The Federal Reserve Bank of Atlanta eyed up an extreme but perhaps not unlikely hypothetical scenario this week: What would happen to the U.S. economy if a "massive geomagnetic solar storm" crippled the domestic power grid, potentially for a "long, long time"?

The answer: nothing good.

"One could anticipate serious disruption of electronic payments such as [automated clearing house transactions], cards and wire transfers in the affected areas and beyond," Steven Cordray, a payments risk expert at the Atlanta Fed, wrote in a blog post Monday.

That means electronic payment systems in general – including those involving credit and debit cards – would be compromised in the event of a solar storm. No electronic money transfers, no online banking – possibly no "online," period.

Solar storms occur when solar wind from the sun wreaks havoc on Earth's magnetosphere. Assessing exactly what a big solar storm would do to life as we know it is complicated and depends on how severe the hypothetical storm is.

But a big one wouldn't be a walk in the park. On a basic level, a large solar storm could effectively knock out telecommunications channels and disrupt computer networks, navigation systems and other electronic devices.

These storms aren't nearly as predictable nor as regular as weather events like hurricanes, but scientists would have a few days' – or, in a worst-case scenario, a few hours' – notice before one strikes. That said, a 2011 Department of Homeland Security report noted that the U.S. government "lacks comprehensive, national-level geomagnetic storm risk management assessments and strategies" for a large-scale storm that could, for example, knock out power "along the entire Eastern Seaboard."

Perhaps to remedy that, the White House last year released a National Space Weather Strategy and National Space Weather Action Plan to "clearly articulate how the federal government will work to enhance national space-weather preparedness."

It's worth noting, though, that a major solar storm hasn't made its way to Earth since 1859, in an occurrence referred to today as the Carrington Event. The world in the late 1850s wasn't exactly the technological hub it is today, so damage was minimal, relatively speaking. But there were still some potentially concerning consequences.

"Telegraph lines sparked, and telegraph operators could send and receive messages without the use of electric batteries. The Northern Lights lit up the sky in all of North America," Cordray wrote. "One report estimates that a Carrington-level storm today could result in power outages affecting as many as 20 to 40 million Americans for a duration ranging from 16 days to two years at an economic cost of up to $2.5 trillion."

The report Cordray referenced was published in 2013 by the London-based insurance hub Lloyd's and offers a sobering assessment of what a large-scale solar storm could do to the U.S. economy. America's 10 largest cities – led by New York, Los Angeles and Chicago – have a combined population of nearly 25.6 million people, according to the Census Bureau. So a worst-case-scenario solar storm could knock out power to nearly twice that group.

"Weighted by population, the highest risk of storm-induced power outages in the U.S. is along the Atlantic corridor between Washington, D.C., and New York City," the Lloyd's report said. "The duration of outages will depend largely on the availability of spare replacement transformers. If new transformers need to be ordered, the lead time is likely to be a minimum of five months."

Commerce as we know it, especially in the digital sphere, could largely grind to a halt. So how would people get food, gasoline and other essentials they'd need to survive for months without power – and potentially months without the ability to use a credit card?

Cordray predicted we'd still be able to make purchases, but we'd have to do it the old-fashioned way – with cash.

"Cash would have fewer problems if it were on hand to distribute to the affected population. Perhaps cash, accompanied by ration books, could be used to mitigate hoarding," he wrote. "Cash solves the problem for small-scale, low-value payments during a long-term power outage."

Cordray also joked that he plans on "going to the bank to cash out [his] accounts and then on to the hardware store to buy a gas-powered electric generator." But how concerned should the world be about a potentially crippling solar storm? Is this some imminent threat that people should be aware of?

Maybe, Cordray said, citing another research paper published in the journal Space Weather that predicted there's about a 12 percent chance of "another Carrington event" occurring by 2022.

The Lloyd's study offers an even bleaker assessment, claiming such a storm is "almost inevitable in the future." The paper indicates Carrington-level events are thought to take place roughly every 150 years.

The last one was observed around 157 years ago.