According to this website about volcanoes at the Smithsonian Institute there are about 20 active volcanoes at any moment, and about 50-70 active in any one year. A problem with facts about volcanoes is that they do not operate on a human scale. While humans live for about 80 years, to a volcano that would seem like an instant in time. Geologists frequently talk about volcanic activity in "small" units of 10,000 years. Many volcanoes are millions of years old.
At the website of the San Diego State University Geology department, there is a page about Climate Effects of Volcanoes. Concerning greenhouse gas emissions, they say, "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. Volcanoes contribute about 110 million tons/year, whereas other sources contribute about 10 billion tons/year."
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