That is a phrase that gets thrown around with disturbing frequency. But what does it mean?
To begin with, the greenhouse effect is a good thing. When electromagnetic radiation (including visible light) given off by the sun reaches the earth, a variety of things happen. On average, about 26% of the radiation is reflected back into space by the particles making up Earth's atmosphere. 19% is absorbed by particles in the atmosphere and 4% is reflected from the earth's surface back out into space. Therefore, only 51% ever reaches the Earth. It is this radiation that is responsible for the greenhouse effect.
When this 51% reaches the earth, it is first absorbed by the ground then re-radiated back out into the atmosphere. Ordinarily, one might expect this radiation to exit through the atmosphere back out to space. However, gases like CO2, H2O and methane absorb the re-radiated energy and trap it within the atmosphere, sending it back down to the surface. As a result, the earth is able to retain more energy than it otherwise would. The effect is often metaphorically compared to a giant greenhouse, which is where the name, the Greenhouse Effect, comes from. Those gases responsible for the trapping of radiation (heat) are referred to accordingly as Greenhouse Gases. Without them, Earth's temperature would change from its average 15°C (about 59°F) to the much colder -18°C (about -1/3°F).
The problem with the Greenhouse Effect started when human civilization hit the Industrial Age in Europe and North America at the dawn of the 19th centiry. Industrialization requires vast energy resources, many of which can only be drawn from fossil fuels like coal and oil, which emit CO2 as a by-product when burned. With the internal combustion engine at the end of the 19th century and the advent of industrialization in the non Euro-American world during the 10th, the amount of CO2 being released into the atmosphere greatly increased. As a result of higher levels of CO2 trapping heat in the atmosphere, scientists believe that the Greenhouse Effect will be exacerbated from its natural levels, heating the earth more than is natural and moving the temperature range into areas that would make sustaining life extremely difficult. However, the computer models for such predictions are, at best, uncertain - for example, the models have no way to account for the increased cloud cover (and therefore increased reflection of radiation back into space) that would result from higher atmospheric gas concentrations.
The modeling is also based on rough estimtes of greenhouse gas emissions, which are only guesses. Data from China and India is hard to come by and many 2nd and 3rd world countries lack the means to keep good track of their industrial emissions. Therefore, no one is sure the quantity of greenhouse gases being released and therefore the resultant damage is impossible to accurately predict.
In summary, no one is sure what is going to happen to the Greenhouse Effect, because the effect is unclear due to a marked ignorance as far as what human civilization has truly done to intensify the Greenhouse Effect. All that is known is that humans are interfering with what was once a stable equilibrium.
Source:
The Greenhouse Effect
Global Warming PowerPoint