The extent of the economic damage can be estimated in one massive sum: $1 trillion. That’s according to a recent study by specialist insurance company Lloyd’s and University of Cambridge’s Centre for Risk Studies.

The report looked at two scenarios. One where a group of terrorists or “disgruntled insider” hackers break into the power system and bring 50 of the almost 700 generators in the northeastern U.S. offline, resulting in a blackout that lasts about 4 days. The damage: $243 billion in immediate and tangential economic loss.

The second scenario is where things get even worse. A group of hackers target the U.S. power grid and take out twice as many generators for the same amount of time. The economic damage more than quadruples to $1 trillion, or about 6% of U.S. GDP.

“This scenario shows the huge impact and havoc that could result from a major cyber attack on the U.S.,” said Tom Bolt, director of performance management at Lloyd’s. “The reality is that the modern, digital, and interconnected world creates the conditions for significant damage.”

Such a blackout would reach across 15 states, including New York City and Washington D.C., leaving 93 million people without power. The damage includes factors like a rise in mortality rates as health and safety systems fail, a decline in trade as ports shut down and transportation chaos as infrastructure collapses.

While such an attack is technologically possible, noted Lloyd’s, it isn’t necessarily likely. The report sets the “benchmark return period,” or an estimate of the likelihood of such an event, at 1:200. Lloyd’s, which is in the business of insurance, warns that the industry could lose upwards of $71 billion from such an attack.

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