Volcanoes
Volcanic action belches more gasses in a second than all of mankind could make in a year.
It's not even close. In an average year all volcanoes together emit about 1% of the CO2 that human emissions do. Even the biggest eruption in the past century, Pinatubo in 1991 only emitted around 42 million tonnes of CO2 compared to 23 billion tonnes by humans in that year. Lately human emissions have been hovering around 30 billion tonnes/year.