How fast could a climatic change forced by a civilization happen?

Question

A Civilization with semi advanced Technology should be able to start climatic changes on a earth like Planet (same land mass to water surface ratio).

What would the fastest way be and How?

Some little restrictions:

No radiation, no burning down of all the forests, no burning of resources at all (electric based transportation) and no Magic. The Planet should be intact after the treatment and while the treatment is in progress it should be possible to live on it in some fortificated places to observate the Progress. Fauna and Flora are doomed but will be replaced after.

Definition of "advanced Technology": Normal energy resources like Atomic Power Plants and Solar technology. No fancy stuff like Fusion. But the civilization is able to build massive Buildings (up to some km in height). Space travel is established so there are also power sources in orbit.

Start Point: -10C° ~ 10C° at equator. -40C° everywhere else (poles are not relevant)

End Point: about 20C° to 30C° everywhere and blue sky afterwards (so no massive cloud factory).

TL;DR : A Icy planet transformed in a caribic summer vacation post card shoot in the fastest possible way.

Answer 1

Orbital Reflectors

Large mirrors in orbit (you've stated they can build 1 KM buildings, in space that should translate into really large structures), designed to reflect additional sunlight down to the planet.

You'll need a lot of them, but that's purely a resource issue, and the more you add the more additional sunlight will hit the planet.

You can go with a ton of semi-powered mirrors that only have station-keeping ability (they can stay in the same orbit). These are cheap, but the downside is that more than half the time they won't be doing any useful reflection - this is the quantity over quality strategy.

Alternatively you can have fewer mirrors, but give them the ability to adjust their angle to maximize the amount of sunlight reflected. This way they can reflect sunlight for longer periods, up to 100% of their orbit if you set it up correctly.

As for how fast this will be: it depends. The more mirrors you throw at the project, the faster it will change things. But you're still probably looking at a minimum of 100 years, if I had to make Wild Ass Guess. I can't think of a faster non-destructive way though.

Answer 2

Pumping Out Supergreenhouse Gases

Dichlorodifluoromethane, $CCl_2F_2$, or CFC-12, better known as Freon-12, alongside Sulfur hexafluoride $SF_6$ are two extremely potent greenhouse gases, thousands to tens of thousands of times more powerful, molecule for molecule, than CO2.

That means that you can get a similar effect with much smaller atmospheric concentrations. Instead of parts-per-million you can have parts per billion and get the same effect. On Earth, you have $2.996×10^{12}$ tons of CO2 in the air, so to get the same effect you'd need only a few hundred million tons of the super-greenhouse gas. Our civilization outputs 10,000 tons of $SF_6$ every year as a byproduct of industrial manufacturing. A determined campaign to create more could easily output truly industrial amounts into the planetary atmosphere in a few years.

Most of these gases have a lifetime in the atmosphere measured in the thousands of years, before they are disintegrated by reactions or by UV radiation, so unless you trigger a positive feedback, you'll have to have a generator forever outputting small amount of the gas to compensate for this loss-rate.

Ending the Ice-age

Since you specify that your planet starts off as a iceball wasteland with a narrow barely livable band around the equator, you need to do something about all that ice, since ice has a high albedo (reflects off heat) while ocean has a low albedo (dark blue absorbs better), so melting the ice would help push and keep the world outside of its ice-age. Additionally, being able to have more water vapor stay in the warmer atmosphere should contribute massively to the greenhouse effect (water vapor on Earth is reckoned to account for about 80-98% of the greenhouse effect, depending on whom you ask).

The cheapest way is to simply cover your glaciers with a dark layer at the top to remove the albedo issue. This way, you amplify the solar absorption many-fold. You might have to periodically redo this to compensate for runoff.

The next lowest level would involve building a vast number of fission reactors which are simply used to generate the largest amount of heat, with a pipe network gradually extended to melt the vast glaciers, in the same way some houses have electric bathroom tile-warming.

Finally, if you have basic space technology, you can deploy light-weight reflector mirrors, but they'd still be stymied by the ice albedo issue unless you already addressed that.

Dealing With the Mess

Your ocean levels will increase tremendously, your glaciers will destroy the landscape as they melt, and the permafrost ground will collapse as it melts and it's battered by the ocean and runoff. Even under optimistic scenarios, it'll be centuries before the earth settles into a new stable configuration.

Answer 3

Volcanoes

This is the way that comes to mind first. Drop bombs in every volcano you can find, empower full enough to cause an eruption, and detonate them simultaneously.

This does obviously have its difficulties: it is probably quite hard to stop a bomb detonating once it's in lava. The alternative is you schedule all the drops for the same time. As long as they are reasonably close together it shouldn't matter too much.

This works because when the volcanoes erupt they'll produce huge ash clouds which can cause climate change by blocking the sunlight. A quick google search for volcanic winters should bring up plenty of these - an ash cloud can easily block out sunlight for a month or two, by which time most plant life is dead and thus other life can't support itself.

Answer 4

Combining the Mirror array of @Dan Smolinskle with part of the comment of @MichaelKjörling:

Provided you have large methane pockets in permafrost: If your mirrors would be set up so as to heat up the specific regions enough to free those methane deposits, that should help:

Methane is a greenhouse gas, reflecting infrared from the planet back to the planet, thus preventing "heat from escaping". According to [Wikipedia][1], greenhouse gases are supposed to be resposible for some +14°C.

Not being any kind of expert in that field, i would assume that additional surfaces not covered in snow (which reflects a lot of infrared) should help you reach a few more degrees.

I guess with such a setup you would need far less mirrors than for trying to heat up the planet with the mirrors alone.

Answer 5

I would go with a combination of the space mirrors and gm plants. Robot drones used to coat the planet in a blanket of various gm algae / fertilizer mixture - this being dark in nature would help with the lowering of the albedo. So the plants push micro roots into the surface ice extracting some water - they will on clear nights be getting extra light. By using the varying types of Chlorophyll to maximize the spread of the available spectrum you could maximize photosynthesis. The result great floating algae blooms. When alive the algae consumes some of the by products of photosynthesis, it uses other to build cell wall , when it splits , what ever is left is stockpiled. It might be designed to split say 10 times - after the 10th division - the splinting part of division process is halted - the cell then stockpiles more and more products in the expectation of a division , until it ruptures - this provides a massive boost to the nearby cells as a "flood" of nutrient becomes available.

The same kind of process can be used in massive vats in the big buildings to create the fertilizer, as well as the first gen to be sprayed.