Is global warming a linear process? Is global warming linear in the sense that doubling the emission of Greenhouse gases will result in a doubling of global warming. Is there a threshold, i.e. is there are certain concentration of GHG where global warming is triggered and additional gases do not lead to a faster global warming?
No. Consider water vapor. Water vapor is a greenhouse gas. As the temperature increases, more water is evaporated. Since water vapor is a greenhouse gas, the temperature will increase. Clouds, however, form from water vapor, reflect sunlight.

The earth is not as linear as people would like.