarly devastating outcome. With too many heat-absorbing greenhouse gases in the atmosphere, too much radiation gets trapped, causing the Earth to warm beyond its ideal temperature.
Basically, Earth is not able to expel enough heat to keep itself cool, resulting in what's become known as "global warming." It's the greenhouse effect gone wild.
The problem is mostly the increased concentration of carbon dioxide (CO2) in the atmosphere. CO2 (in addition to water vapor) is the most substantial greenhouse gas.
In Earth's natural state, CO2 levels are kept in balance mostly by the activity of plant life. But as humans started burning more and more CO2-releasing fossil fuels, levels have shifted out of balance. We're emitting too much CO2 for the plants to handle (especially with the added effects of deforestation). Before the Industrial Revolution in the late 1700s, the concentration of CO2 in the atmosphere was about 280 parts per million (ppm). In the mid-1900s, that number was 300 ppm, and at the start of the 21st century, we have levels around 360 ppm [source: NASA].
This increase has led to a 1.8 degrees F (1 degree C) increase in Earth's temperature -- which may not seem like a lot until you realize that scientists predict dire, irreversible global effects at a 2 degrees C temperature increase, including melting ice sheets, rising sea levels and accompanying flooding, extreme climate patterns and widespread destruction of wildlife habitat [source: Times].
In agreements like the Kyoto Protocol, the world has begun to take action to try to curb this increased greenhouse effect. It remains to be seen whether these efforts will succeed in averting disaster or will prove to be too little too late.
For more information on the greenhouse effect, global warming and related topics, look over the links on the next page.