Ox and Snake - The Improbable Empire, #1

Two young men decide to risk the uncertainties of time travel, rather than fall in line with the expectations of others. Their journey runs into trouble in the first minute and things get worse from there. They discover a situation so bad on a 15th century Earth that they are both tempted to time slip again and leave the death and horror behind. The Earth is under attack, strikes on the planet have set off massive volcano eruptions and the resulting ash and vapor clouds are causing an ice age.
The population has been murdered, leaving less than a percent alive and clinging to an ugly struggle for survival. Can anything be done to help them? Or should sensible young men start over elsewhen?