[note: originally posted to Science as a joke, enough people commented that the idea wasn't documented well enough, that I didn't provide enough references, etc. sheesh...]
The windmill is a Dutch icon. Giant but quaint wooden mills turn slowly in the North Sea wind, a sign of Dutch ingenuity and as much a part of the sub-sealevel Netherlands landscape as dykes, canals, and red-light districts.
But this image may be slowly changing, and windmills may no longer be the sole providence of the Dutch. Despite the enormous infrastructure and installed base of oil consumers, energy suppliers are starting to realize the need for alternative energy for long-term fulfillment of an exponentially-growing demand for low-cost power.
Modern multi-megawatt wind turbines are sleek white pinwheels, standing hundreds of feet tall, with blades longer than the wingspan of a 747. Large scale installations, with hundreds of turbines, are popping up wherever a high plains mesa or sea-side cliff presents the opportunity to harness the "free" power of the wind, to generate cheap, pollution-free energy.
It's hard not to love wind-power. Aside from the cost of the tower itself, (as well as research and development), wind is free. Better still, it's globally available, naturally available, and inexhaustible. All we have to do is tap into the power of the wind that's already blowing all around us, and if we need more, we can just add more turbines, right?
You can't get something for nothing. Like death and taxes, it's one of life's unavoidable truths. Everything we do comes at some cost. So what's the price we pay for harnessing the wind's energy? Drag. Newton's third law, folks. Simple aerodynamics. Putting a giant propeller into the wind slows it up, diverts it. You get enough of these things, it's like putting up a giant wall.
Crazy, you say? Absurd! What's a few little windmills going to do to something so big and powerful as the jet stream? Dipping your finger in the Mississippi doesn't sway the current from its course. What do we have now, a few thousand windmills globally? Hardly enough to make a difference. They said the same thing about oil in the 19th century.
If the current trend of wind power continues, and conflicts over middle-east oil fields spark more violence, wind could overtake fossil fuel as the preferred energy source over the next few decades. And as it does, windmills may become as much a part of the landscape as telephone poles and Starbucks franchises.
And for each new windmill, an increase in drag. Maybe not much at first, but sooner or later, scientists will start to measure the first consequence of what will come to be known as Global Drag: the earth spinning a tiny little bit slower, a natural result of bad aerodynamics.
I know what you're thinking. What about mountains? Wouldn't mountain growth have the same gradual effect on drag? Absolutely. But mountains are part of a far more balance ecology. As rock is pushed upward by the slow collision of tectonic plates, opposing forces of wind, water, and small animals carry material off the top of the mountain. The net effect on wind resistance is minimal.
But you can't stop man from too much of a good thing. As millions of windmills are erected on every hilltop and open plain, the cumulative drag will gradually cause the Earth's rotation to slow, resulting in longer days, leading to-- guess what-- increased energy needs. The whole thing is a vicious cycle, and if history is any indicator, one that we will repeat until it's too late.
Don't get me wrong; wind power is a great alternative to oil. It's cleaner, and even in the long term, probably less destructive. But, we need to recognize that the only real long term solution to the energy crisis is conservation. How many more natural recourses do we have to exploit and deplete before we learn this simple lesson?
(The Danish Wind Industry has a really neat page full of actual info about wind turbines, including refutations of common windmill fears. For instance, birds almost never collide with windmills. But they don't even mention the possibility of stopping the earth. Careless omission? I doubt it...)