when science fiction invades global politics

The UN is musing about a giant sunshade to cool the planet by reflecting 1% of the light that reaches us. They really don't want to know the price tag...

When it comes to dealing with global warming, we've discussed why a genuine plan is probably not going to emerge from the endless partisan bickering on the issue and we're unlikely to really get our act together as the average global temperatures creep upwards. But what if we had more time? What if there was some sort of giant sunshade in space, reducing the amount of energy and sunlight hitting our planet?

The less sunlight makes its way to Earth, the less heat greenhouse gases can trap, which means that the planet will cool pretty quickly thanks to this hypothetical sunshade. All we'd have to do is launch a big enough reflector into orbit and park it at one of the L1 Lagrangian point so it constantly faces the Sun, blocking, oh say, about 1% of sunlight from its permanent orbit 1.5 million kilometers away. Hey, wait a minute… Haven't we seen this before?

At any rate, the United Nations' Convention on Biological Diversity is actually thinking of discussing the merits of this idea and the media's impression so far is that it will try to ban orbital sunshades. Since no one really knows the full impact of a geo-engineering project, who knows what other effects this reflector may have, say the concept's critics. And besides, add environmental groups, wouldn't polluters just say that the whole global warming thing is now solved and they can keep burning fossils fuels, thinking that next time it starts getting a little warm, we'll just launch another giant sunshade?

While both those objections are valid in their own ways, they miss the real point here. The actual process of building and launching an object large enough to deflect somewhere in the neighborhood of 1% of the sun's light is logistically and prohibitively expensive, something a little back of the envelope math quickly shows us. So before we start getting carried away with a grand plan of building our own little piece of a reverse Dyson shell, let's consider the numbers involved.

We get about 174 × 10¹⁵ W from the Sun, or 174 petawatts, absorbing about 1,370 W per m2 on a typical day, so to block 1% of incoming sunlight, we'd need to deflect just under two petawatts. Using the figures provided, we can estimate that our sunshade will need a reflective surface that covers 1.2 million square kilometers, or about the area of Peru. Obviously, we couldn't launch a reflector as big as an average sized nation with only a handful of rockets. In fact, if we could launch an inflatable, lightweight mirror that stretches a square kilometer when unfurled, that would already be a feat of engineering. A feat we have to repeat over a million more times to meet our goal.

And even if we could somehow bring the costs of such a launch down to that of NASA's new interplanetary missions, about $700 million or so, the total effort would cost us $840 trillion. Which by the way happens to be twelve times more than the entire global economy generates on an annual basis. That's right, blocking just 1% of the Sun's energy will cost you all the money in the world about twelve times over. The UN may as well be discussing a global ban on antimatter production while they're at it because it will be just as realistic of a project as a giant sunshade to cool down the planet.