A single large volcanic eruption can have a strong effect on the planetary climate for a number of years. Piecing together records from tree rings, ice cores, cave stalactites, and other sources, researchers have observed several periods in Earth's recent history where cooler temperatures, or "volcanic winters" appear to be linked to massive volcanic eruptions. The physical material and the sulphur dioxide released by large eruptions can suppress the Earth's temperature by several degrees for two or three years.