It's tempting to think that wind power is entirely free of greenhouse gas emissions, but energy has to be used to make, assemble, maintain and ultimately decommission turbines and wind power projects. Even given all that though, a new assessment of the environmental impact of wind power, published in Environmental Research Letters, shows that wind power is a huge improvement over fossil fuel power plants it replaces.
Assuming a 20-year lifecycle for onshore wind farms, and a 25-year timeframe for offshore projects, the paper concludes,
the total emissions of wind electricity range between 4% and 14% of the direct emissions of the replaced fossil-fueled power plants. For all impact categories, the indirect emissions of displaced fossil power are larger than the total emissions caused by wind power.
The paper's authors note that these figures are in high range of other analyses done on the subject from other sources.
For the average TreeHugger reader the paper's calculations are a bit wonky and it's probably good enough to file away the 4-14% stat in the green power talking points part of your memory.
But here's a smidgeon of some of the interesting analysis done, on what part of the complete lifecycle of a commercial wind farm contribute most to the overall emissions (and it's really different for onshore and offshore projects).
For onshore projects the wind turbine itself is the single largest emission source at 60-69% (very roughly evenly divided between making the tower, the nacelle and the rotor). In an offshore project though, the turbine itself drops to just 19-35% of the overall emissions, with the installation and decommissioning creating the majority of impact (19-35% and 18-52% respectively). Interestingly, the foundations for the turbines can be up to 11% of the overall emissions for an onshore project, and up to one quarter of the emissions for an offshore project.
For the statistically inclined, read the original research: Environmental implications of large-scale adoption of wind power: a scenario-based life cycle assessment