Nouriel Roubini gives an audience no shortage of scenarios to keep them up at night, but his number one worry right now is the looming threat of Iran building nuclear weapons. Specifically, it’s the risk of a confrontation between Israel and Iran, or the United States and Iran, Roubini told Michael Milken in front of hundreds of onlookers at the Milken Institute Global Conference in Los Angeles Wednesday. Roubini, a notorious doomsayer and chairman of Roubini Global Economics, expects both Iran’s push into nuclear weapons and the pushback from Iran and Israel to ramp up in the second half of 2012.