claim: A drop of volcanic activity in the early 20th century may have had a warming effect.
evidence: Notable eruptions in the historical records are the 1991 eruption of Mount Pinatubo which lowered global temperatures by about 0.5 °C (0.9 °F) for up to three years, and the 1815 eruption of Mount Tambora causing the Year Without a Summer.
category: Climate change (general concept)