What is the impact of CO2? We do not actually know its magnitude (on global temperature). We certainly know that the relationship is not one of direct proportionality. Climate literature shows that its interpretation (in terms of causation) is made exceptionally difficult by many other factors which if given a fair discussion would render virtually every popular account unreadable and most definitely outwith the reach of any rational policy making. Doubters should examine the Vostok core data and temperature data for 1880-2016. For sure, there's a story there but to claim there's a policy directive arising from this data is simply unjustified. To be clear - simplistic denial of <any> link is of course mere ignorance.

Answered in the 2013 IPCC report, on page 1008 (page 56 in the PDF):

FAQ 11.2 | How Do Volcanic Eruptions Affect Climate and Our Ability to Predict Climate?

http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chap...