Smoke from burning cities would release 16 to 36 teragrams of black carbon into the atmosphere, blocking out sunlight and cooling the global surface.

A world without nuclear weapons, widely considered a desirable if challenging goal over the long term, could pose new uncertainties and risks, experts from the United States and Japan said during a day-long symposium at AAAS.

The legal framework does not yet exist that would allow the pursuit of a nuclear-free world, they said. Nor has there been enough clear-headed analysis of the potentially de-stabilizing impact of a world with zero nuclear weapons where terrorists or rogue states might not play by the rules.