We will run out of oil if we keep on using more and more motor vehicles (surprising/should/unless)
-> It is surprising that we should run out of oil unless we stop using more and more motor vehicles. (Or: It would be surprising if we didn't run out of oil unless we stop using more and more motor vehicles.)

More and more trees will die if pollution increases (on condition/should/unless)
-> More and more trees will die on condition that pollution increases. (Or: More and more trees should die unless pollution decreases.)

I can buy the house if the house is not expensive (as/should/unless)
-> I can buy the house as long as it isn't too expensive. (Or: I can buy the house unless it is too expensive.)

If there is an emergency, ring this number (in case)
-> In case of an emergency, ring this number.