The Department of Energy has a new report (pdf) out taking a look at the vulnerability of the U.S. power grid to blackouts. One notable fact: Outages caused by severe weather have become much more common over the past decade:

Power outages of all sorts are becoming more frequent since 1992, but the real action has come from weather-related outages.

The report notes that "thunderstorms, hurricanes and blizzards account for 58 percent of outages observed since 2002 and 87 percent of outages affecting 50,000 or more customers." (The rest are caused by things like "operational failures, equipment malfunctions, circuit overloads, vehicle accidents, fuel supply deficiencies and load shedding — which occurs when the grid is intentionally shut down to contain the spread of an ongoing power outage.")

Since the big uptick in 2003, the report estimates, weather-related outages have cost an average of about $18 billion to $33 billion per year, adjusted for inflation. (Obviously in some years, like last year with superstorm Sandy, the costs are much higher.)

It's not entirely clear from the chart why these sorts of blackouts are becoming more common. Is severe weather on the rise? Or is the grid getting older and becoming more vulnerable to storms? There's certainly some evidence in favor of the second hypothesis. Here's a useful look at when various U.S. transmission lines were built:

Much of the existing power grid was built in the 1970s and 1980s — and there's been a slowdown in construction since then. As a result: "Seventy percent of the grid’s transmission lines and power transformers are now over 25 years old." As the dashed lines show, there are plans for upgrades in the future, but those haven't happened yet.

The rest of the Energy Department report lays out the case for modernizing the grid and making it more resilient in the face of storms—that involves everything from "smart-grid" technology to replacing wooden poles with concrete or steel structures to elevating substations in flood-prone areas. (The report notes that burying power lines underground, a favorite topic of discussion, represents "significant challenges, including additional repair time and much higher installation and repair cost.")

That certainly won't be cheap. But the report argues that in certain parts of the United States — particularly in places where severe weather events are expected to become more common as a result of climate change — grid upgrades could avoid even higher costs over the long run.