GLOBAL WARMING

To understand global warming , you have to know what greenhouse effect is.

some gases in the earth's atmosphere can trap heat from the sun and keep the earth warm . this is called greenhouse effect .

carbon dioxide is a kind of greenhouse gas . due to increasing human activities , more fossil fuels are burnt and more carbon dioxide released to the atmosphere . therefore , more heat is trapped . the global temperatures increase and this is called global warming .

http://en.wikipedia.org/wiki/Global_warming