Wind farms affect local nighttime temperatures, not global warming

Posted on May 1, 2012

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On April 29, a paper about how wind farms affect surface temperatures was published online by the journal Nature Climate Change. The authors of the paper found that wind farms increase the nighttime surface temperature within and immediately downwind of the wind farm because the turbines mix up cold surface air with warmer air from up higher off the ground. What the authors did not find, however, was that wind farms were having any global effect on climate disruption. But if you only read articles and blogs from Forbes, Fox News, The Star Ledger, the UK’s Daily Mail, The National Review‘s Planet Gore blog, The Free Republic, etc., you’d never know that.

In fact, if those were your only sources of information, you’d believe that the paper was all about how wind farms were yet another cause of global warming, when in fact it says nothing of the sort.

The fundamental point of the paper is shown in the figure above (Figure 2a from the paper) – there’s a high degree of correlation between areas of increased nighttime surface temperatures (red blotches) and the location of large wind farms (black “plus” symbols). Looking closely at the figure reveals that the red areas are very tightly associated with the turbines and extend only a small distance downwind (north in this case) of the wind farm itself.

The figure shows that the increase in nighttime surface temperatures affects the wind farm and only a few miles downwind, meaning that the change is local, not global, in extent.

As for why the surface temperatures would increase (and why they’d increase at night more than during the day), it helps to think first about what happens to the air without a wind farm near by.

If you’ve ever taken an early morning walk (or run, hike, bike ride, etc.) in hilly terrain, you’ve probably experienced how the air near the ground is colder early in the morning than the air is high up off the ground. I’ve got a running route that has hollows where the temperature can be five or more degrees colder than the air up on top of the little ridge 20 feet higher and 50 feet away.

The reason this happens is because at night, the ground cools off fast as it radiates heat away into the air and space. As the ground cools off, it cools off the air right above it, and the cold air sinks because it’s slightly denser than hot air is. If there’s a hollow where the cold air can’t escape, the cold air sits there until a breeze disturbs it or the morning sun heats it up again. And because it’s natural for denser cold air to sit underneath less dense warm air, the cold pockets are very stable so long as they’re not disturbed.

This state of affairs occurs even when there aren’t any hollows to collect the cold air, and even a mass of nighttime wind will tend to have cold air near the ground and warmer air high above the ground.

What the paper says is that wind turbines mix up the cold and warm layers of air by dragging cold air from the ground up high into the air (where it’s normally warmer) and by dragging warm air from high up down to the ground (where it’s normally colder). This redistributes the heat energy held in a given volume of air by roughly equalizing the temperature of a large volume of air. However, because the turbines are only redistributing the energy already present in the air, it doesn’t affect global warming in any way.

In case this isn’t clear, let’s look at an analogy culled from my 21st birthday bar tour – the B-52 shot (pictured at right). Notice that there are three discrete layers of liquor – Kahlua on the bottom, Bailey’s Irish Cream in the middle, and Grand Marnier on the top. The Kahlua is the densest, so if the Bailey’s is poured carefully, it will sit on top of the Kahlua. Similarly, the Grand Marnier is the least dense, so it will essentially float on top of the Bailey’s if it’s poured carefully enough. While you get the beautifully stratified layers of the shot when the three liquors are poured carefully, if the pour is done badly or you covered the shot with your hand and shook it up, you’d end up with a gray-brown cloud that didn’t look nearly as cool.

And regardless of whether you drink it properly stratified or all mixed up, the B-52 contains the same amount of alcohol – merely mixing the layers up doesn’t do anything to the overall alcohol content (we’ll assume that the person ordering the B-52 didn’t want it flaming in this case).

Similarly, at night, cold and dense air settles to the ground just like the Kahlua layer of the B-52, with a layer of warm air (the Bailey’s) on top of that, and (for purposes of this analogy) a layer of even warmer air (Grand Marnier) above that. Adding a wind farm to the area mixes those three layers up just like shaking a B-52 does. What the wind farm doesn’t do, however, is change the amount of alcohol heat energy present in the affected volume of air (more energy in the hot air, less in the cold) – it just mixes all the layers up and makes the heat energy about the same throughout the entire volume.

While it’s clear that the wind farms don’t affect global warming physically, they could affect one of the measurements of global climate disruption. The most common way to measure climate disruption is by measuring the surface temperature multiple times every day and tracking the change of those measurements over the course of years. The problem is that the paper indicates that wind farms increase the nighttime surface temperature, and by a significant amount (about 1.3 °F), simply by mixing up the different layers of air.

If enough of the Earth’s surface, or enough areas near to climate monitoring stations, becomes covered by wind farms, then the wind farms would add an error that would appear to show the surface warming slightly faster than it really is.

Thankfully, however, scientists now know that wind farms might be making nighttime surface temperatures artificially warmer in their immediate vicinity. And since they know this, the scientists at the Goddard Institute for Space Studies (GISS), the Climatic Research Unit (CRU), the Berkeley Earth Surface Temperature project, and NOAA’s National Climate Data Center can correct for the artificial warming in data coming from affected climate monitoring stations.

There are a lot of unanswered questions raised by the paper. The authors conclude that changes in the Earth’s surface reflectivity (aka “albedo”) didn’t have a measurable effect, but it’s not clear that this claim is well supported in the paper. The results apply to the region of Texas that the authors studied, but does it apply to the wind farms along Interstate 80 in Wyoming, or to offshore wind farms off Nantucket? The authors calculated a per-decade trend in nighttime surface temperature based off of nine years of data, but nine years isn’t enough to say whether it will be an actual trend or whether it’ll be a one-time increase that stabilizes after the wind farm is completely built out.

But even with those unanswered questions, it’s clear that the people reporting that wind farms increase global warming are wrong. And in case anyone doesn’t believe the explanations offered above, here’s a statement from the authors themselves:

Overall, the warming effect reported in this study is local and is small compared to the strong background year-to-year land surface temperature changes. Very likely, the wind turbines do not create a net warming of the air and instead only redistribute the air’s heat near the surface (the turbine itself does not generate any heat), which is fundamentally different from the large-scale warming effect caused by increasing atmospheric concentrations of greenhouse gases due to the burning of fossil fuels. [emphasis added]

Image Credits:
Nature Climate Change, DOI: 10.1038/NCLIMATE1505
Wikipedia

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