How much would we have to cut emissions?

[tolbiny]

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Lets say, hypothetically MMGW is correct, furthermore our current rate of emissions will cause a significant increase in temperature by 2020, and we want to prevent this.

Is there a linear relationship between C02 and temp increase so that a 10% reduction in emissions will result in a 10% lower temp increase?

What is the lower bound of temperature increase for significant climate problems, what is the lower bound for catastrophic changes?