Some climate change denial groups say that because CO 2 is only a trace gas in the atmosphere (roughly 400ppm, or 0.04%) 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.[111] The scientific consensus, as summarized by the IPCC's fourth assessment report, the U.S. Geological Survey, and other reports, is that human activity is the leading cause of climate change.