Volcanic gases can either warm or cool the planet, depending on their composition. For example, sulfur dioxide combines with water and forms sulfuric acid aerosols, which reflect incoming solar radiation and cool the planet’s surface. It has been estimated that volcanoes have helped keep global temperatures down by two to three degrees. For context, during the most recent Ice Age, the global average temperature was about five degrees cooler than the current average.

Source: one, two, three