In the novel Saturn's Children, what happened to cause the runaway greenhouse effect that killed all the "pink goo" (humanity) on Earth? Was it something humanity did to ourselves, or was it something the aristocrats did to enslave the rest of robot-kind?
The runaway greenhouse thing happened after humanity had already become extinct. And at least according to general knowledge among robots, the extenction had no specific cause:
Their gradual withdrawal from public life was barely noticed at first. We busied ourselves following their instructions, maintaining their domed cities, building new homes for them on the far-flung planets and moons of the solar system, providing for their every need. Only a few arbeiters slaving in the bowels of insurance companies and government bureaucracies noticed that the population adjustment downward from the claustrophobic spike of the Overshoot was continuing; that fewer and fewer of our progenitors were replicating themselves via the weird, squishy process to which they devoted their organs of entertainment. And arbeiters don’t have enough free will to take independent action— such as telling someone who could do something about the problem. By the time people started paying attention, it was too late to arrest the crisis. Attempts were made to organize a captive breeding population, but the natural objections of the population in question to being so manipulated—combined with our own innate reflexive obedience— foiled all such programs.
If I remember correctly (couldn't find the quote), the common theory about the reason for the population decline is that humans, having created robots to do everything strenuous and dangerous for them, lost the impulse to do anything creative, including making more humans.
But it could also be seen as a logical extrapolation of birth rates in most developed countries, which are already today insufficient to maintain the population.
As for the greenhouse thing, that seems to have been the result of carelessness:
And I suppose it’s understandable that, without the guidance of our Creators, certain people who were entrusted with maintaining specific programs let them drop. But how they missed the onset of a runaway greenhouse effect—well, it was the scandal of the century! At first there was denial, and then there were recriminations, followed by assertions aplenty that it signified nothing. But when the Gulf of Mexico came to a rolling boil, heads rolled in their turn.