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Last December, four U.S. scientists published a paper in the Journal Chaos entitled Critical points and transitions in an electric power transmission model for cascading failure blackouts. "Detailed analysis of large blackouts has shown that they involve cascading events in which a triggering failure produces a sequence of secondary failures that lead to blackout of a large area of the grid," the authors found. They presciently concluded that "large blackouts are much more likely than might be expected from [conventional statistical analysis]" and are "suggestive of a complex system operating close to a critical point."

At 4:10pm on Thursday, Ontario and seven states hit that "critical point." Within seconds, workers in New York City, Toronto and thousands of other communities found themselves staring at blank computer screens. Many were forced to walk home in sticky weather -- generally to dark, uncomfortably hot homes. Some are still without power as of Saturday morning. Their only consolation is that the biggest power outage in North American history evidently had nothing to do with terrorism.

At first, Ottawa claimed the cause of the blackout was a lightning strike at a power generation plant on the U.S. side of Niagara Falls. New York state officials, on the other hand, blamed the problem on a transmission problem in Ontario, as did New York City Mayor Michael Bloomberg. Then yesterday, the North American Electric Reliability Council, which oversees the continent's transmission grid, reported the culprit was "a nine-second event," in which the current on a set of high-voltage lines in Ohio reversed directions. That news no doubt pleased Ontario Premier Ernie Eves. His government's handling of energy issues has been a source of controversy for years. And a finding that the blackout originated in Ontario would have hurt him in the upcoming election.

But the question of where the crisis originated is not as interesting as why it mushroomed the way it did. On both sides of the border, technical glitches such as fires and maintenance problems happen all the time. Sometimes, this causes people to lose power: In the last three decades, blackouts occurred in North America at the rate of more than one every two weeks. But in almost every case, the damage was contained locally. On Thursday, however, a single breakdown centered somewhere in the Great Lakes area spread to about 25-million people.

In California, Ontario and elsewhere, much attention has lately been focussed on a shortage of electricity generation. But Thursday's crisis seems to have had little to do with this. Dozens of plants did shut down in Ontario and New York State -- but only because the transmission grid into which they usually feed their power had been overloaded. "To supply power to everyone, you have to do two things: You have to generate it, and you've also got to transmit it," says University of Wisconsin scholar Ian Dobson, who co-authored the above-cited article in Chaos. "I think, [on Thursday] there was plenty of generation available. The problem was 'cascading failures' in the transmission system."

The problem of cascading failures is not acute in transmission systems with plenty of slack built in: If one line or substation goes down, power is simply shifted to another part of the grid, and all the end-users experience is an imperceptible flicker in their lighting. But when that other part of the grid is operating near capacity itself, then it may fail too -- and so on, until the entire system suffers catastrophic collapse.

Although no one could have predicted when or where Thursday's crisis would hit, the problem has been building for decades. "The transmission grid has simply not been substantially improved for 20 or 30 years," says Oak Ridge National Laboratory expert Benjamin A. Carreras, one of the Chaos article co-authors. "[U.S.] power consumption increases at 2% per year. The transmission isn't keeping up. There may be improvements in existing lines, but these are relatively small ... Every company has their own interests. Since deregulation, everybody wants to hook up to the existing system to sell their electricity, but no one wants to pay for upgrades."

Ontario, too, has neglected its transmission system. As National Post reporter Paul Vieira notes in today's edition, the old Ontario Hydro monopoly was known to divert funds earmarked for transmission to other projects, including nuclear power.

Over coming weeks, there will be plenty of finger-pointing among politicians. But the increasing congestion of North America's electrical grid is a problem that goes back many years, and impugns Canadian and U.S. governments both. Avoiding another massive blackout will require that all sides co-operate to expand an electrical delivery system that it clearly straining under the load.