The remaining carbon budget left to limiting warming to 1.5C is so small that its effectively impossible at this point (~235 GtCO2 for a 66% chance of avoiding; 395 for a 50% chance). So how do we create plausible scenarios to limit warming to 1.5C? More negative emissions! 1/4 
The amount of negative emissions ranges widely across 1.5C scenarios in the SSP database – from 400GtCO2 to 1,600GtCO2 (10-40 years of current emissions). The use of negative emissions effectively increases the size of the remaining carbon budget by between 100% and 400%. 2/4
So while carbon budgets can be a useful tool for communication, in the case of the 1.5C target they are not particularly useful for understanding mitigation pathways! 3/4
The worlds not going to cut all emissions to zero in 20 years, so if you want 1.5C you need to root for large-scale low cost negative emission technology later in the century, or go with sulphate geoengineering to suppress temperatures until atmospheric CO2 levels can be reduced.
