This is according to scientists from NOAA and Scripps Institution of Oceanography at the University of California San Diego, on 4 June.
Monthly CO2 values at Mauna Loa first breached the 400 ppm threshold in 2014, and are now at levels not experienced by the atmosphere in several million years.
Actually, the rate of increase during 2020 does not appear to reflect reduction in pollution emissions due to the sharp, worldwide economic slowdown in response to COVID-19.
The reason is that the drop in emissions would need to be large enough to stand out from natural CO2 variability, caused by how plants and soils respond to seasonal and annual variations of temperature, humidity, soil moisture, etc.
If humans were to suddenly stop emitting CO2, it would take thousands of years for our CO2 emissions so far to be absorbed into the deep ocean and atmospheric CO2 to return to pre-industrial levels.
Namely, if emissions reductions of 20-30% were sustained for six to 12 months, then the rate of increase of CO2 measured at Mauna Loa would be slowed.