Of course, there is no threshold at 400 ppm. If there is one, it’s at 350 ppm. As Hansen and friends discuss in Young People’s Burden: Requirement of Negative CO2 Emissions
We advocate a stricter goal, based on restoring Earth’s energy balance and limiting the period when global temperature is above the range of the Holocene; temperature stability of the Holocene has allowed sea level to be stable for the past several millennia in which civilization developed. This goal leads to a CO2 target of 350 ppm, …
Humans use the decimal system. That’s a convention without any physical significance. 400 ppm might look special to humans, to nature it does not. To nature 400 ppm is just an arbitrary point well into the danger zone on a curve of global exponential growth of a little over 2% increase in atmospheric carbon dioxide starting with the industrial revolution.
The 70 year rule implies that the amount of CO2 emitted by humanity into the atmosphere doubles every 35 year. Keep that in mind when you hear talk about the great things we’ll do by 2050, almost 35 years from today.