CAMBRIDGE, MASSACHUSETTS — A new Harvard study simulated the effects of switching the country's entire power grid to wind power to look at the effect turbines have on local temperature.

Wind turbines can alter local temperatures by increasing the mixing of air at and above the surface, Ars Technica reports.
Researchers Lee Miller and David Keith from Harvard University used a high-resolution climate model of the United States to simulate what would happen if enough wind turbines were placed in the middle third of the country where the winds are highest to meet all current power needs.
Their results showed the continental U.S. would get warmer by about 0.2 degrees Celsius on average, while the regions with the wind-turbines would warm by around 0.5 degrees Celsius. The study was published in the journal Joule.

The study found that temperature fluctuations are greater at night and smaller during the day because surface warming and Sun driven convection are greater than the effect of the wind turbines.
At night when the air is calmer, wind turbines mix warmer air down toward the cooling surface.

An important point is that the wind turbines are not generating heat, rather they are moving existing heat around.
Greenhouse gases from fossil fuels on the other hand continually accumulate and last for centuries.