2) If underwater volcanoes suddenly decided to erupt at at a higher rate we would have detected it from space via OCO2 and OCO3 and GOSAT gosat.nies.go.jp/en/ which can monitor CO2 outgassing and pinpoint it.
What are the Carbon Markets? 1 Carbon Credit = a digital certificate proving a company or an environmental project prevented the emission (pollution) of 1 ton of CO2eq (carbon dioxide)