Volcanic eruptions reduce global warming: smoke reflects the sun's rays. But that is not the only effect that smoke has, since sulfur dioxide from this smoke returns to the ground in the form of acid rain, increasing sulphurous bacteria from the wet media and decreasing methane bacteria. This reduces methane production and reduces the greenhouse effect.
A geophysicist from the open university of the UK comes to this conclusion by performing a simulation in a swamp of Scotland that threw sulfide and saw that in two years the emission of methane was 40% lower.