Natural Factors
Though a large part of today's changing climate is due to human actions, one can't put all the blame on people. Natural factors come into play as well when dealing with climate change.
Volcanic Activity
Volcanoes, vents in the Earth's crust which spew forth lava, rock fragments, ash, and gas, are natural factor involved in climate change. The lava and rock fragments they eject are of no global concern, the ashes and gas are another matter. Because of their light weight, they can be shot high into the atmosphere and drift around the Earth.
As an example of what a volcano can do, think of the 1883 Krakatoa eruption. Krakatoa, a volcano in Indonesia, hurled an estimated 45 cubic kilometers of debris into the air, darkening skies up to 442 kilometers around the volcano. Within 13 days, sulphur dioxide and other gases mixed into the atmosphere and increased the albedo of the clouds, reflecting sunlight away from the Earth, cooling the world and killing crops. For many years following, global temperatures dropped by an average of 1.2 degress Celsius. As you can see, the effect of volcanic activity on the climate can be catastrophic.
Luckily however, volcanic eruptions of that scale are relatively rare. Volcanic eruptions also tend to be episodic and in the long scheme of things, don't affect the climate for very long. However, volcanoes are still a natural factor influencing climate.
Orbital Changes
Other natural factors that can influence climate can include the Earth's orbit, it's axis, and fluctuating solar radiation levels. These are all important due to the fact they effect how much energy the Earth is receiving.
As mentioned before when talking about what affects climate, the axis of the Earth and the latitude change how much energy is received. In the graph to the right, you can also see the CO2 level rise and fall consistently as the years go by. Because most of the Earth's vegetation is in the Northern hemisphere, CO2 levels rise when the north experiences winter... due to the Earth's axis.
Solar output levels shift as time goes by, as shown by surface activity and sunspot cycles. It is self-explanatory how the sun influences climate - the sun is the driving force behind things such as plant growth, winds, and the greenhouse effect. Of course changes in its energy output will change the climate.
Finally, let's mention the Milankovitch cycles. The Serbian scientist Milankovitch proposed that 3 major parts of the Earth's relationship with the sun change over time, namely axial tilt, eccentricity, and precession. We've already talked about axial tilt, but eccentricity is how elliptical the Earth's orbit is, and precession is how much the Earth's axis "wobbles" as it orbits the sun. All 3 of these factors are under cycles tens of thousands of years long. As they change, they affect how much solar heat the Earth receives, and thus affect climate patterns.
A painting of the 1883 Krakatoa eruption.
The different motions of Milankovitch cycles