The graph below shows the increase in downwelling longwave radiation as CO2 increases. The red section shows the increase from 350 PPM to 400 PPM CO2.
As you can see, increased CO2 does almost nothing to the radiative balance of the atmosphere near the surface, and Hansen’s claims of increased heatwaves due to CO2 have zero basis in science. The greenhouse effect at the surface is dominated by water vapor, not CO2.
This graph was generated using RRTM – the model which Trenberth uses.