Volcanic activity is the cause of global warming

Myth:

Global warming is the result of volcanic activity, not the burning of fossil fuels. 

Fact:

Volcanoes have caused global warming in the past, when they erupted en masse and emitted vast quantities of CO₂, but this has not happened for many millions of years. The level of CO₂ emanating from volcanoes each year now is less than 1 percent of that emitted by humans – not enough to have any impact on temperatures. Moreover, present day carbon emissions contain relatively little of the isotope Carbon-13, meaning they had their origin in plants rather than volcanoes. [Plants preferentially select Carbon-12 rather than Carbon-13 during photosynthesis.]

In fact, volcanic activity in modern times has a cooling effect, not a warming one, as it increases the particulate matter in the atmosphere, thereby increasing albedo (the reflection of the sun’s rays). Major eruptions can reduce the Earth’s temperature by as much as half a degree for a period of several years, but these effects are temporary. The last major eruption that had an impact on global temperatures was that of Mount Pinatubo in 1991. It which cooled the atmosphere by 0.6°C for about 15 months.

. [Source: Skeptical Science]

Sources:

Michon Scott and Rebecca Lindsey, “Which emits more carbon dioxide: volcanoes or human activities?” US National Oceanic and Atmospheric Administration, 15/6/16

“Do volcanoes emit more CO₂ than humans?” Skeptical Science. https://skepticalscience.com/volcanoes-and-global-warming.htm

“Are volcanoes causing global warming?” Paleontological Research Institution. https://earthathome.org/quick-faqs/do-volcanoes-cause-global-warming/

Gregory A. Zielinski, “Use of paleo-records in determining variability within the volcanism–climate system,” Quaternary Science Reviews, 21/4/2000