Coal plants are shutting down because of a lack of cooling water. Hydropower dams are struggling to generate electricity because reservoir levels are dropping. Western wildfires are damaging power lines, causing blackouts in cities like San Diego.

A recreational boat cruises Lake Mead, the reservoir created by the Hoover Dam. (Jonathan Gibby/For the Washington Post)

There are all sorts of ways that nature can wreak havoc on the U.S. energy system, making it difficult to keep the lights on. But as humans heat up the planet, these sorts of disruptions are likely to become even more common in the decades ahead.

That's the upshot of a big new assessment (pdf) from the Department of Energy, which argues that large swaths of America's aging energy infrastructure — from nuclear reactors and barges transporting coal to oil rigs and power lines — are at risk from the effects of global warming. Heat waves, droughts, flooding, and wildfires could all put a heavy strain on key energy services in the decades ahead, costing Americans billions.

We don't even have to speculate to see the impacts. The report notes that dozens of weather events in recent years have shown how vulnerable the energy sector is to even a moderately hotter climate (the United States has warmed about 1.5°F over the past century):

The full key for the map is above on page 2 of the report, but I'll draw out a few highlights:

(4) - In the summer of 2011, Texas had to declare several power emergencies after a record drought and a series of multiple 100-degree days forced a number of power plants to shut down due to a shortage of cooling water.

(7) - In the fall of 2007, wildfires in the Southwest damaged more than two dozen transmission lines, causing nearly 80,000 people in San Diego to lose power, some for weeks. Many climate models expect Southwest wildfires to become bigger and more severe as the planet warms.

(13) - In the summer of 2012, reduced snowpack in the Sierra Nevada mountains caused California's hydroelectric power generation capacity to drop by about 8 percent. Climate models suggest that snowpack will shrivel further as the Earth warms.

(28) - In May of 2011, nearly 20 percent of barge terminals along the Ohio River had to close because of flooding, which made it harder to transport coal and petroleum.

That's just the past decade. The report warns that many more aspects of the energy system could be seriously affected if climate change does, in fact, bring the impacts that many scientists expect. A few examples:

-- Many coal, natural gas, and nuclear power plants will have to shutdown partially or fully during the summer if the air gets hotter and certain rivers start to warm, since they won't be able to cool as efficiently.

-- If climate change does bring more frequent coastal storms or hurricanes — and this is still being debated — that could hamper oil and gas production in places like the Gulf of Mexico.

-- If the air heats up, then transmission lines in the electric grid won't be able to carry as much current and will operate less efficiently. Solar panels will be able to squeeze out less power if ambient air temperatures rise.

-- Gas fracking operations in places like Texas could face restrictions if water becomes scarce.

-- Oil and gas operations in Alaska may prove vulnerable to melting permafrost, which could damage existing infrastructure. But that said, it's certainly not all grim news for the oil industry: The report notes that offshore drilling operations will likely expand if the Arctic ice recedes.

There's also a separate, but related consideration: If the United States keeps getting warmer, then many Americans will use air conditioning more often. Combine that with the risk of more frequent power-plant interruptions, and the Western United States will need an additional 34 gigawatts of generation capacity by 2050 to keep the lights on, according to a study by Argonne National Lab. That will cost consumers some $45 billion. (On the flip side, some parts of the United States will have fewer heating needs in the winter, particularly the Northeast.)

Now, the DOE report does suggest that there are ways to adapt to many of these changes. Transmission lines can be hardened against wildfires. Fracking firms can pursue more water-efficient ways to drill for natural gas. Hydroelectric dams facing lower reservoir levels can install more efficient turbines. But this all costs extra.

The report also stresses that utilities and other energy companies will likely need to take many of these adaptation steps regardless — even if humans do succeed in curbing emissions and slow the pace of global warming. That's because some of the expected impacts of climate change are already locked in for the years ahead, based on the greenhouse gases we've already put into the atmosphere.