Checking numbers on CO₂ forcing

Although I am no more a climate scientist than Robert Adair is, when I saw his letter (Physics Today, March 2012, page 8 ) nearly dismissing carbon dioxide’s role in global warming, I couldn’t help checking his numbers. Because of what I was taught as an astronomer who once studied planetary physics, I was struck by his paragraph comparing Earth’s surface mean energy transfer of 492 W/m² to the present CO₂ forcing of “only” 1.6 W/m². What’s relevant, of course, is solely the effect of the differential heating represented by the extra 1.6 W/m², not at all whether there is a much larger total number to compare it with. That comparison seems to be a misleading justification for Adair’s claim that “CO₂ forcing leads directly to only a fraction of the 0.8 °C global temperature increase in the past century.”

Just assume that the extra 1.6 W is distributed throughout a column of water a square meter in cross section and a mile deep (and that may overestimate the relevant Earth-average surface thermal inertia). You’ll find that the heating exceeds 0.7 °C per century, in nice agreement with the conclusions of “radical” climate scientists about the CO₂ greenhouse effect.