To what extent does volcanism contribute to Global Warming?
None. Humans release more than 100 times more carbon dioxide than volcanoes. The rate at which volcanoes emit carbon dioxide does influence carbon dioxide, but over thousands of years. When the rate of carbon dioxide from volcanoes doubles, the amount of carbon dioxide slowly increases. This causes the rate at which carbon dioxide is absorbed by the oceans to also increase.
humans emit 100 times more CO2 than all volcanoes. Large eruptions do cool the earth for a few years, but not a long term trend.
Volcanic eruptions affect climate in two main ways. First, they release the greenhouse gas carbon dioxide, contributing to warming of the atmosphere.
A lot, those that just say we emit more are diverting attention.