Also, inhabitable land & crop failure will happen with an increase of 3 to 4 degrees (and this is the optimistic outcome), so i don't see why it's not worth or your "truly catastrophic" characterization.
For an example of what can happen from simply a large volcanic eruption, see the Year Without a Summer in 1816: https://en.m.wikipedia.org/wiki/Year_Without_a_Summer
But that was not a super volcano. It was just one large eruption.
We have only increased by about 1 degree in the past 100 years. We are still cooler than the Medieval Warm Period when Vikings were farming parts of Greenland, and cooler than the Roman Warm Period, when they were growing wine grapes in Scotland. It's doubtful that CO2 increase of even doubling would cause catastrophic crop failure. After all, the CO2 levels were thousands of PPM for most of Earth's history, and plants evolved under those conditions before the Pliestocene. Dinosaurs were very large creatures that were supported by a rich ecosystem of plant life. CO2 is plant food. Only at extremely high levels of 7000+ ppm (20x modern levels) has there ever been any geological evidence of catastrophe and even that event is hard to disentangle cause and effect.