This may sound odd, but it is caused by the wind turbines affecting the "atmospheric boundary layer". "Boundary layers" are a concept from fluid dynamics; when gas or liquid flows over a surface, there is a rule which say that the speed of the fluid must fall to zero at the surface (the fluid cannot "slip" at the surface). This creates a region close to the surface where the speed of the fluid is changing from zero at the wall, to it's normal flow speed far away from the surface. (See here for more information).
This effect happens in the atmosphere, albeit on a vast scale. The atmospheric boundary layer may be a few hundred metres thick. Wind turbines disrupt this boundary layer - by extracting energy from the wind (and also momentum), the velocities within the boundary layer downstream of the turbines are altered. This change, according to the research, promotes the production of turbulence within the boundary layer. This turbulence produces mixing of the warm air closer to the surface with colder air at higher altitudes, causing the temperature effects describe above.
This effect is currently not well understood. A quote in the article claims:
“There may be an effect, and the wind industry would welcome any research into this. My view is that any effect would be far less than the benefits in reducing fossil fuel consumption.”This is where things get difficult. What is the basis for "his view"? A hunch?
Last year, an interesting paper was published in the Journal of Earth System Dynamics by Miller et al. (Link). They also reported that large scale wind power generation could affect global surface temperatures. But they went further - rather than look at isolated areas (as in the above article), they analysed the global climate system, using a top-down thermodynamics based approach.
They modelled the effect that large scale wind power generation would have on global surface temperatures. Critically, they found that generating large amounts of energy from wind could have the same effect on surface temperatures as a doubling of atmospheric CO2 levels.
(The reasons for this are complex, but similar to those given above - extraction of momentum from the atmospheric boundary layer has knock on effects. Read the paper for more)
Yes, you read that correctly. If we generated large of amounts of power using wind turbines, it could have the same effect on the planet as if we doubled CO2 levels, which is precisely what we had hoped to avoid by building wind turbines.
The one caveat to this is that Miller et al. based their simulations on the extraction of very large amounts (20-60 TW) of wind energy. For comparison, current total global energy use is of the order of 10-15TW. I.e. we would have to replace the entire planet's energy consumption with wind generated energy. So we can probably proceed with wind energy on current scales without too much worry.
However, this research serves as an important reminder that to meet our energy needs with renewables requires vast developments, which may themselves have important (and potentially negative) implications for the global climate system.
