The SST was seen as particularly offensive due to its sonic boom and the potential for its engine exhaust to damage the ozone layer. Both problems impacted the thinking of lawmakers, and eventually Congress dropped funding for the US SST program in and all overland commercial supersonic flight was banned over the US. Presidential advisor Russell Train warned that a fleet of 500 SSTs flying at for a period of years could raise stratospheric water content by as much as 50% to 100%. According to Train, this could lead to greater ground-level heat and hamper the formation of ozone. In relation to stratospheric water and its potential to increase ground temperatures, although not mentioning Concorde as the source of the "recent decline in water vapor is unknown", in 2010 the National Oceanic and Atmospheric Administration noted that Stratospheric Water Vapor levels in the 1980s and 1990s were higher than that in the 2000s, by approximately 10%, with Susan Solomon of NOAA calculating that it is this change which is responsible for the slow down in the rise in surface temperatures from global warming by about 25 percent when compared to the warming rate in the 1990s. Russell Train's other, water-ozone concern, was however countered by Fred Singer in a letter to the journal "Nature" in 1971, "which upset those who claimed that supersonic transports might seriously affect stratospheric ozone".