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DENVER -- It's a real-life threat no one wants to imagine, but one that many acknowledge is very real.

An electromagnetic pulse is a short burst of electromagnetic energy that could shut down the nation's power grid in a matter of seconds.

The threat is so real there is a US Congressional EMP Leadership Caucus sponsored group called The EMP Task Force on National and Homeland Security studying the problem. Glenn Rhoades is the Colorado coordinator of that task force.

"It would be so subtle, so silent, you would not know what was going on. It would just be a click. With an EMP device you wouldn't hear any noise," he said.

Rhoades said most experts believe it's a matter of when not if, and he believes it's an event that could ultimately kill more than 200 million Americans.

An EMP threat could come from several sources. A solar storm could hit Earth, overwhelming the power grid. North Korea could drop a bomb on the grid or a cyberterrorist could hack into the system and shut down the grid.

Even more worrisome? Rhoades said the country is not prepared to respond to an EMP event.

"Everything that has electricity and uses electricity can and will be affected," he said. "Unfortunately, we'd have mass casualties within a few months, kind of like what happened with Hurricane Katrina at the Superdome, only on a massive scale."

Rhoades said an EMP event would be more devastating because it would be difficult for first responders and others to help because most all communications systems would be down. He said repairing and rebuilding the grid could take years without electricity to fix and transport it.

He said food and water would be affected, but sewage would perhaps be the biggest concern.

"Once the sewage pumps start failing, your neighbors' sewage would start coming into your house and you wouldn't be able to shelter in place," he said. "Most people would die from diphtheria, cholera and typhoid, water-related diseases."

Howard Singer is a physicist at the National Oceanic and Atmospheric Administration in Boulder.

He and his colleagues keep a constant eye on solar storms and are prepared to issue warnings about EMPs that would give electric utilities at least a few hours to react and possibly reconfigure their transmitters to lessen the impact.

He said there have been a few close calls over the years. In 1859, a large solar storm hit Earth, knocking out telegraph lines. Scientists call it the Carrington Event, one of the largest geomagnetic storms on record. But it hit long before electricity was more widespread so the impact was minimal.

But in 2012, Singer said a similar bullet was dodged when a storm narrowly missed Earth.

“If it had happened nine days earlier that would have been aimed right at Earth and we would have felt the full impact of the storm," he said.

That full impact is hard to imagine, although a partial shutdown of the grid has occurred.

In 2003, New York and much of the Northeast were paralyzed for days by a sudden blackout that stopped 50 million people in their tracks. Trains stopped, computers crashed and traffic lights went out.

The culprit was overloaded transmission lines that brushed up against some foliage.

"This is going to be an interesting lesson for our country and we’ll have to respond to it," said then-President George W. Bush.

But 13 years later, few cities or states have done anything. Colorado Springs is one of the few exceptions.

El Paso County Commissioner Peggy Littleton has created a network of communication hubs where people could gather if the grid were to shut down. Those hubs would be used to disseminate information to the public and first responders.

"As we know, the most critical thing people want to know during a crisis is information, right?" she said. "This is obviously not an easy thing to move forward because it’s not a sexy subject. People are not wanting to think about the unthinkable."

The EMP Task Force on National and Homeland Security estimates there is about a 12 percent chance of a massive EMP event every decade, yet Colorado's Emergency Response Guide includes no plan on how to react.

"Do you know we have nine pages devoted to a deep space asteroid hitting Colorado? Yet we have nothing for an extended lights out," Rhoades said.

Rhoades believes the power grid would be shut down for a lengthy period of time, and he believes the threat is something Colorado's electric utilities need to address.

"Xcel Energy is committed to protecting the electric power grid and ensuring a reliable supply of energy for the communities we serve," Xcel Energy Executive Vice President Marvin McDaniel Jr. said in a statement.

"Maintaining physical and cyber security is a complex task that requires constant vigilance, and XCEL Energy works closely with federal and state officials, as well as other utilities to maintain and enhance critical infrastructure and cyber security protections and to prepare our system to recover quickly from any reliability problems we may experience."