According to many Skeptics, volcanoes release more Carbon Dioxide into the atmosphere than humans therefore, any human release of the gas is trivial and does not contribute to Global Warming. However, according to the United States Geological Survey (USGS), humans currently release 100 times more CO2 than volcanoes. They state:
Do the Earth’s volcanoes emit more CO2 than human activities? No.
…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.
Figure 1: The proportion of volcanic emissions of CO2 (Small gray illustration) compared to human emissions (Large gray illustration). Courtesy of Skeptical Science.
Also, in a detailed scientific paper by the American Geophysical Union, we read:
Which emits more carbon dioxide (CO2): Earth’s volcanoes or human activities? Research findings indicate unequivocally that the answer to this frequently asked question is human activities. However, most people, including some Earth scientists working in fields outside volcanology, are surprised by this answer. The climate change debate has revived and reinforced the belief, widespread among climate skeptics, that volcanoes emit more CO2 than human activities [Gerlach, 2010; Plimer, 2009]. – EOS, Transactions, American Geophysical Union, Volume 92, Number 24, June 14, 2011 Pages 201-208
See also this article in Scientific American.