The exact size of the carbon budget cannot be specified with high confidence since it depends on many uncertain factors, including emission pathways for non-CO2 climate forcers. This said, the remaining budgets for the 1.5ºC and 2ºC targets have been estimated at about 200 and 800 Gt of CO2 . With unchanged present emissions at about 40 Gt CO2/year these budgets would be exhausted in as few as 5 and 20 years, respectively. Consequently, most of the IPCC emission scenarios able to meet the global two-degree target require overshooting the carbon budget at first and then remove the excess carbon with large negative emissions, typically on the order of 400‑800 Gt CO2 up to 2100.
At the same time as negative emissions appear to be indispensable to meet climate targets decided, the large future negative emissions assumed in climate models have been questioned and warnings have been raised about relying on very large and uncertain negative emissions in the future. With the future climate at stake, a deeper and fuller understanding of the various aspects of negative emissions is needed.