That is if we ignore the Jevons effect: "In economics, the Jevons paradox occurs when technological progress or government policy increases the efficiency with which a resource is used (reducing the amount necessary for any one use), but the rate of consumption of that resource rises due to increasing demand." https://web.archive.org/web/20150529111127/http://sspp.proqu...
The width of this slim bright side you talk about - is it epsilon?
The harm done by ships travelling the northwest passge might outweigh any emission savings (there - not in total). Carbon dioxide emissions are a rather non-localized problem while soot, spilled oil, invasive species, noise pollution and you name it, are very localized problems.
https://en.wikipedia.org/wiki/Northwest_Passage