Regarding the claim that was made against Terrance Gerlach’s volcanic CO2 calculation I notice that link posted in support of it doesn't work. I also notice that it includes the number "1991". If that number is a year, then we should be looking for calculations that are much more recent. https://news.agu.org/press-release/human-activities-emit-way-more-carbon-dioxide-than-do-volcanoes/ has an article dated to June 2011 (though it fortunately a link in that article to a supporting document no longer works). It says in part the following.
'On average, human activities put out in just three to five days the equivalent amount of carbon dioxide that volcanoes produce globally each year. So concludes a scientist who reviewed five published studies of present-day global volcanic carbon dioxide emissions and compared those emissions to anthropogenic (human-induced) carbon dioxide output.
“The most frequent question that I have gotten (and still get), in my 30 some years as a volcanic gas geochemist from the general public and from geoscientists working in fields outside of volcanology, is ‘Do volcanoes emit more carbon dioxide than human activities?’” says Terrance Gerlach of the U.S. Geological Survey. “Research findings indicate unequivocally that the answer to this question is ‘No’—anthropogenic carbon dioxide emissions dwarf global volcanic carbon dioxide emissions.”
Although geoscientists continue in their efforts to improve estimates and reduce uncertainties about how much carbon dioxide is released from mid-ocean ridges, from volcanic arcs, and from hot spot volcanoes, agreement exists among volcanic gas scientists regarding the significantly smaller emissions of volcanic carbon dioxide compared to anthropogenic carbon dioxide.'
I am confident that scientists have a way of measuring how much CO2 is emitted from volcanoes, such as by taking samples at volcanoes to measure the concentration of what is being emitted and the volume of what is being emitted. Likewise at industrial sites and from tail pipes of vehicles the emissions from those locations can be measured right at the source.