Let’s assume that global warming is sufficiently bad that DOOOOM is on the horizon unless we do something about it. Further assume that we’ve reached the tipping point, and that no conceivable reduction in carbon emissions is going to make a difference. What do we do?
The obvious answer: solar shields in space. Vast constructs, or vast numbers of smaller objects, located either in Earth orbit or in the L-1 point between the Sun and Earth, intercepting some percentage of sunlight, shadowing Earth. This is the obvious answer because it would require – and thus drive the creation of – a vast, efficient space industry. Done right, some meaningful part of those solar shields would actually be solar power stations, beaming energy down to Earth in the form of microwaves.
Sadly, the useful suggestions of a space industry tends to make the sort of people who demand that Something Must Be Done snicker. So, what can we do on the surface?
First, the goal: the current energy imbalance (actually more of a power imbalance) is something like 500 terawatts. This means that sunlight comes to Earth, some gets absorbed, some gets reflected back out into space. Of the fraction that gets absorbed, some gets stepped down and radiated out into space as infra-red. But due to greenhouse effect, less energy gets reflected & emitted than come in. That 500 TW imbalance leads to a slow temperature rise.
So how do we fix that energy imbalance? The straightforward answer: reflect at least 500 terawatts of sunlight out into space, energy that *currently* is absorbed. Some people have suggested putting reflectors or bright white insulators on the polar ice to help keep them from melting, but this doesn’t make sense: ice is already white. You’re putting a reflector on a reflector. You might provide some small amount of local cooling, but from a planetary standpoint the effect of X square kilometer of reflector at the poles would be trivial compared to the effect of those reflectors in lower latitudes. Putting the reflectors on deserts might seem sensible…but sand is also quite bright when seen from space. But you know what’s dark? Take a look:
Oceans are dark. Almost black, in fact… the albedo of ocean water is abut 0.06, meaning that it reflects about 6% of the light that falls on it. So if you want maximum effectiveness on a planetary scale, put reflectors over ocean water. And that’s what I suggest: floating reflectors.
To be effective, the reflectors must be cheap, easily produced in vast area and not destructive to the environment. If the reflectors have the albedo of ice – say, 0.7 – a rough hand-wave can be made as to what the surface area of the reflectors would need to be. I’m assuming reflectors that simply float flat on the ocean surface. At noon with the sun directly overhead, one square meter of reflector would reflect 70% of the sunlight that falls upon it, which is approximately 1000 watts/square meter. Of course, that reflector does not spend the whole day with the sun directly overhead; half the day is of course spent in darkness. And during daylight, the sun start off on the horizon, goes to the zenith then drops back down to the horizon. So, over a full day-night cycle, assume that a one square meter reflector reflects the equivalent of 10% of the sunlight that might fall on it if it always faced the sun. And with an albedo of 0.7, that’s 0.7 X 10% = 7%. So a one square meter reflector could account for, daily averaged, 70 watts of sunlight reflected away to space.
70 watts isn’t a whole lot, especially compared to that 500 terawatts imbalance. It would require a minimum of 7.14 trillion square meters of reflectors. That’s a whole lot. It works out to a square patch of ocean some 2,670 kilometers on a side. Paving over a chunk of the ocean that big would do substantial damage; photosynthesis would be shut down in the darkness. But it need not be one giant solid patch. Instead it could be a *lot* of smaller patches.
My suggestion for the reflector would be something simple and dumb… essentially a one square meter slab of styrofoam. Obviously styrofoam itself is not a good choice: it’s not biodegradable, and it is produced from carbon sucked out of the ground. Far better would be a slowly-degrading foam made out of plant material. Thus carbon is sucked out of the atmosphere, turned into plants, turned into foam and eventually dissolved into the ocean (or nibbled up by ocean critters). Assuming such a foam can be produced with similar properties to styrofoam, it would have a density of about 0.1 grams per cubic centimeter. Thus a one square meter slab five centimeters thick would mass five kilograms. An alternative would be a synthetic white plastic sheet material, perhaps similar to styrene at 0.9 grams/cubic centimeter. A sheet one meter square by 3 mm thick would mass 2.7 kilos. 7.14 trillion of these would mass 19.3 billion tons. A modern supertanker can carry something like 320,000 tons of petroleum; if “reflector tankers” could carry the same mass of feedstock goop, then a mere 60,000 tanker sorties would be required to transport the feedstock to mid-ocean reflector production facilities. Of course there will be land locations where ocean currents are such that a factory built on shore could simply spit reflectors out to sea and they’ll drift away.
Now, one might argue that trillions of reflectors made by tens of thousands of supertanker hauls is kind of nuts and, well, one would not be wrong. But unless I’ve done the math wrong (not altogether unlikely), that seems to be what’s required. There are some mitigating factors here that can be a little helpful:
- Assuming CO2 and methane emissions stabilize, then as the reflectors start to accumulate, ocean temperatures will begin to lower. This will, sooner or later, lead to lower arctic temperatures and more pack ice, leading to more reflection of sunlight near the poles.
- The feedstock will require a whole lot of agricultural production. This will lead to a reduction in food production for humans; famines will follow and potentially a statistically important die-off. So… huzzah?
- During the manufacturing process, the reflectors can be seeded with substances such as iron, needed by ocean life. Each reflector would thus serve as a source of nutrients for plankton and algae and such; restoring life to the oceans would help the oceans absorb more CO2 from the atmosphere.