Overall, volcanoes release less than 2 percent of the equivalent amount of CO2 released by human activities. Quite small. However, about once every 20 years there is a volcanic eruption (e.g., Mount Pinatubo, El Chichon) that throws out a tremendous amount of particles and other gases. These will effectively shield us enough from the Sun to lead to a period of global cooling. The particles and gases typically dissipate after about 2 years, but the effect is nearly global.