It wouldn't happen overnight. But it could happen.

A new report to the US Congress warns one or two nuclear weapons detonated high in the atmosphere above the country would - within a few months and years - lead to the death of 90 per cent of the country's population.

It's a worst-case post-apocalyptic nightmare scenario common to computer games and movies.

Nothing would work anymore.

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Society would collapse.

The rule of the gun will replace the rule of law.

And it's exactly what North Korea has been threatening.

DAY ZERO

Nuclear weapons analyst Dr Peter Pry, who testified before a US Homeland Security committee earlier this month, says his work validates past warnings that an EMP (electromagnetic pulse) attack could immediately wreak nationwide havoc.

It's a side-effect of nuclear warheads that are exploded in the ionosphere - an area high in Earth's atmosphere filled with particles charged by the planet's magnetic field and radiation from the Sun.

A gamma-ray pulse, produced by a nuclear blast, causes the ionosphere to 'short-circuit' with the ground below. And this - like a solar-flare - can bring entire economies to their knees in a few hundred nanoseconds.

The more advanced the economy, the more dramatic the effect.

The nuclear blast itself isn't likely to kill anybody, or irradiate anything.

But up to 500,000 people would be dead within minutes.

That's how many people are in commercial airliners in US skies at any one time.

With their avionics fried, they'd fall from the sky.

High-speed rail vehicles would cease to be guided. Cars would stall.

The lights would go out and computers would die - and so would every piece of medical equipment in a line-of-sight of the nuclear fireball, high in the sky.

Then there would be the fires. Gas pipelines feeding cities and industry could ignite from fried control systems, unleashing widespread firestorms.

How many would die in the first day is anyone's guess.

But that would just be the beginning of the end.

Our civilisation is only as resilient as our power grid, insists a US nuclear warfare expert. Photo / 123RF
Our civilisation is only as resilient as our power grid, insists a US nuclear warfare expert. Photo / 123RF

AN INDIRECT KILLER

Dr Pry, a strident critic of the Obama administration and a long-term advocate of the EMP threat, insists it's disturbingly easy to send a nation back to the stone age.

"Rogue states or terrorists can blackout national electric grids and other life-sustaining critical infrastructures, topple electronic civilisation, and kill millions from sea to shining sea, with a single high-altitude nuclear detonation, generating an EMP field covering North America," he says in a commentary published by The Washington Times.

It's actually easier than directing a nuclear warhead into the heart of Washington itself.
All it would take, he says, is for something like a timer, a GPS receiver or an altimeter to be paired with a nuclear warhead on a ballistic missile.

Once the warhead reached an altitude of 300km, its pulse could fry the electrical grid of all 48 mainland US states.

"The US can sustain a population of 320 million people only because of modern technology," Dr Pry told Forbes. "An EMP that blacks-out the electric grid for a year would remove the critical infrastructure necessary to support such a large population."

But is such an attack likely? Or even possible?

Or is it another scare campaign?

Nuclear weapons experts are split.

Some argue even this task would be at the extreme edge of North Korea's capabilities.

The missile must deliver its warhead - intact - to the correct altitude, and in roughly the right place, to have its desired effect.

And North Korea hasn't tested anything offering such capability. Yet.

Nor is there any evidence it has the technology to optimise the gamma-ray output of a nuclear device.

DAY THREE

The United States would still be at a standstill.

Few would know what was going on.

But the population would begin to panic.

Government would have already devolved to local councils and sheriff's offices. The National Guard - and private militias - would be milling aimlessly about in the streets.

The only people with power would be those with generators that survived the pulse.

The water supply would be drying up everywhere from country towns to the most major cities. There'd be nothing to pump it through the system.

The fresh food in grocery stores - if it hadn't already been eaten - would have begun to go off.

No replacements stocks would be being trucked in. The vehicles wouldn't be working, and even if they did, none of the computer and communications systems for distributing and paying for it would be active.

In just a few more days, nuclear reactors - their control and safety systems dead - would begin to melt down.

