Stopping global warming requires net greenhouse gas emissions to fall to zero and remain at zero thereafter. Put simply, all emissions must either cease, or be completely offset by the permanent removal of greenhouse gases (particularly carbon dioxide - CO2) from the atmosphere. The time taken to reduce net emissions to zero, and thus the total mass of greenhouse gases in the atmosphere, will determine the final equilibrium temperature of the Earth. Almost all analysis concludes that reducing emissions rapidly enough to remain within a 1.5°Celsius carbon budget is practically impossible. Consequently, to limit global warming to 1.5°Celsius above pre-industrial times, greenhouse gas emissions must be reduced to net-zero as soon as possible, and then CO2 must be permanently removed from the atmosphere to bring the total mass of greenhouse gases in the atmosphere below the 1.5° Celsius carbon budget.