Reading Comprehension

Read the passage and answer the question.

When a large volcano erupts, it can briefly cool the entire planet. Powerful eruptions inject sulfur gases high into the stratosphere, where they form a haze of tiny droplets. This haze reflects part of the incoming sunlight back into space, lowering temperatures at the surface for a year or two. The effect fades as the droplets gradually settle out of the air. After one major eruption in 1991, global average temperatures dropped by about half a degree Celsius. Scientists study such events to better understand how particles in the atmosphere influence climate.