Wind Farms and Climate

Do wind farms affect the local and global climate? My first guess would have been “no,” but a couple of papers just out find evidence that they do affect at least the local microclimate. New Scientist reports on a paper in the Journal of Geophysical Research – Atmospheres that considers a hypothetical wind farm consisting of a 100-by-100 array of wind turbines, each 100 meters tall and set 1 kilometer apart, placed in the Great Plains region of the United States. During the day, the model suggests that wind farms have very little effect on the climate because the warmth of the sun mixes the lower layers of the atmosphere. But at night, when the atmosphere is stiller, the wind turbines have a significant effect, bringing down to ground level the warm night air and higher wind speeds that are normally found at an altitude of 100 meters.
UPDATE: An upcoming paper in the Proceedings of the National Academy of Sciences, by David W. Keith et.al. (not yet available online), finds that “the climatic impact of wind power is currently negligible in comparison with other anthropogenic climate forcings.“ Looking ahead to a noncarbon world where wind power grows to 2 terawatts, they project a peak seasonal change of about 0.5 degrees C–but near-zero change in global mean temperature (the signs of the seasonal changes cancel). Overall, they find, the carbon dioxide reductions from using wind power still prevail over any global effects on the environment.