John Cook says that volcanoes produce harmless amounts of CO2 :
Published reviews of the scientific literature by Moerner and Etiope (2002) and Kerrick (2001) report a minimum-maximum range of emission of 65 to 319 million tonnes of CO2 per year. Counter claims that volcanoes, especially submarine volcanoes, produce vastly greater amounts of CO2 than these estimates are not supported by any papers published by the scientists who study the subject.
http://www.skepticalscience.com/volcanoes-and-global-warming.htm
Then today, we hear that CO2 from volcanoes caused mass extinctions :
Professor Jim Zachos, of the University of California, said that 55 million years ago volcanic activity caused around 4,500 gigatons of greenhouse gases to be released into the atmosphere over thousands of years. This caused the planet to warm by 6C (10.8F), forcing whole ecosystems, including early mammals, to adapt, migrate or die out in certain areas.
The numbers are not incompatible between the two authors. At 319 Million tons per year, it would take 14,000 years to emit 4,500 gigatons. What is different is how the same message is spun.
Alarmists take the same numbers to generate opposite conclusions when convenient. When it suits the argument, volcanoes are harmless producers of CO2. Otherwise, volcanoes are deadly producers of CO2.
How is it that the same amount of CO2 emissions can be typical equilibrium conditions, or wildly catastrophic and requiring hundreds of thousands of years to recover?