Nuke the Yellowstone Caldera
Hundreds of cubic kilometers of magma at high pressure. A five kilometer cap that limits eruptions to only every million years or so. A well-placed explosion that destroys that cap in the space of a few seconds. A lava plume ten times taller than Mt. Everest, followed by perpetual and global night that lasts for years. This one requires a nuke slightly larger than 1-megatons - 20-megatons ought to be sufficient.