The general climate of our planet is controlled by the delicate energy balance between incoming radiation from the sun and outgoing radiation from Earth. Chemical and physical processes in the atmosphere are critical to this balance.
Specifically, the presence of greenhouse gases such as water vapor, carbon dioxide and methane have the effect of letting most solar radiation pass through the air to the ground, but reflecting some of the outbound terrestrial radiation back into the lower atmosphere.
Without this greenhouse effect, it is estimated that the average temperature around the world would be somewhere near 0°F. This would mean the entire planet would be frozen, and life as we know it would not exist.
It stands to reason that, in the past as well as in the future, changes to the chemical makeup of the atmosphere, whether natural or anthropogenic, can affect this balance.