The causes are manifold and many of these are beyond human control.

Causes of Global Warming

Astronomical:
Changes in the Sun's output
Changes in the eccentricity of the Earth's orbit
Precession of the Earth's axis
Nutation of the Earth's axis

Geological:
Shift of continents, etc, due to plate tectonics, including chage of ocean current paths
Frequency and volume of volcanic eruptions, including volcanic carbon dioxide
Release of methane from methane hydrates in bottom of ocean

Biological:
Changes in carbon dioxide & methane levels due to rotting of soil detritis depending upon changes in moisture content and aeration (By the way, this particular source is HUGE and poorly known.)
Algal blooms, etc

Human:
Fossil fuel combustion
Deforestation and albedo changes
Overfishing of the oceans

The Earth's temperature has been bouncing around for eons, as these graphs show:

Source:
http://www.chatham.edu/PTI/Pgh_Env_History/Real01.htm

Now, the question is:

How much Global Warming is due to human actions?

Answer: Nobody knows.