Transcendent is a science fiction novel in Stephen Baxter's expansive “Xeelee Sequence” (Raft, Ring, Timelike Infinity, Vacuum Diagrams, etc.) and the third book in the “Destiny's Children” subseries, following Exultant and Coalescent. Like most of those books it has a more-or-less standalone story, but you'll get a lot more out of it if you're familiar with its universe; I'd recommend at least reading Exultant and Coalescent first, and the short story collection Vacuum Diagrams wouldn't hurt. (I recommend Vacuum Diagrams as the best entrance point for new readers to the Xeelee Sequence a whole, actually.)
Earth in the 2040s is a world drastically diminished. Global warming has raised sea levels drastically shifted climates around the world, devastating the biosphere and displacing entire nations. The cost of fossil fuels and desperate efforts to contain further environmental damage has made the average person dramatically less mobile, with automobiles a memory, air travel a luxury of the ultra-privileged.
Michael Poole is a middle-aged engineer who has never recovered from the death of his wife Morag over a decade ago. He's plagued by ghostly visions of his dead wife- except that these visions began not only before she died, but long before Michael had even met her. His life is thrown into turmoil when his son Tom is nearly killed by the sudden eruption of methane gas from the thawing ground of Siberia. Michael comes to a disturbing realization- the colder regions of the world are filled with such deposits, frozen into ground that is now unfreezing, and the release of enough in a sufficiently short period of time could further increase the greenhouse effect, releasing more gas and driving the temperature still higher in a vicious cycle that could render the world uninhabitable.
Michael sets out trying to find a way to prevent this catastrophe, struggling with his grief over his dead wife, his strained relationship with his son, and the challenge off mustering support for a geoengineering project of massive scale in a world that has grown increasingly decayed and fatalistic. And, meanwhile, the ghostly image of his dead wife is appearing more frequently.
500,000 years in the future, the human race is united across the galaxy in a vast Commonwealth ruled by a collective consciousness of immortal post-humans called the Transcendence. Their technology is so advanced that they can peer into the distant, and each of the Commonwealth's countless trillions of inhabitants is required to Witness the life of a single human who lived and died long ago. Michael Poole's Witness is a girl named Alia who is unexpectedly pulled from the life she knows when she is chosen for the rare honor of becoming a transcendent herself. An agent of the transcendence takes her on a journey across the galaxy, teaching her the lessons each of its members must learn.
Along the way, she starts to learn more about the origins and nature of the Transcendence, and about its goal of “redeeming” the suffering-filled history of the human race before its continuing evolution puts it beyond humanity forever- but doubt starts to set in as Alia starts learning what this “redemption” will entail. The Transcendence, on the cusp of godhood, can do more than just look into the past...
I greatly enjoyed Transcendence. It's filled with the imagination and awe-inspiring sense of scale commonly seen in Baxter's work and the Xeelee Sequence in particular. The speculative elements are interesting, and the dual setting lets Baxter incorporate an unusually diverse set of ideas, from the social and economic effects of life without automobiles to human evolution after hundreds of thousands of years and alien environments and the cosmological theories that form the basis for the Transcendence's ability to see into the past.
The two plot threads alternate chapters, and Baxter does a good job of balancing them and keeping both interesting. I repeatedly had the experience of wishing a chapter would go on longer because I didn't want to leave that plot behind yet, only to get drawn into the other thread in the next chapter strongly enough that by the end of it I didn't want to return to the thread I had just regretted leaving. That's always a good sign.
Both the near-future and far-future settings for the story are interesting. Technology in Michael Poole's era has continued to advance, with developments including sophisticated virtual reality interfaces and holographic projections, a new source of power based on Higgs fields, and true artificial intelligence that has become so ubiquitous that people who specialize in working with computers have more in common with psychologists than programmers.
But much has been lost. The automobile has been abandoned and air travel is too expensive for anyone but the most privileged, making the accessible world much smaller for the average citizens of industrialized countries. The disruption of the ecosystem brought by rapid global warming has led to mass extinctions on a scale never seen in human history, while declining birth rates in many countries have turned formerly bustling cities into near-ghost towns. The world of the 2040's has an eerily quiet, empty atmosphere- not unlike outer space in a lot of hard science fiction, actually- that Baxter uses to good effect.
Meanwhile, Alia's thread fills in an era that has been largely blank in the Xeelee Sequence until now. The tens of thousands of years of bloody turmoil that raged as humanity waged war for domination of the entire galaxy are over, and the nightmarish totalitarian government called the Coalition that drove those conflicts is long gone. (See Exultant and the short story collection Resplendent for more on this era.) Hundreds of thousands of years of both natural evolution and genetic engineering in a vast diversity of environments has given rise to myriad sub species of humans and post-humans, at peace under the rule of the Transcendence.
We see a number of different environments- Earth, still bearing the scars of an alien occupation nearly half a million years in the past, an ancient generational starship converted into a mobile space-going city, and planets with environments that have radically reshaped humanity.
My favorite element is the Transcendence itself, a group intellect of godlike power and intelligence that is nevertheless as burdened by its evolutionary history as a human being, even if that inheritance is intellectual and psychological rather than genetic. The result is something not only interesting but quite poignant, which is not usually a word I use to describe far-future post-human superintelligences, but it really works well here.
I definitely recommend Transcendent to anyone who's a fan of Stephen Baxter and the Xeelee Sequence. I also recommended it to fans of far-future science fiction in general, though as said above it's not the Baxter book to start with.