The consideration for SST's won't be so much environmental concerns as economics. The airlines chose the 787 over the Sonic Cruiser; the Sonic Cruiser would go at Mach .92 for the same cost as the 767, but airlines wanted to cut cost, since the demand for ultra-high speed wasn't great. With 787 technology, we don't need to worry about emissions when going that fast, its all operating costs, seat revenue, and effeciency. Delta has the biggest gas bill this year, over $2.5 Billion. If passengers don't really demand for supersonic travel, Delta, or any airlines, won't go for an SST and raise the gas bill to $3 Billion, they'd rather have it at $1 Billion.