Overall, volcanoes release about 1% 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 number 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.

By comparison, human activities emit a Mount St. Helens eruption of CO2 every 2.5 hours and a Mount Pinatubo eruption of CO2 twice daily. The largest possible eruptions come from super volcanoes like Yellowstone or Mount Toba (which erupt very rarely, about every 100,000-200,000 years or more), but the total annual CO2 emissions from human activities is like one or more Yellowstone-sized super eruptions going off every year. So, the human impacts are quite large compared to volcanoes.