A genetic tweak creates the blue-green glaucophil, more efficient at processing sunlight into energy for plants than chlorophil (this latter having evolved at a time when there was a redder bias in the sunlight reaching the earth). It is not long before blue, rather than green, becomes the dominant vegetative colour. This in turn changes the planet's Albedo: and although this is only a marginal alteration, it is enough to alert extraterrestrial civilisations -- this being the trigger for which they have been waiting.
The development of a hugely sophisticated virtual world, modelled closely on the actual world, works as a laboratory in which social and political ideas are tested. The presence of this 'test' helps fine-tune a balance of right-wing and left-wing ideological ideas, identifying precisely the right mix of welfare and laissez-faire, which in turn leads to decades of unprecedented growth, stability, environmental and cultural balance. Some, however, brow anxious that this virtual world cannot accurately model the real world because, unlike the real world, it lacks a recurive virtual model of itself within itself. Altering the mdoel to include such a model-within-the-model fractures the coding, perhaps because the recursive model is already seeded with a model-within-a-model-within-a-model, which in turns opens an infinite regress. The model's processing needs collapse the worldweb, and precipiates collapse, civil war and disaster. Or ...?
Harry Gill. A punishment for crime, the Chills, alters the body's perception of how cold it is -- even though you are warm enough to remain healthy, you feel bitterly cold. Harry is brought to a lab attempting to chill something below absolute zero (which can be achieved by holding all its constituent atoms perfectly motionless, and then shrinking them all such that the distance between them increases). The experiment succeeds; and cold enters the bones of the world.