Sensitivity of carbon budgets to permafrost carbon feedbacks and non-CO ₂ forcings

The near proportionality between cumulative CO 2 emissions and change in near surface temperature can be used to define a carbon budget: a finite quantity of carbon that can be burned associated with a chosen 'safe' temperature change threshold. Here we evaluate the sensitivity of this carbon budget to permafrost carbon dynamics and changes in non-CO 2 forcings. The carbon budget for 2.0 of warming is reduced from 1320 Pg C when considering only forcing from CO 2 to 810 Pg C when considering permafrost carbon feedbacks as well as other anthropogenic contributions to climate change. We also examined net carbon budgets following an overshoot of and return to a warming target. That is, the net cumulative CO 2 emissions at the point in time a warming target is restored following artificial removal of CO 2 from the atmosphere to cool the climate back to a chosen temperature target. These overshoot net carbon budgets are consistently smaller than the conventional carbon budgets. Overall carbon budgets persist as a robust and simple conceptual framework to relate the principle cause of climate change to the impacts of climate change.