Anti-nuclear campaigners have marked the 64th anniversary of the world's first atomic attack with a plea for Australia to reconsider its uranium exports.
A single nuclear bomb dropped by the US on the Japanese city of Hiroshima on August 6, 1945, killed about 140,000 people.
They died instantly or in the days and weeks that followed as radiation and horrific burns took their toll.
Three days later, the US dropped a second nuclear bomb, on Nagasaki. It killed 70,000 people.
Japan surrendered on August 15, ending World War II.
Steven Starr, senior scientist with Physicians for Social Responsibility in the US, and the Sydney-based People for Nuclear Disarmament have lobbied the UN to pass a resolution eliminating high-alert nuclear weapons, which can be launched in two to three minutes.
Mr Starr hopes the resolution will help prevent potential attacks caused by error and miscalculation.
"The presidents (of Russia and the US) are followed around by guys with briefcases ... they're a way to transmit a launch order in 30 seconds. (The cases) are hidden away. People don't know they exist," Mr Starr said.
Mr Starr said a nuclear missile could travel between Russia and the US or vice versa within 30 minutes.
He said accidental nuclear war was a real threat to the human race.
"The US and Russia now have 900 missiles armed with 220 strategic nuclear warheads on high alert which can be launched with only a few minutes' warning," he told a forum at NSW parliament marking Hiroshima Day.
"The detonation of 4,440 of these warheads ... would cause up to 180 tonnes of soot from burning cities to rise right into the stratosphere.
"The smoke would block about 70 per cent of the sunlight in the northern hemisphere and 35 per cent of the sunlight in the southern hemisphere."
He warned that the nuclear "night" would trigger a global ice age, destroying agricultural regions, resulting in worldwide starvation on an unimaginable scale.
People for Nuclear Disarmament spokesman John Hallam told the forum Australia's good work pushing for nuclear non-proliferation was being undermined by its uranium exports, which, he said, should be reconsidered.
Mr Hallam played down the North Korea nuclear threat.
"North Korea has 10 to 12 warheads, the United States has 10,000-plus and it's the United States and Russia that have the capability to make the planet uninhabitable in roughly 40 minutes," he said.
In the UN General Assembly last year Australia voted for a resolution to eliminate high-alert nuclear weapons.
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