Inspired by this question and my comment on one of the answers.

If the earth were flung out of the solar system, it would rapidly become far too cold for any unprotected life to survive on the surface. Any remaining humans would have to live in pressurized and heated domes. However, if the earth was ejected from the solar system on a shallow enough trajectory, it's possible that governments or groups could put together such domes before the temperature dropped too severely. However, such domes need to be kept heated, oxygen needs to be provided, and food needs to be grown for long-term survival. Therefore, I nominate Iceland as the location most likely to support a long-term surviving colony. The reason for this is Iceland's reliance on geothermal power, just about the only power source likely to remain viable over the long-term as the earth drifts away from the sun. According to Wikipedia, Iceland used 79.7PJ of geothermal power in 2004, which, if my math is right, gives them about 2.5GW. (If anyone manages to find more recent numbers, please comment and I'll add them).

Given these facts, how likely is it that Iceland could maintain a colony of this type indefinitely, and how large is the maximum colony size that could be maintained?

Iceland can hold out indefinitely

• It would take 2-3 months before the average temperature on the surface to be below zero (the reason for this is that the ocean has tremendous heat capacity, inland regions will fall much faster). That gives them a reasonable amount of time to prepare a living space. Iceland would have slightly more time due to the natural heating due to geothermal and their proximity to the ocean.


• Due to its molten core, the earth's surface will stabilize at an average temperature of -160 C on the surface. David Stevenson, Caltech professor of planetary science This is not cold enough for the atmosphere to condense, so the need for pressurized domes is incorrect.


• It would take approximately two years to get down to this steady state temperature. And -160 C is not impossible for a human with appropriate equipment to survive and work on the surface.


• The best option for the Icelanders is to tunnel beneath the surface to maximize the benefits of the geothermal heating (without having to turn it into electricity). That way, they can use the electricity they produce for cultivating crops.


• As for how large this colony can be, it probably depends more on their digging speed (to produce adequate farmland area) than it does on actual energy constraints. Just because it produces that much geothermal energy doesn't mean that that is all the geothermal available to it as it expands.


• Food stockpiles will obviously give them more time to increase their living spaces before they have to depend on their own farming to sustain them. Additionally, if they have a submarine, then they will be able to operate limited fishing operations (and later mining for frozen fish), as the frozen surface of the ocean will insulate the water, keeping it liquid for many years.

Iceland is doomed.

Sorry, but that's the way it is. Granting 2 to 3 months as a lead time to zero Celsius, that is simply not adequate time to prepare. Much has been said about digging shelter, but Iceland simply does not have deep soil which might make "digging" possible. Instead, we're talking about strip mining, and Iceland has no equipment suitable for the job. If we hand-wave that issue away, digging a hole is only the first step. Once you have a hole, you need to erect a structure which will support the overburden which will cover the structure, and that is a major undertaking. At the least, production of reinforced concrete (the best material for large structures of this type) will take weeks to months to organize - and if the refill process has not started shortly after the temperature drops below freezing it will not proceed at all. Frozen dirt is pretty much indistinguishable from solid rock for these purposes.

Deep mining techniques (boring/tunneling machines) won't work, either. On the one hand they are slow, and on the other hand, Iceland doesn't have any of that equipment, either.

But let's say that a certain amount of structure has been accomplished before the Big Freeze. Food production is now an issue, and either "normal" farming or hydroponics will require massive preparation. Keep in mind that, at this point, it has started to snow, and very heavily. This will continue until the oceans are frozen over, although the area of worst snowfall will gradually move south as more and more of the ocean freezes. Due to proximity to the north polar ice cap, things will get bad at exactly the wrong time, and moving equipment will be a nightmare.

And how will farming be lit? It's true that Iceland has a relatively large geothermal generating capacity, about 700 MW, but this is concentrated in two areas of Iceland, one the west and one to the north, and 70% of the power produced is used in aluminum processing, so the distribution lines to the rest of the country are much smaller than is necessary to allow full use of the capacity. Any use of the aluminum power will require construction of new substations, and that is not remotely a 2-3 month enterprise.

So the amount of habitable underground shelter is very limited. How sustainable is it?

The answer is, not. Food production is not sustainable. The geothermal generators will need spare parts and eventual (less than 50 years) replacement. Iceland does not have the industry required to produce such replacements, and there is simply no reason to project the existence of an alternate source.

And just in case some are thinking about fish as a food source: forget it. Within a few weeks to months the sunlight levels over any unfrozen water will be too small to support diatoms, which will cause a cascade of extinctions, with the largest carnivores dying off within a year.