While various opinions, facts, and myths exist regarding the role of humankind in climate change, there is little argument about the impact of numerous naturally occurring events that play a role in changing climate. One of these events is volcanic eruption, which the U.S. Geological Survey states can affect climate change significantly.
During eruptions, volcanic gases like sulfur dioxide can cause global cooling, while volcanic carbon dioxide has the potential to promote global warming. Volcanic eruptions may even cause global freezing; 70,000 years ago, the Lake Toba eruption apparently caused a six-year global freeze. See Do Volcanoes cause Climate Change?
Mount Pinatubo's eruption was small compared to eruptions during the eighteenth and nineteenth centuries. Take for example the 1883 eruption of Krakatoa in Indonesia. On August 27, Krakatoa erupted in four catastrophic explosions, propelling ash 50 miles high into the mesosphere.
"as a result (of the eruption)... weather patterns
did not return to normal until five years later."
The sound of the eruption was heard 3,000 miles away on the island of Rodrigues, Mauritius, and barographs around the world recorded the shockwave and show that it reverberated around the world seven times See How Krakatoa made the biggest bang. As a result of this eruption, the average global temperature fell by roughly
1° F, and weather patterns did not return to normal until five years later.
These are just two examples of eruptions in the past 130 years, which is an extremely short period in geological terms. Given the amount of volcanic activity in recent years and the high potential for eruptions to affect the climate, the role of volcanic activity in climate change should not be ignored.
The most significant climate impacts from volcanic injections into the stratosphere come from the conversion of sulfur dioxide to sulfuric acid. This forms sulfate aerosol, which increases the reflection of solar energy back into space, having a net cooling affect on the Earth's atmosphere.
An example of this occurred in 1991 with the eruption of Mount Pinatubo in the Philippines, the second largest terrestrial eruption of the twentieth century (Source: The Cataclysmic 1991 Eruption...). This eruption injected 20 million tons of sulfur dioxide into the stratosphere at an altitude of more than 20 miles, temporarily reducing the normal amount of sunlight reaching the Earth's surface by roughly 10%, and causing temperatures worldwide to drop by 0.7° F (0.4° C). See Mt Pinatubo's Cloud Shades Global Climate