A Song of Ice and Fire
As Ball rightly points out, the worst part comes from the effect that such huge volcanic eruptions would have on the climate. As aforementioned, the world would be gripped by a volcanic winter, and the concept of “seasons” would disappear, as seen most famously after the 1815 eruption of Tambora. The year after was known in the Northern Hemisphere as the “Year Without Summer.”
Volcanoes have freed the world from the grips of ice ages in the past. In this case, “ice age" refers to one of perhaps five periods in Earth’s history where the world was extremely frigid and where glaciers were extremely widespread. This doesn’t refer to a glacial maximum, where the wobble of the Earth on its rotational axis causes its orbital path to stretch further away from the Sun, enough to encourage glaciers to encroach on lower latitudes.
During periods of continental break-up, where supercontinents like Pangaea or Rodina split apart, volcanism kicks into gear and massive amounts of carbon dioxide are released into the atmosphere. This quickly warms the world, and ice cover dramatically retreats towards the poles.
Although there is some global cooling initially, as huge quantities of sunlight-reflecting sulfur aerosols are released into the sky, the very long-term expulsion of carbon dioxide will overcome this effect – something observed immediately following the end-Permian mass extinction 252 million years ago when over 90 percent of all life on Earth died off. A similar trend was seen at the end-Cretaceous mass extinction 66 million years ago when the non-avian dinosaurs bit the dust.
Volcanic ash would cool the world, but over the long run, the carbon dioxide would warm it. Ammit Jack/Shutterstock
If all the volcanoes in the world were to erupt at once, this is almost certainly what would happen in the long run, but to a far more severe degree than life on Earth had ever previously experienced. Not only that, but huge unstable reservoirs of carbon in the world's oceans and Arctic would thaw out, be turned by microbes into carbon dioxide and methane – a more potent but shorter-lasting greenhouse gas – and would be unleashed into the sky.
This would accelerate planetary warming, ultimately encouraging more trapped icy methane reserves to escape into the atmosphere in a potentially deadly cycle. Warmer oceans also hold less carbon dioxide, so much of this would also effuse skywards.