I think you are mistaken. In nature graphs are never linear. They are curves, special type of curvers. They start slow, then they can grow lineary for a while, and then they switch to exponental growth. This is the case w/ everything if you take a time to look. Epidemics, population growths, Vulcanos, even stars.

The very same thing start on Earth w/ climate change. Temp raises slowly, more CO2 (and more importantly, CH4) in atmosphere. Temp reises more. Atmosphere is hotter, can absorb more H2O vapors as well. Now, when you start hitting first threshold, hydrates (CH4) will be released from shallow ocean bottoms, more CH4 in atmosphere hotter.. cycle accelerates hitting runaway (exponental growth). KABOOM, earth might be doomed.

Its simplified model, not accounting for huge vulcanos on polar, with might explode and make nuclear winter with will block the heating. Complicated stuff, thats why we need scientists :)