Some climate change denial groups say that because is only a trace gas in the atmosphere (roughly 400ppm, or 0.04%, 4 parts per 10,000) it can only have a minor effect on the climate. Scientists have known for over a century that even this small proportion has a significant warming effect, and doubling the proportion leads to a large temperature increase. The scientific consensus, as summarized by the IPCC fourth assessment report, the U.S. Geological Survey, and other reports, is that human activity is the leading cause of climate change. The burning of fossil fuels accounts for around 30 billion tons of each year, which is 130 times the amount produced by volcanoes. Some groups allege that water vapor is a more significant greenhouse gas, and is left out of many climate models. While water vapor is a greenhouse gas, the very short atmospheric lifetime of water vapor (about 10 days) compared that of (hundreds of years) means that is the primary driver of increasing temperatures; water vapour acts as a feedback, not a forcing, mechanism. Water vapor has been incorporated into climate models since their inception in the late 1800s.