Historical Context
The Earth has been both warmer and colder than at present, having had both more and less greenhouse gases in the atmosphere through purely natural processes.
Millions of years ago various proxies and modeling show that CO2 levels were very different than now. At times they were 10 times higher than present, while at other times they were very low. For example, in the Precambrian, a 200-million year period of intermittent, widespread glaciation extended close to the equator. Scientists have termed this period as Snowball Earth.
Snowball Earth appears to have been ended by a colossal volcanic outgassing which was not balanced by weathering of rock and deposition of carbon on the sea floor. This raised the CO2 concentration of the atmosphere abruptly to 12%, about 350 times modern levels. No volcanic carbon dioxide emission of comparable scale has occurred since.
During the last 10,000 years, human civilization developed during the relatively steady atmospheric composition and climate of the Holocene. In the modern era, emissions to the atmosphere from volcanoes are balanced by deposition of sediments on the sea floor and subsequent subduction.