Because electricity and heat account for 41 percent of global carbon dioxide emissions, curbing climate change will require satisfying much of that demand with renewables rather than fossil fuels. But solar and wind come with their own up-front carbon costs. Photovoltaics require much more aluminum—for panel frames and other uses—than other technologies do, according to a 2011 study at Leiden University in the Netherlands. Alloys for wind turbines demand lots of nickel. Those metals are carbon culprits because they are produced in large amounts by high-energy extracting and refining processes.

The demand for metals, and their already significant carbon footprint, may grow with a switch to green energy. Given all the resources needed for new infrastructure, an analysis last year found that large solar installations take one to seven years to “break even” with coal power on the greenhouse scorecard. Wind farms take from less than one year up to 12 years.* All the more reason to make the switch sooner than later.

*Clarification (9/30/2013): This sentence was edited after publication to note the low-end estimate for wind energy's greenhouse payback time, which is more reflective of modern wind turbines.

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