The sequestered CO2 can be estimated as the difference between total anthropogenic CO2 emissions and the change in CO2 atmospheric concentration. The number oscillates, and total CO2 emissions estimates are a rough number. But claiming that 80% is sequestered sure sounds like an exaggeration. Don't forget emissions include sources other than fossil fuel combustion and cement manufacturing.
Tom Wigley had a guest post on this not long ago, he argues it is more like 80%.
https://rogerpielkejr.substack.com/p/net-zero-does-not-mean-what-you-think
He makes a strong point, but it is not widely accepted.
The sequestered CO2 can be estimated as the difference between total anthropogenic CO2 emissions and the change in CO2 atmospheric concentration. The number oscillates, and total CO2 emissions estimates are a rough number. But claiming that 80% is sequestered sure sounds like an exaggeration. Don't forget emissions include sources other than fossil fuel combustion and cement manufacturing.