Zeke Hausfather
@hausfath
The IPCC 6th Assessment Report features a new emissions scenario – SSP1-1.9 – limiting warming to 1.5C in 2100 with limited overshoot. It requires the world reach net-zero by 2055.
However, under the hood it assumes a huge amount of negative emissions over the 21st century.
The assessed "carbon budget" to limit warming to 1.5C in the IPCC report is around 500 GtCO2.
However, the SSP1-1.9 scenario emits 700 GtCO2 during the 21st century, blowing way past the remaining carbon budget. At the same time, it deploys 430 GtCO2 of negative emissions.
Relying on negative emissions allows the scenario to have a more plausible emissions reductions pathway; a similar scenario not using any net-negative emissions would likely require getting to global net-zero emissions in the 2040s.
However, the technologies to do this are still largely untested at anything close to the scales required. We are essentially betting that we can get back to 1.5C through planetary-scale engineering late in the century after overshooting the target.
Data on the positive and negative emissions assumed in various scenarios can be found in the SSP database:
SSP Databasehttps://tntcat.iiasa.ac.at/SspDb/dsd?Action=htmlpage&page=welcome