The burning of fossil fuels accounts for around 30 billion tons of CO 2 each year, which is 130 times the amount produced by volcanoes.[112] Some groups allege that water vapor is a more significant greenhouse gas, and is left out of many climate models.[111] While water vapor is a greenhouse gas, the very short atmospheric lifetime of water vapor (about 10 days) compared that of CO 2 (hundreds of years) means that CO 2 is the primary driver of increasing temperatures; water vapour acts as a feedback, not a forcing.[113] Water vapor has been incorporated into climate models since their inception in the late 1800s.[114]