Wow, it's freezing out - literally. Last Saturday was 70 degrees, now we've got freeze warnings and a few snowflakes drifted by this morning. Global warming sure has our normal weather patterns on the run. One of the most important tasks for our society in the years ahead is to get a handle on that we're doing to heat the climate up. If we don't it's going to just get warmer, and history teaches us that when the ecology changes, species die out.
Now, I'm not implying that humans are likely to die out as a result of global warming. In fact, I think it's pretty darn unlikely. What I do think is going to happen is that people are going to start moving around more. It may be called chasing job opportunities, the country going to the city, or whatever, but by and large there's going to be a population shift from the warmer climes northward. It will get harder to grow traditional foods; crops will fail, pollinators (such as bees) which are already stressed will get even worse and may collapse, and people will adapt by moving out of the old spot and into a new one. We're already seeing a significant species shift in a food plant here in the Northeast. The sugar maple, long a staple of Vermont industry, is dying out in Vermont, and production is moving north to Canada.
The thing about migration is that on a long-term basis, no "civilized" society on record has ever successfully resisted a major migration. ("Civilized" here means that they left books and records documenting the migration(s).) The fall of the western Roman Empire didn't come about because an organized national or imperial enemy fought a military campaign and defeated Rome's legions. No, what happened was that people moved around the Asian steppes, and the losers were pushed into Europe with nowhere to go except into Roman territory, so as a result the Huns, Vandals and Goths brought a mighty empire down. And these were the losers, but it may be that they had the strength of desperation.
Now, there is one society that has some sort of record of success in dealing with incoming migrations. That is the Chinese society, which dealt with various waves of invaders by assimilating them. You could argue that they were unsuccessful in dealing with the various Europeans in the 19th century, but the Europeans weren't migratory - they were military adventurers imposing temporary outside rule. There are definitely some interesting lessons in the Chinese example for societies that are looking how to deal with incoming migrations and still maintain social cohesion, but that's not really what I'm talking about today.
Instead, it's important to consider what migration means. It doesn't just mean immigration in the conventional sense, in which mostly poor people slip across a border and try to fit in. The sort of migrations that global warming will be causing are going to include some significant clashes, simply because leaders aren't going to sit quietly while their population leaves (or starves). It's not that likely that the United States will invade Canada - Canadians are too nice, they'll just take us all in. However, the most fertile parts of China are mostly in the south of the country, and as those become more prone to drought, and as Siberia becomes more temperate, it's likely that the Chinese government of 50 to 100 years from now is going to be very interested in what is now very empty territory, and China is going to be one of the most powerful countries in the world.
Likewise with India, except that they have a very forbidding northern border. If I were to characterize the military that would make those two countries most secure and successful in the second half of the 21st century, I would expect China to be the predominant land power, and for India, in a fit of irony, to rule the sea.