Global cooling - volcanic eruption: Mount Pinatubo
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.
Case study: Mount Pinatubo
On 15 June 1991 Mount Pinatubo in the Philippines erupted, ejecting vast amounts of ash and gas high into the atmosphere, so high that the volcano's plume penetrated into the stratosphere.
Over the course of several eruptions a massive 10 cubic km of material was ejected, making it the second biggest eruption of the 20th century.
Pinatubo also injected about 15 million tons of sulfur dioxide into the stratosphere.
Sulfur dioxide reacted with water to form a hazy layer of aerosol particles made up of sulfuric acid droplets.
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.