We also know one important point from linear stability analysis. For the Earth's climate system to be stable at all, it has to respond to perturbations in forcing by opposing the change, not augmenting it.
Or maybe flipping between two extremes? How about that for a "null hypothesis"? Do you know that once there were trees growing in the Antarctic?
Oh, and since we're talking hypothetical shit without any real connection to the actual science, you MUST also mention the possibility of the Venus effect - runaway greenhouse warming. Why do you think it's impossible? After all, the Sun is growing hotter and this time _just_ might be nonlinearly different! If we think about the nonlinear systems then we MUST fail on the side of caution and stop ALL the CO2 emissions NOWNOWNOWNOW!!!!!!!111111
And reality check - we actually do know quite a bit about actual climate processes and we can model them with a significant precision. The models show that the global warming won't be catastrophic for the environment itself, but it'll cause wild upheavals in the human economy.
"If I do not want others to quote me, I do not speak." -- Phil Wayne