Volcanic eruptions are often discussed in relation to climate change because they release CO2 (and other gases) into our atmosphere. However, human contributions to the carbon cycle are more than 100 times those from all the volcanoes in the world - combined.
In comparison, while volcanic eruptions do cause an increase in atmospheric CO2, human activities emit a Mount St. Helens-sized eruption of CO2 every 2.5 hours and a Mount Pinatubo-sized 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 to 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.
Essentially, CO2 emissions from human activities dwarf those of volcanoes.
Climate scientists bring up volcanic eruptions to better understand and explain short periods of cooling in our planet’s past. Every few decades or so, there is a volcanic eruption (e.g., Mount Pinatubo, El Chichón) that throws out a tremendous number of particles and other gases. These will effectively shield us enough from the Sun to lead to a short-lived global cooling period. The particles and gases typically dissipate after about 1 to 2 years, but the effect is nearly global.
Comparatively speaking, greenhouse gas warming coming from human activities (primarily driven by the human burning of fossil fuels) will endure for millennia, even longer than nuclear waste.