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 F at the height of the impact.
The large 1783-1784 Laki fissure eruption in Iceland released a staggering amount more sulfur dioxide than Pinatubo (approximately 120-million ton vs. 20). Although the two eruptions were significantly different in length and style, the added atmospheric SO2 caused regional cooling of Europe and North America by similar amounts for similar periods of time.
Carbon dioxide (CO2) is a greenhouse gas and is the primary gas blamed for climate change. 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. In 2015, human activities were responsible for a projected 32.3 billion metric tons (gigatons) of CO2 emissions. 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.
Published scientific estimates of the global CO2 emission rate for all degassing subaerial (on land) and submarine volcanoes lie in a range from 0.13 gigaton to 0.44 gigaton per year. The 32.3-gigaton projected anthropogenic CO2 emission for 2015 is about 73 to 248 times larger than the respective maximum and minimum annual global volcanic CO2 emission estimates.
There is no question that very large volcanic eruptions can inject significant amounts of carbon dioxide into the atmosphere. The 1980 eruption of Mount St. Helens vented approximately 10 million tons of CO2 into the atmosphere in only 9 hours. However, it currently takes humanity only 2.5 hours to put out the same amount. While large explosive eruptions like this are rare and only occur globally every 10 years or so, humanity's emissions are ceaseless and increasing every year.
There continues to be efforts to reduce uncertainties and improve estimates of present-day global volcanic CO2 emissions, but there is little doubt among volcanic gas scientists that the anthropogenic CO2 emissions dwarf global volcanic CO2 emissions.
For additional information about this subject, please read the American Geophysical Union's Eos article "Volcanic Versus Anthropogenic Carbon Dioxide" written by USGS scientist Terrence M. Gerlach.
Yearly CO2 emitters
||Billion metric tons per year (Gt/y)|
|Global volcanic emissions (highest preferred estimate)||0.26|
|Anthropogenic CO2 from fuel combustion 2015+||32.3|
|Worldwide Road Transportation 2015+||5.8|
|Approximately 24 1000-megawatt coal-fired power stations *||0.22|
|United States 2015+||  4.99|
CO2 emission events
|Mount St. Helens, 18 May 1980||0.01 Gt|
|Mount Pinatubo, 15 June 1991||0.05 Gt|
|Number of Pinatubo-equivalent eruptions equal to 2015 global anthropogenic CO2||646|
|Number of Mount St. Helens-equivalent eruptions equal to 2015 global anthropogenic CO2||3230|
2015 global anthropogenic CO2 multiplier (ACM)**
|Number of days for anthropogenic CO2 to equal a year's worth of global volcanism||2.9|