Global warming

There are many theories surrounding the causes for global warming, but the main and proven reason is the effect of greenhouse gases that are emitted into the atmosphere.  It has been due to the increase in the atmosphere of greenhouse gas concentrations like water vapor, carbon dioxide (CO₂), methane and ozone.  The largest contributor of the greenhouse effect are fossil fuels being burned which then emits carbon dioxide into the air.

When sunlight reaches Earth's surface, some is absorbed and warms the earth, but the rest usually is radiated back to the atmosphere at a longer wavelength than the sun light. Some of these longer wavelengths are absorbed by greenhouse gases in the atmosphere before they are lost to space. The absorption of this long wave radiant energy warms the atmosphere.  Because of this the greenhouse gases then radiate the heat back to the earth and cause the "global warming" effect due to the fact that the heat trapped by the gases could not be released back into space.

http://timeforchange.org/cause-and-effect-for-global-warming
http://news.ufl.edu/2012/01/09/glacial-patterns/
http://climate.nasa.gov/causes