C1.3. Limiting global warming requires limiting the total cumulative global anthropogenic emissions of CO2 since the preindustrial period, i.e. staying within a total carbon budget (high confidence). By the end of 2017, anthropogenic CO2 emissions since the preindustrial period are estimated to have reduced the total carbon budget for 1.5°C by approximately 2200 ± 320 GtCO2 (medium confidence). The associated remaining budget is being depleted by current emissions of 42 ± 3 GtCO2 per year (high confidence). The choice of the measure of global temperature affects the estimated remaining carbon budget. Using global mean surface air temperature, as in AR5, gives an estimate of the remaining carbon budget of 580 GtCO2 for a 50% probability of limiting warming to 1.5°C, and 420 GtCO2 for a 66% probability (medium confidence). Alternatively, using GMST gives estimates of 770 and 570 GtCO2, for 50% and 66% probabilities, respectively (medium confidence). Uncertainties in the size of these estimated remaining carbon budgets are substantial and depend on several factors. Uncertainties in the climate response to CO2 and non-CO2 emissions contribute ±400 GtCO2 and the level of historic warming contributes ±250 GtCO2 (medium confidence). Potential additional carbon release from future permafrost thawing and methane release from wetlands would reduce budgets by up to 100 GtCO2 over the course of this century and more thereafter (medium confidence). In addition, the level of non-CO2 mitigation in the future could alter the remaining carbon budget by 250 GtCO2 in either direction