smoking cigarettes while walking over ice and Tundra in the far north of Canada and Siberia can be fatal

Why?

Because as the methane is released as the planet warms (even in the Tundra of the north) there are times when the methane vents enough through Ice or tundra to combust with a cigarette. When this happens often the person dies or they cannot even find his or her body when this happens. So, walking on melting ice or tundra can be fatal sometimes when you are smoking right then. 

And the other factor here is that Methane is even more harmful to our atmosphere than other chemicals in the air for some reason too. So, as the planet warms the methane trapped in the ice or under the tundra will tend to vent more and more over the years especially during this century.