If you haven't seen Infinity Wars, you probably shouldn't read any farther! Fair warning, MASSIVE spoilers ahead. Seriously, don't read any farther unless you've already seen the movie or don't care if it gets spoiled.
Alright, ready for some spoilers?
The short version? Thanos wants to kill half the population of the universe at random. And, well, he succeeds. With a literal snap of his fingers once he acquires the Infinity Gauntlet. And despite that seeming batshit insane, it's actually one of the better thought out plans in the Marvel Cinematic Universe. Thanos isn't just crazy, (he's definitely a little crazy), but actually has a serious plan in mind. He's acting out of his own perverse sense of altruism. Thanos comes from a world (Titan, no relation to our moon) that died due to overpopulation and resource over-consumption. Thanos had tried to save it by proposing half the population be sacrificed, but, not surprisingly, the population rejected his plan. Its death drove him to want to do the same to the universe, to keep that catastrophe from repeating itself.
There seem to be a couple of important specifications to Thanos' plan. First off, only sentient species appear to be targeted. We don't see any plants or animals dying after Thanos snaps his fingers- which makes sense, because destroying them goes against his plan. This is due to a concept called carrying capacity. An ecosystem has the ability to support only so much in the way of life before it collapses. A lot of factors go into determining how large the carrying capacity of an ecosystem is. This can include how much water is available, how much sunlight for plants, how harsh the weather is, how fertile the soil is, so on and so forth. As carrying capacity for plants increases, the number of plants generally increases along with it, which in turn raises the carrying capacity for herbivores, which increases the carrying capacity for carnivores, both of which raise the carrying capacity for scavengers, all of which... you get the picture. If Thanos had wiped out half of all plants, he would be severely exacerbating resource overuse. (Of course, there are also questions regarding where the sentience line is drawn- would dolphins or great apes count? Do you need to have a technological civilization?)
Thanos went from purple to pink for Infinity War. [Image source]
Next, Thanos seems to be killing half of each species separately, rather than merely killing half of all sentient life. This isn't shown explicitly in the film, but given his prior method of invading planets and killing half their populations militarily, it seems unlikely he'd be so sloppy as to not specify half of each sentient species. This could result in, well, a random distribution that favors one species over another quite severely, crippling some while not significantly reducing the resource consumption of another. In a universe with as many different sentient species as the MCU, this seems likely, so it's a safe assumption that Thanos thought of this.
There's an important question to ask here: how much does this actually benefit the various ecosystems involved? It turns out that the answer is a lot. To take an example from our world, if you were to start living an extremely environmentally conscious life (in regard to carbon emissions, specifically), you might be able to save 5 metric tons of carbon per year. (This is if you stop driving entirely, recycle, go vegan, etc.) However, if you instead were to decide to have one fewer child than you'd planned, you'd save 58.6 metric tons of carbon per year. Alarmingly similar calculations apply to many other metrics of environmental health. Humans are big, put a lot of pressure on our ecosystems, and are really good at finding ways to amplify the pressure we put on ecosystems.
This refers to Earth, not one of the alien worlds from Infinity War. Just in case you were wondering. [Image source]
Thanos is also thinking of this on a huge scale, which is really important. He's not worried about individual countries, he's worried about planets and galaxies. This is, frankly, the right scale to be thinking at. I've seen a lot of people praising individual countries for their environmentally friendly policies. There's nothing wrong with that, it's important to have individual nations doing everything they can- but it often breezes over the oh-so-common exportation of environmental stress and pressure. Take Japan, for example. Japan is frequently praised for its amazing forestry management- it's the most heavily forested industrialized nation. This is thanks to forestry management policies dating back all the way to the Tokugawa Shogunate. What people often overlook, however, is that Japan still uses a ton of wood- almost entirely from various tropical nations. What Japan has done is export its deforestation. This method of shifting the damage is utterly normal across the globe. There's really no evading the cold equations dictating how many resources an individual human needs at baseline.
An objection I've heard to Malthusian fears (that is, the fear that populations will grow out of control, causing disaster- much like Thanos' worry) is that once a civilization reaches a certain quality of life for its citizens, its population growth slows or even starts to drop. And that has happened to a number of civilizations around the globe, including France and Japan. Unfortunately, there are also plenty of other industrialized nations, like Canada and the US, that still have ample population growth. (In fact, there's a type of Christian in the United States known as Quiverfulls, who seek to take over the world by having way more babies than everyone else. They're responsible for a big chunk of the population growth in the US.) We don't have any confirmation that halted population growth is an inevitable outcome of industrialization. (Lowered seems likely, however.) Thanos can probably point to countless examples of civilizations where that doesn't occur- most notably his own, which was quite advanced when it died. Even if, however, it was an inevitable outcome eventually, it's still a huge problem- right now, on Earth, if everyone lived at the standard of living that people in the US live at? We'd be drastically over the carrying capacity of the planet. While people in industrialized nations do tend to have fewer children, they each use so absurdly much more in the way of resources that it's more than counterbalanced. The average American puts as much pressure on Earth's carrying capacity as an entire family in the developing world- or more.
