Over the long term, planets do not provide stable environments for alien civilizations. Over the very long term, millions of years, continents move. On shorter terms, such as tens to hundreds of thousands of years, ice ages might occur. On Earth, they do. If an alien civilization was trying to last tens of thousands of years on a planet like Earth, they would have to be ready for what might be called extreme climate change.
Ice age is defined as a period when there are large glaciers on a planet. We are now in an ice age. So far, five ice ages have been found on Earth, and probably more have occurred, but investigations have not yet turned up evidence of them. Ice ages on Earth have lasted tens to hundreds of millions of years, and are broken up into periods of maximal ice, glacial, and gaps between them, interglacials. Our current ice age started about 2.5 million years ago, and the current interglacial, about 10 thousand years ago. Ice ages on Earth are not well understood, as there are a number of feedback mechanisms in the atmosphere and ocean, possibly involving the continents as well, which cause ice to grow and cover large areas and then disappear entirely. The second known ice age, called the Cryogenian, appears to have been the largest with ice extending down to the equator. The nickname, Snowball Earth, is applied to this period, which lasted somewhat over 200 million years, around 700 million years ago.
The glacial and interglacial alteration is thought to be caused by the variation in Earth’s orbit. The orbit is affected by other planets in several cycles, which are not harmonically related, that last tens of thousands of years. During this alteration, glaciers go from extending far south to being confined to the polar regions.
To try and deduce whether planets in other solar systems which could give rise to intelligent life would also have ice ages is made difficult by the lack of a coherent theory of ice ages. There are many competing factors which affect whether ice forms at the poles and spreads or simply melts and stays away for very long periods. The Earth’s temperature is a major variable here, and it is affected by events such as volcanism, photosynthesis, the release of methane from land and continental shelf deposits, the position of the continents and interference with the circulation of waters between cold polar areas and warm tropical areas, plus many other factors.
A planet which gives rise to life may have to have an ocean and land masses, as well as the right type of atmosphere. If the planet is in a part of the habitable zone of its star where, if there were no greenhouse gases in the atmosphere, it would have ice in the polar regions or everywhere, and if there were a large amount of greenhouse gases in the atmosphere it would not, it seems reasonable to expect it will have ice ages. CO2 is absorbed by rocks and the ocean, and released under some conditions, and this is commonly accepted as one of the feedback mechanisms which cause ice ages. Methane is another gas which can be absorbed or released, and which has a strong greenhouse effect. The more mild effects of glacial and interglacials come from simply having multiple planets to affect one another, plus some axial tilt, which also would be a common occurrence among exo-planets in general. So, since it seems possible that an alien civilization would arise on a planet with these effects, we can ask about how the civilization would deal with them.
Aliens would certainly notice the beginning of a switchover from interglacial to glacial, as there would be countless signs available. Glaciers would be advancing, seasons would change length, average temperatures would be decreasing, and precipitation would be altering its patterns. If they had already existed for some thousands of years, they would have established an infrastructure on their planet, consisting of cities, transportation systems, energy stations, and more. These might be affected by the switchover.
The alien civilization would certainly understand ice ages and the fluctuation between glacials and interglacials. They would likely be able to estimate well when it occurred, and what would be the extent of the changes. For example, they would understand what the extent of glacier formation would be in the next glacial period. What would they do about it?
It is hard to see how an alien city could be built strong enough to withstand contact with an approaching glacier. Glaciers move, and scoop up large boulders and move them. The pressure that a moving glacier would put upon the polar-facing walls of the city would likely be much more than they could withstand. Glacials last for tens of thousands of years on Earth, possibly they also would on other inhabited planets, and why would an alien civilization want to have a city buried beneath a glacier, even if it could be built strongly enough to withstand its motion?
One alternative would be to move a city southward during the centuries before the time when glaciers would approach it. Recall that alien civilizations likely do not have resources to spare, and abandoning a city would represent a large loss of materials. It is possible that they could deliberately reduce their population, and evacuate out of the northern affected cities without having to build new ones, if their warning time was sufficiently long compared to the average lifetime. Another alternative is to predict the maximum extent of the glaciers during the whole ice age, and only build the infrastructure of the civilization in those areas of the continents where glaciers will never come.
There is another, more extreme, alternative and that is when the alien civilization understands all the mechanisms of ice formation and manipulates the atmosphere so that glaciations remains confined to some portion of the planet which they can avoid building in. In other words, they engineer their own climate. This would likely have to be done by adding or subtracting greenhouse gases from the entire atmosphere. They would want to use greenhouse gases which were extremely effective, so that the total mass required could be minimized. Another requirement would be for the residence time of the gases in the atmosphere to be large, so they would not have to be replaced quickly. Since interglacials and glacials last over periods of thousands of years, even long-lived greenhouse gases would disappear from the atmosphere before they had to be forcibly removed.
Could an alien civilization take over their own macroscopic climate control? One problem would be finding the resources to spare for such a task. This problem depends on the choice of chemical to be used, and how much would be required, and how often it would have to be replaced. At this point in time, we have on Earth only some ideas related to carbon dioxide, and this would likely not be a good choice, as the quantities are too large.
Recall that Earth’s oxygen was created by photosynthesis, and a good fraction of the whole atmosphere was replaced, albeit over a hundred million years. Biological methods of much, much more rapidly producing much more effective gases could be conceivable. These questions are far beyond our understanding at this time.
Ice ages are not the only large problem that an alien civilization, with the goal of lasting for a million years or more, would have to face on an exo-planet that didn’t cooperate by being totally stable. Each of them should be discussed to see if there are any so difficult that the concept of a million-year-old civilization is impossible to maintain. This would answer why there are no aliens here in a unique way: They could not survive long enough on their own planet to explore the galaxy.
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