What If All the Volcanoes in the World Erupted At Once?

Ocean Acidification

The magma that spews from the volcanoes will eventually make its way into the seas and oceans. When this happens, the water in these seas and oceans will turn acidic. Sealife will be greatly damaged, causing great harm to the various food chains. The lack of sunlight also means that the planet will begin to cool down significantly. This can bring about the new Ice Age.