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Volcanic eruptions can intensify global warming by adding CO₂ to the atmosphere. However, of greater significance is the haze effect - caused by ash and gases released during an eruption, which results in global cooling.

It was thought for many years that the haze effect resulted from ash particles in the upper atmosphere blocking out solar radiation. However, scientists now believe that the sulfur-rich gases released in many eruptions have a greater impact. Sulfur combines with water vapour in the stratosphere to form dense clouds of tiny sulfuric acid droplets. These droplets absorb solar radiation and scatter it back into space.

Observational evidence shows a clear correlation between historic eruptions and subsequent years of cold climate conditions. Examples include:

• Laki, Iceland - erupted in 1783 - Europe and eastern USA recorded the lowest ever winter average temperature in the following year, almost 5°C below average.
• Tambora, Indonesia - erupted in 1815 - resulted in an extremely cold spring and summer in 1816, which became known as the year without a summer. Snowfalls and frost occurred in June, July and August and alpine glaciers advanced down mountain slopes to exceptionally low elevations.
• Krakatoa, Indonesia - erupted in 1883 - the second largest eruption in history. For months after the eruption the world experienced unseasonably cool weather and brilliant sunsets.
• Mount Pinatubo, The Philippines – erupted in 1991 - 10 cubic km of material and 15 million tons of sulfur dioxide was ejected, making it the second biggest eruption of the 20th century. Over the course of the next two years, strong stratospheric winds spread these aerosol particles around the globe. Data collected following the eruption showed that the mean world temperatures decreased by about 1°C over the two years following the eruption.