Banter and General Comments, page-52

Dunno where you got your numbers from but last time I checked preindustrial CO2 concentrations were about 25% lower than todays (sub 300ppm) which is actually quite a substantial difference. The sun is pumping so much energy onto this planet that changes in IR opacity that might seem trivial to the layperson can actually produce a substantial effect. Remember, without the sun we'd be at -270 degrees or so, so if you increase the absorption of solar energy by 1% you get a full 3 degrees. Furthermore you don't need 1% of the atmosphere to be CO2 to get 1% increased absorption, just like how a car doesn't need to be 1% paint by weight to be 1% red - a thin coating is enough to make it quite red indeed. There are a lot of compounding effects at play