A study published this week in the journal Nature Climate provides evidence that wind farms are responsible for localized temperature increases of up to 0.72 degree per decade. While some people equate this with wind farms "causing" global warming, the reality is that just about everything humans do to our environment causes climate change.
What The Study Says
Let's start with what the study says: over an eight-year period (2003 to 2011), researchers analyzed satellite data over a region in west-central Texas that includes four of the world's largest wind farms (amounting to more than 2,350 individual turbines). They found "a significant warming trend of up to 0.72 degree per decade" between areas with wind farms and nearby areas without, with the most significant warming occurring at night during the winter.
I know, it sounds bad. But let's take a look at what's going on here: in addition to harvesting energy from wind, turbines also generate wind, or at least, their blades cause turbulence as they spin. This turbulence extends over and beyond the span of the blades, which creates air movement outside of the naturally windy altitude that's causing the blades to spin in the first place. At night, when air near the ground is cooled by the Earth and air higher up is still warm, a wind turbine might cause mixing of these two layers that wouldn't otherwise happen, leading to higher ground temperatures.
I know, it still sounds bad. But, the important thing to understand here is that the wind turbines are mostly (I'll come back to the mostly) not creating these higher temperatures in the sense that the turbines are not adding 0.72 degree per decade of heat to the atmosphere. Rather, they're mixing warm air higher up with cooler air lower down such that if you measure the lower down air, you get a temperature increase. From what I can tell, the study is based on MODIS data products measuring land surface temperatures, which implies that if you were to conduct this same study except measure the atmosphere a hundred meters up instead, you'd actually see a cooling effect over time. In other words, the system as a whole isn't gaining energy, the energy is just being shifted around from one place to another by the turbines.
Understanding Wind Turbines & Heat
Now, let's get back to that "mostly" from the previous paragraph. We'd be remiss if we didn't mention that wind turbines do add heat to the atmosphere. They're mechanical, they've got moving parts, moving parts mean friction, friction means heat, and that heat has to go somewhere. Needless to say, this is a very small amount of heat, even if you have a very large number of turbines, but it's there. Also, turbines require service roads, and if those roads are made of asphalt, you've just replaced some amount of vegetation (that generally has a cooling effect) with a surface that retains heat (and consequently has a warming effect).
The thing is, though, everything works this way. Everything has some sort of effect on local climate. Build a road? Climate change. Build a house? Climate change. Plant a tree? Climate change. Harvest a field? Climate change. Whenever you mess with anything that could potentially change wind patterns or heat absorption or surface reflectivity or moisture emissions, you're making small changes to your local climate, and if you make enough of them, they add up to the point where they become measurable or even noticeable by humans without the need for measurement.
For example, you may have heard of the urban heat island effect, which can boost the temperatures of city areas by several degrees Centigrade compared to their rural surroundings. This is caused by (among other things) a combination of lack of vegetation, massive amounts of heat-absorbing concrete and asphalt, and tall buildings that trap reflected light and block wind. Interestingly, studies have shown that even massive localized urban heat island effects don't seem to have any noticeable impact on global climate change overall, which means that we definitely don't need to worry about turbine farms driving up global temperatures.
Anyway, the point that I'm trying to make here is that altering our environment by definition changes it. And if you make enough small alterations, like the addition of more than 2,000 wind turbines, those changes will be measurable. But when it comes down to it, do we really need to be concerned about the effect that wind turbines have on local climate? I'd argue that what we need to think about is the overall costs of wind turbines (financial, environmental, etc.) relative to the overall benefits of wind turbines. Yes, it's true that turbine farms do temporarily raise nighttime ground temperatures a small amount in a localized area, but they're also providing a source of energy that isn't continuously generating greenhouse gas emissions, which have the potential to verifiably effect global temperatures.
We certainly shouldn't be so naive as to think that "green" technologies have no negative environmental impacts at all, but at the same time, we should also be able to understand how these impacts are relevant to long-term global climate change in the context of clean(er) energy production.