Advocates for nuclear power may think that they've found an abundant source of carbon-free energy. But have they considered the carbon emissions that would result from the burning of the world's cities if, say, the expanded use of nuclear power led to increased nuclear-weapons proliferation and, hence, to nuclear war? That's just one of many possibilities that Stanford's Mark Jacobson deals with in a new paper (mentioned at ClimateProgress earlier this week) that tries to parse out the full life-cycle carbon footprint of each supposedly carbon-neutral energy source. Coal plants that sequester their carbon end up having the biggest carbon footprint, mostly because not all the carbon-dioxide gets captured before it enters the atmosphere. Nuclear power turns out to be the second-most carbon intensive, and not just because of the climate risk posed by all those burning buildings.

Even if solar farms and nuclear plants produce no emissions when they operate, building them in the first place is an energy-intensive process that can actually have an impressive carbon footprint. This implies a certain level of carbon emissions per kilowatt-hour of electricity—with the exact level dependent, of course, on how much electricity the plant produces and how long it lasts. By this metric, photovoltaic panels, which require a lot of energy to manufacture and don't produce much electricity once they're installed, are quite possibly the most carbon-intensive source of alternative energy, producing 19-59 grams of carbon-dioxide equivalent per kilowatt-hour of electricity. (This is, needless to say, still a huge improvement over coal, which produces about 1000 grams of carbon-dioxide equivalent per kilowatt-hour.) Wind farms are the least carbon-intensive source of alternative energy, producing only 2.8-7.4 grams of CO2 equivalent per kilowatt-hour.

But, in addition to the carbon emitted during the construction of a power plant, it's also important to take into account how long that plant takes to build. During the time a new power plant is under construction, a utility will continue to generate power using coal or natural gas. Zero-carbon power plants that take longer to build therefore carry a carbon opportunity cost: If the utility had opted for an alternative-power installation with a shorter construction time, it could have cut back on its fossil-fuel generation sooner. Wind and solar are the fastest forms of alternative energy to deploy, so they carry no opportunity cost. Geothermal, hydroelectric, and tidal-power installations take longer to build. But nuclear power takes by far the longest—anywhere from ten to 19 years from planning to operation—so it has the highest level of opportunity-cost emissions. When you combine these implied emissions with the emissions produced in the process of building the plant and mining the required uranium (plus the off-chance of a big carbon release resulting from a nuclear war), nuclear power produces as much as 180 grams of carbon-dioxide equivalent per kilowatt-hour. That's still a lot less than coal. But for a supposedly carbon-free power source, it's pretty lousy.

--Rob Inglis