Often repeated but definitely incorrect. A single large volcano (Pinatubo) produced about 42 Mt of CO2. see http://pubs.usgs.gov/pinatubo/gerlach/ In contrast man produces 82 Mt a day (about 30Gt a year)
Regardless of the bare numbers, the burden of proof is on the AGW adherents that a particular storm or warming/cooling trend is a result of human activity vs. naturally caused. The trouble is that to them, the lack of storms or sudden changes is also a sign of AGW. In fact, anything that happens is a sign of AGW. That is not proper science because for every theory, there must be a way to support as well as disprove it. For AGW there is only support no matter what the evidence is. That’s how you can tell bad science.