Natural gas emits just half the greenhouse gases per BTU emitted from burning coal. The largest portion of America's electricity is by burning coal.
Between 7 percent and 8 percent of energy is lost in getting it from external power plants to customers, says Durst, so the less power plant electricity a building has to buy, the less greenhouse gas emissions it is probably supporting.
Fox and Durst say they even found a new way to make the building's concrete 10 percent stronger.
They're making the building's concrete with a 45 percent admixture of blast furnace slag: a glassy material left over after the smelting of ore.
Normally, slag piles up as industrial trash.
When they found they could use slag as a substitute for nearly half of the building's concrete, Fox explains, they knew they had discovered yet another green advantage in the fight against global warming:
"Cement manufacturing produces massive greenhouse gas emissions. Something like 8 percent of global CO2 emissions come from cement factories," he said.
Fox and Durst know that this one building cannot by itself make any big difference, but they hope it can help lead the way, inspiring other builders.
The two are hopeful that when the tower opens for business in 2008 it will be "carbon neutral" -- the gold standard that must be met, say scientists, for any building that wants to be part of solution for global warming,
"It's not a simple calculation," Durst said. "We can't be sure yet that we're going to be completely carbon neutral, but we're working on it."