The Year 2512

For starters it wasn't Portuguese conquistadores who descended on South America 500 years ago but the Spanish. The Portuguese had more important operations in Asia at the time. It wasn't until the 17th that the Portuguese really started to exploit part of what today is known as Brazil, part because a chunk of Brazil used to be part of the Spanish Empire, not the Portuguese.

Of course we are not going back to hunter-gatherers, that's stupid on the face of it. Even after a massive collapse the first thing survivors will do is try to salvage whatever machinery they can find and start anew, first with crops and eventually industry. The "dark ages" weren't really dark, the north of the Roman Empire had always been underdeveloped which is why after the fall it continued to be a number of backwater kingdoms while Venice remained as a center of scientific and cultural development.

Also I don't get why OP comes with all these amazing sometimes insane technologies but rules out any attempts at geoengineering which rely on technology which we already have (but we don't deploy because the costs are still too high and the political motivation too low to support it).

And his understanding of the roots of racism is egregious, he completely ignores the vast political and economical reasons for recent and still enduring racism in many parts of the world. For example the Rwandan genocide background lies on which tribe had the upper hand during the colonial age and was thus wealthier and more powerful. It had next to nothing to do with religion or patriarchy.

Wetland plants (like Mangroves) already transport substances from their leaves to their roots.

If your nation consists of 1/6th of the world's population, you bear 1/6th the burden of the global warming triggered by your fossil fuel use, but get 100% of the energy output from your fossil fuel use. Conversely, if you unilaterally decide to build a giant shade for the earth, you foot 100% of the bill yet only receive 1/6th of the overall benefits.

If we choose to protect biodiversity today, if we choose to keep our climate optimal for naturally evolved life, then our future in 500 years can be a lot better than anything Stross has described.

For example, I think we will indeed have AI, but it will turn out that AI, like a human baby, has to be raised by parents. The process of "programming" an AI is just like the process of "programming" a person - it's messy, and it takes a lot of personal effort and fortitude.

I also think it is likely that the rules of our universe allow only one successful colonization attempt per home planet, or perhaps home system. This limits the spread of any Life to linear (rather than geometric), and it also means that the odds of life meeting each other on the same planet are extremely small. (It goes without saying that there is Life in a lot of places, but that it rarely succeeds in the singular effort of colonizing another system).

More and more of our productive efforts will be created and consumed in virtual environments. But that doesn't meant that the physical will be ignored. "Slow space" is important, and beautiful in it's own way, and we evolved with it. We're not going to leave it behind.

Social constructs will become self-aware, and driven by ever greater understanding of ourselves and our relationship to the world. We will truly choose our beliefs, and aggregate (virtually and physically) with others of the same belief. Indeed, I can imagine a world government who's primary purpose is not adjudicating national disputes, but rather protecting the rights of new adults to choose what they believe, and to live their lives wherever those beliefs are most closely held.

The US and all nations will evolve according to what they really are: explicit belief systems that exploit energy/food/material resources in order to protect/promote their belief system. We will probably see a spectrum of US-like nations, that differ only on second-order considerations. Basically, what we have today, only more explicit and with far more porous borders.

It seems very unlikely that the majority will actually modify their bodies with computer interface technology. The inherent risks are far too high when a device doesn't have a physical off switch. Consider the nightmare scenario where a hacker hacks into your visual cortex, and Rick rolls you. this is far worse than blindness - it could drive you insane (really).