Volcanic Gases and Climate Change Overview - updated 10/22/2015

http://volcanoes.usgs.gov/hazards/gas/climate.php

Volcanoes can impact climate change. During major explosive eruptions huge amounts of volcanic gas, aerosol droplets, and ash are injected into the stratosphere. Injected ash falls rapidly from the stratosphere -- most of it is removed within several days to weeks -- and has little impact on climate change. But volcanic gases like sulfur dioxide can cause global cooling, while volcanic carbon dioxide, a greenhouse gas, has the potential to promote global warming.

The most significant climate impacts from volcanic injections into the stratosphere come from the conversion of sulfur dioxide to sulfuric acid, which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the reflection of radiation from the Sun back into space, cooling the Earth's lower atmosphere or troposphere. Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree (Fahrenheit scale) for periods of one to three years. The climactic eruption of Mount Pinatubo on June 15, 1991, was one of the largest eruptions of the twentieth century and injected a 20-million ton (metric scale) sulfur dioxide cloud into the stratosphere at an altitude of more than 20 miles. The Pinatubo cloud was the largest sulfur dioxide cloud ever observed in the stratosphere since the beginning of such observations by satellites in 1978. It caused what is believed to be the largest aerosol disturbance of the stratosphere in the twentieth century, though probably smaller than the disturbances from eruptions of Krakatau in 1883 and Tambora in 1815. Consequently, it was a standout in its climate impact and cooled the Earth's surface for three years following the eruption, by as much as 1.3 degrees at the height of the impact. Sulfur dioxide from the large 1783-1784 Laki fissure eruption in Iceland caused regional cooling of Europe and North America by similar amounts for similar periods of time.

For more information about sulfur in the atmosphere, please see Volcanic Sulfur Aerosols Affect Climate and the Earth's Ozone Layer.

While sulfur dioxide released in contemporary volcanic eruptions has occasionally caused detectable global cooling of the lower atmosphere, the carbon dioxide released in contemporary volcanic eruptions has never caused detectable global warming of the atmosphere. This is probably because the amounts of carbon dioxide released in contemporary volcanism have not been of sufficient magnitude to produce detectable global warming. For example, all studies to date of global volcanic carbon dioxide emissions indicate that present-day subaerial and submarine volcanoes release less than a percent of the carbon dioxide released currently by human activities. While it has been proposed that intense volcanic release of carbon dioxide in the deep geologic past did cause global warming, and possibly some mass extinctions, this is a topic of scientific debate at present.

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Added Oct. 22, 2015

http://www.geology.sdsu.edu/how_volcanoes_work/climate_effects.html

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Volcanic eruptions can enhance global warming by adding CO2 to the atmosphere. However, a far greater amount of CO2 is contributed to the atmosphere by human activities each year than by volcanic eruptions. T.M.Gerlach (1991, American Geophysical Union) notes that human-made CO2 exceeds the estimated global release of CO2 from volcanoes by at least 150 times. The small amount of global warming caused by eruption-generated greenhouse gases is offset by the far greater amount of global cooling caused by eruption-generated particles in the stratosphere (the haze effect). Greenhouse warming of the earth has been particularly evident since 1980. Without the cooling influence of such eruptions as El Chichon (1982) and Mt. Pinatubo (1991), described below, greenhouse warming would have been more pronounced.

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Observational evidence shows a clear correlation between historic eruptions and subsequent years of cold climate conditions. Four well-known historic examples are described below.

LAKI (1783) -- The eastern U.S. recorded the lowest-ever winter average temperature in 1783-84, about 4.8OC below the 225-year average. Europe also experienced an abnormally severe winter. Benjamin Franklin suggested that these cold conditions resulted from the blocking out of sunlight by dust and gases created by the Iceland Laki eruption in 1783. The Laki eruption was the largest outpouring of basalt lava in historic times. Franklin's hypothesis is consistent with modern scientific theory, which suggests that large volumes of SO2 are the main culprit in haze-effect global cooling.

TAMBORA (1815) -- Thirty years later, in 1815, the eruption of Mt. Tambora, Indonesia, resulted in an extremely cold spring and summer in 1816, which became known as the year without a summer. The Tambora eruption is believed to be the largest of the last ten thousand years. New England and Europe were hit exceptionally hard. Snowfalls and frost occurred in June, July and August and all but the hardiest grains were destroyed. Destruction of the corn crop forced farmers to slaughter their animals. Soup kitchens were opened to feed the hungry. Sea ice migrated across Atlantic shipping lanes, and alpine glaciers advanced down mountain slopes to exceptionally low elevations.

KRAKATAU (1883) -- Eruption of the Indonesian volcano Krakatau in August 1883 generated twenty times the volume of tephra released by the 1980 eruption of Mt. St. Helens. Krakatau was the second largest eruption in history, dwarfed only by the eruption of neighboring Tambora in 1815 (see above). For months after the Krakatau eruption, the world experienced unseasonably cool weather, brilliant sunsets, and prolonged twilights due to the spread of aerosols throughout the stratosphere. The brilliant sunsets are typical of atmospheric haze. The unusual and prolonged sunsets generated considerable contemporary debate on their origin.They also provided inspiration for artists who dipicted the vibrant nature of the sunsets in several late 19th-century paintings, two of which are noted here.
For a more thorough description of the 1883 eruption, see Krakatau.

PINATUBO (1991) -- Mt. Pinatubo erupted in the Philippines on June 15, 1991, and one month later Mt. Hudson in southern Chile also erupted. The Pinatubo eruption produced the largest sulfur oxide cloud this century. The combined aerosol plume of Mt. Pinatubo and Mt. Hudson diffused around the globe in a matter of months. The data collected after these eruptions show that mean world temperatures decreased by about 1 degree Centigrade over the subsequent two years. This cooling effect was welcomed by many scientists who saw it as a counter-balance to global warming.

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