Each wind turbine creates behind it a "wind shadow" in which the air has been slowed down by drag on the turbine's blades.
Keith’s research has shown that the generating capacity of very large wind power installations (larger than 100 square kilometers) may peak at between 0.5 and 1 watts per square meter. Previous estimates, which ignored the turbines' slowing effect on the wind, had put that figure at between 2 and 7 watts per square meter.
If we were to cover the entire Earth with wind farms [...] the effect of that on global winds, and therefore on climate, would be severe—perhaps bigger than the impact of doubling CO2.
In short, we may not have access to as much wind power as scientists thought.