Causes of climate change involve any process that can alter the global energy balance between energy coming from the Sun and energy leaving the Earth. There are many natural causes of climate change, but recently we have become concerned with the effect mankind's pollution of the atmosphere may be having on the global climate.
The Earth has a natural greenhouse effect which keeps it much warmer that it would be without an atmosphere. Greenhouse gases in the atmosphere trap infrared heat energy trying to escape back to space. In doing so they raise the temperature of the lower atmosphere and the Earth's surface in contact with it. During the last 200 years, mankind has been releasing substantial quantities of extra greenhouse gases to the atmosphere, through the burning of fossil fuels and deforestation. These extra gases are trapping more heat in the atmosphere, and it is now suspected that the observed warming of the Earth by about 0.6C since the late 19th century is due to this man-made enhancement of the natural greenhouse effect.
This climatic trend has become known as "global warming", and may be distinguished from historical and prehistorical climate changes that have occurred naturally. The term "global warming" is usually reserved for the observed global climate change during the last 100 to 150 years that is believed to be related to mankind's enhancement of the greenhouse effect.