Thomas Robert Malthus, the English thinker who first popularized the dangers of overpopulation. [Image source]
Of course, there are ways to live more sustainably and put less pressure on your ecosystem, as mentioned above. They don't solve the problem, but they do mitigate it. Would it be possible for Thanos to simply use the Infinity Gauntlet to enforce more environmentally sustainable lifestyles across the universe, or to perhaps boost the resources available around the universe? For the first- well, it seems unlikely. The Infinity Gauntlet seems to require you to have a clear idea of what you want to do, and sustainable lifestyles are, well, extremely variable. They are highly dependent on the local ecosystems, carrying capacity, local infrastructure, culture, diet, etc, etc. They require a high degree of metis, or local knowledge. It seems unlikely that even with the Infinity Gauntlet Thanos could perform such an intricate action. Power, he's got in spades. But this requires near omniscience, not the near omnipotence the Gauntlet grants him.
As for boosting the amount of resources available? He'd have to violate the Second Law of Thermodynamics, which, essentially, states that entropy always increases. Even as insanely powerful as the Infinity Gauntlet is, it's far from mighty enough to change the laws of physics for the entire universe. Plus, Thanos is clearly intelligent enough to have thought of that one- he's a monster, but a well intentioned one.
So... basically, Thanos' plan works. Just fine, really. (Ignoring all moral issues- it might be well intentioned, but the road to hell is paved with good intentions. This is a profoundly evil act.) But the plan doesn't work perfectly. There are a few problems with it. First off, it's going to kill a lot more than half of the universe. In a plane where the pilot and copilot die? So do you, probably. Most people in busy traffic will probably die. Anyone who has their doctor die mid-surgery? Probably dying too. The list goes on and on, and probably applies to any given alien species as well. The long term death toll will be huge, too. Society will be basically nonfunctional for a time. There will be starvation, disease, rioting, etc, etc. I ran into one estimate that many species will suffer death tolls as severe as 75%. Life will be really bad for a time, and these civilizations will take a lot of time to rebuild and to regain lost knowledge. We actually have an example of this in history- the Black Death in Europe. It killed about a third of Europe, but also solved the starvation, underemployment, and endemic vagrancy running rampant through Europe at the time. But... well, Thanos is probably resigned to all the disastrous short and medium term consequences of his plan. Again, this is a character shown again and again to be extremely intelligent. He knows this is going to happen, and he thinks it's worth that price.
A mass grave dating back to the Black Death in France. [Image source]
Next, however, is the possibility that his action might take a species below its minimum sustainable population. If a species is endangered- like, say, the Asgardians, most of the survivors of which were killed by Thanos, those few who remain? There might not be enough of them, or enough genetic diversity, to keep the species going. Which I'm sure Thanos would regret, but regret doesn't do much for those species. Even the species that survive will likely have severely reduced genetic variance.
Thanos' plan also doesn't fix pre-existing damage to ecosystems. Extinct species are still extinct, invasive species are still running amok, and there's still lots of CO2 in the atmosphere- or whatever problems various alien worlds are having.
The next one, however, is a doozy- what's to stop these populations from growing once again? Well, he's not putting any hard limits on population growth, so it could totally happen in theory. However, earlier in the movie he talks about how several of the planets- including his adopted daughter Gamora's world- have become veritable paradises after his purges. There's a distinct possibility that they could institute strict population growth policies to keep things that way. Or to keep from attracting Thanos' attention again. And if they don't? Well, Thanos is immortal and still has the Gauntlet.
Thanos is trying to solve a very real problem. He might be doing it in an evil and crazy way, but he's still trying to solve it. It's a problem that we need to solve, too. I just hope we can come up with a better solution.
Bibliography:
- An Environmental History of the World, by J. Donald Hughes
- https://www.tor.com/2018/05/08/thanos-snap-avengers-infinity-war/
- https://rapidecology.com/2018/05/14/the-avengers-infinity-war-has-an-important-ecological-lesson/
- https://www.theverge.com/2018/5/4/17319992/thanos-plan-avengers-infinity-war-historical-precedent-science-malthus
- https://en.wikipedia.org/wiki/Laws_of_thermodynamics
- https://en.wikipedia.org/wiki/Thomas_Robert_Malthus
- https://en.wikipedia.org/wiki/Carrying_capacity