"Climbing the Blue" by Stephen Baxter

The story is set on a very unusual Earth: the inhabitants live on a kind of cliff that is stratified in time: the lower you go, the slower time flows and light appears red-shifted and the higher you go, the faster time flows and light appears blue-shifted. A doctor who is investigating the causes of a strange plague is forced to make a decision when his wife is inflicted with the plague. His decision may cause him to become an exile in time but he believes it is a price worth paying.

• Read from Analog SF, July-August 2005 issue