Burt talks about the diminishing increase in greenhouse effect as concentration of CO2 increases...
The generaly accepted theory is that a doubling of atmospheric CO2 and
ignoring all other feedbacks increases temperature by about 1.1 degrees celsius. i.e. going from 300 parts per million to 600 parts per million will give a temperature increase of 1.1C - going from 600 parts per million to 1200 parts per million will give a temperature increase of about 1.1C.
.