If the average temp goes up by 5 degrees, you will not see much in terms of that. What happens is a shift in the weather patters. You know, like more energy in the oceans producing greater numbers and larger sized storms (like that cyclone that just went through Australia?).
The thing I would look at would be stability of weather, not temps per se. You have a shift in the usual seasonal weather pattern (more rain, less rain, warmer winter, colder summer) then it is a sign.
The problem is, as soon as you wait long enoug to verify the pattern, the damage has already begun, and even if you were to stop all greenhouse gas production that day, it would still proceed to get worse for several years before it got better (slowing a moving freight train).
What some scientist worry about is that we have enough climate change momentum to carry us over a stabilization point (like in Calculus). We get out of a self-rectifying "trough" and start skidding off the graph on one side due to secondary and tertiary cumulative effects (algae bloom, etc).
It is just amazing how many people get up in arms whenever someone suggests that their fat asses have to actually do one iota of work (or conservation) for anyone but themselves or "God".
Hell, they will go to WalMart for a SuperSaver on toilet paper, but damn the one that tells them to drive a car that gets 3mpg more than their current ball-sack on the trailer hitch Hemi.