How does the gases released from volcanoes affect global warming?

Volcanoes often produce sulphur gases, which can reflect sunlight.

The last two big volcanoes were El Chichon and Pinatuba, El Chichon was smaller than Pinatuba but contained more sulphur and had a bigger cooling effect.

Large volcanic eruptions have effects which dwarf anything man made, but inconveniently they always cool the earth down, sometimes for many years (there have been much bigger ones in the past than those which have occurred in recorded history).

The CO2 produced is offset by the sulfur compounds that are released , they basically have no effect.