If we burned all known fossil fuel, would CO2 levels only rise 20%?

Spencer on climate sensitivity and solar irradiance | Watts Up With That?

[S.E. Hendriksen] ...Prof. Tom V. Segalstad (Oslo Norway) made that calculation long time ago when the CO2 level in the atmosphere was 380 PPMv… if you burn all known fossil fuel, it will end up with 456 PPMv in the atmosphere (a rise of 20 %), so a doubling of the CO2 in the atmosphere is not possible due burning fossil fuel.

But a CO2 release from the oceans (because of warming) can do it without any doubt.