Really. When Mount St. Helens started erupting in early October 2004, it was pumping out between 50 and 250 tons a day of sulfur dioxide, the lung-stinging gas that causes acid rain and contributes to haze. In fact, the volcano's emissions were so high that if it were a new factory, it wouldn't have been able to get a permit to operate, according to Clint Bowman, an atmospheric physicist for the Washington Department of Ecology. All of Washington State's industries combined "only" produce about 120 tons a day of the noxious gas.
The 1991 eruption of Mt Pinatubo in the Philippines was one of the largest in the past 100 years. The injection into the stratosphere of 14-26 million tons of sulfur dioxide led to a global cooling of about 1 degree a year after the eruption and probably was the cause of the unusually cool U.S. summer in 1992. The climatic impact of Pinatubo was stronger than the warming effects of either El Niño or human-induced greenhouse gas changes during 1991-93.
The eruption of Tambora in Sumatra in 1815--the most violent eruption in modern history--caused the "Year without a summer" in 1816. It snowed in July in New England that year. The climate of the following three years resulted in the worst famine of the century. Food shortages across Europe causing riots and political change in France.
The eruption of Toba 74,000 years ago (also in Sumatra) caused the extinction of 95% of all life on Earth. It was the largest volcanic eruption within the last two million years, equivalent to about one gigaton of TNT and three thousand times greater than that of Mount St. Helens. Human populations may have been reduced to only a few tens of thousands of individuals as a result. We almost didn't make it!
But don't think volcanoes are the cause of global warming--they actually tend to reduce the temperature. Worldwide, people and their activities pump 26 billion tons of carbon dioxide into the atmosphere. The total from volcanoes is about 200 million tons a year — or less than 1 percent of the man-made emissions.