Just to add to DrDave above,
The presence of CO2 (and many other "greenhouse" gases) in the atmosphere is natural - because of the presence of these gases (the physical properties of which DrDave described), the effective temperature of the earth is greater than it might otherwise be - hence life is sustained. The issue at stake within the climate change debate is increases in concentrations of these greenhouse gases above background levels. Consequently, especially with regard to the role of Milankovitch forcing on 100, 41 an 20kyr timescales, any climate change signal will be small, and nested within much greater variability through time.
Hence regarding 'circumstantial' evidence of climate change, then it is necessary to regard it with a degree of caution. For example, shrinking Alpine glaciers are often cited as evidence for climate change, but this ignores more immediate and identifiable explanations, such as adjustment of ice masses from little ice age maxima, or the role or regional variability of climate