The war of words between Donald Trump and Kim Jong-Un has led to fears of all-out nuclear war. Photo / AP
The war of words between Donald Trump and Kim Jong-Un has led to fears of all-out nuclear war. Photo / AP

THE WEAKEST LINK

The electromagnetic pulse between the ionosphere and the Earth's surface - like a solar storm - would pump an enormous excess of energy into exposed power lines and into the national power grid.

This could melt thousands of 100-ton, extra-high voltage transformers.
These complicated pieces of power-regulating equipment are difficult and slow to assemble, and even more difficult to put in place.

Replacing even a handful is an enormous job, taking months and even years. Thousands would be virtually impossible.

And building protective fail-safes (which would allow the current to be routed around the transformer) into existing systems would be exorbitantly expensive.

Dr Pry, who says he has worked with the CIA, bemoans the closure of the Pry Congressional EMP Commission, of which he was Chief of Staff.

"The Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack terminated on September 30, ironically, the same month North Korea tested an H-bomb it described as 'a multifunctional thermonuclear nuke with great destructive power which can be detonated even at high altitudes for superpowerful EMP attack'," he says .

"(The Depeartment of Defence) is letting DOE (Department of Energy) waste millions of dollars on unnecessary studies while DOD sits on a mountain of classified studies proving the EMP threat is real - which is why DOD has spent billions EMP hardening military systems."

YEAR ONE

Dr Pry says the simple scale of such a disaster would overwhelm the world's ability to respond.

Within a year, 90 per cent of the 300 million people living in the 48 adjoining US states could be dead, he warns.

Small pockets of survivors would be scattered across the country.

Disease would be rampant. There would not have been enough living to bury the dead.

Water supplies would be contaminated. Most livestock would be dead through a lack of care - and poaching.

The only food would be what can be grown, and hunted, by survivors.

There would be little fuel. Little communication.

The only hope would come from international relief efforts.

IS IT REAL?

Dr Pry insists there is ample evidence the alarming apocalyptic scenario painted above is a very real possibility.

"A protracted blackout endangering millions will be the inevitable result of the EMP attack described by the North Koreans," he says.

Kim Jong-un certainly appears to believe so.

His state-controlled news agencies have repeatedly referenced EMP in its threats to the United States.

It's also a threat long associated with Russian and Chinese nuclear arsenals.

"The empirical basis for the EMP threat to electric grids and civilisation is far deeper and broader than for cyber-attacks or sabotage," Dr Pry says. "We know for certain EMP will damage electronics and cause protracted blackout of unprotected electric grids and other critical infrastructures."

In his testimony to the Homeland Security, Dr Pry details several decades worth of nuclear testing as the basis of his fears:

• The 1962 US Starfish Prime high-altitude nuclear test that generated an EMP field over the Hawaiian Islands, over 1300 kilometres away.
• Six Russian high altitude nuclear tests in 1961-1962 over Kazakhstan.
• 30 years (1962-1992) of US underground nuclear testing which revealed data about gamma ray output, and other effects.
• Over 50 years of testing using EMP simulators, including tests by the EMP Commission (2001-2008), proving modern semiconductor electronics are far more vulnerable to EMP than 1950-60s era electronics.
Dr Pry also highlights historic power-grid failures which he says can be seen as indicators for a much larger event:
• The Great Northeast Blackout of 2003 - which put 50 million people in the dark for a day, contributed to at least 11 deaths and cost an estimated $6 billion - happened when a power line was damaged by a tree branch.
• The New York City Blackout of 1977, that resulted in the arrest of 4500 looters and injury of 550 police officers, was caused by a lightning strike on a substation that tripped two circuit breakers.
• The Great Northeast Blackout of 1965, that affected 30 million people, happened because a protective relay on a transmission line was improperly set.
• India's nationwide blackout of July 30-31, 2012 - the largest blackout in history, effecting 670 million people, 9 per cent of the world population - was caused by overload of a single high-voltage power line.