I used RRTM LW (the model Trenberth uses) to calculate the effects of increasing CO2 on downwelling longwave radiation, otherwise known as measuring the greenhouse effect of CO2.
As you can see, the effect of increasing CO2 is minuscule.
Even in the mid-latitude winter, going above 400 PPM has very little effect
By contrast, the effect of water vapor is large, and accounts for almost all of the greenhouse effect.
Increasing CO2 has almost no effect on the climate, as we have seen over the past 18 years. Trenberth knows this – this is the same model he uses for his weather and climate models.
Scientists who claim that increasing CO2 is dangerous, are either ignorant or dishonest.