Volcanoes release a fair amount of it. Sometimes they release so much that it causes mass extinctions. The Siberian Traps, the volcanic province that produced a continental-sized lava flow over 1 million years, 252 million years back. This released 85 trillion tonnes of carbon dioxide, triggering a roughly 6-8°C (10.8-14.4°F) rise in global temperatures in a very short space of time.
This likely triggered the Great Dying, a mass extinction that wiped out as much as 96 percent of all life on Earth. It was as close to the apocalypse as you’re going to get.
But this event, although unique, even if it happened today would pale in comparison to the amount of carbon dioxide humanity pumped into the sky every single year – and that’s the crux of it. It’s all about the rate of carbon dioxide emitted over time, not the absolute amounts, per se.
The Siberian Traps produced a lot of carbon dioxide over a geologically short period of time, but humans produce even more in what amounts to be a geological blink of an eye. On average, per year, volcanoes emit 0.3 billion tonnes of carbon dioxide. Humans produce at least 100 times that, and that amount is rising year on year.
In fact, at no point in the last century has a volcanic eruption produced more carbon dioxide than there already is in the atmosphere. As for the future, there is no natural process in which a volcano would emit more carbon dioxide than we can over the course of a year. So this argument will never be correct.
As for the second point, yes, it is also true that volcanoes produce a lot of sulfur dioxide. When this readily reacts with water vapour in our atmosphere, it turns into clumps of sulfuric acid. It’s a reflective substance, which means the more of it there is in the atmosphere, the less solar radiation makes it through and the cooler the Earth will be.
(Incidentally, some geoengineers suggest that we could pump tonnes of sulfur dioxide into the sky in order to offset the man-made warming. While this may work, it would also merely mask the problem rather than deal with the direct cause of it – and let’s not forget about the potentially dangerous acid rain that all that sulfur would turn into, as well as all the other ways in which such a program would potentially alter the climate. Best leave it, we say.)
In terms of volcanic emissions, these aerosols do have a more noticeable effect on our climate. When Mount Pinatubo erupted in 1991, global temperatures cooled by a fraction of a degree for several months.
When the far more explosive Tambora eruption took place in 1815, so much sulfur was released that 1816, in many parts of the world, lacked any significantly warm months whatsoever. In fact, that was dubbed The Year Without Summer.