So, just thinking out loud here, but the current rise in CO2 concentration is around 2ppm/year (confirmation). If that were constant, then 20 years would be an increase of 40 ppm, taking us to around 420ppm CO2.
Just eye-balling the emissions to concentration curves (curves) suggests a linear fit of about 10ppm increase for every 1Gt increase in emissions. That news article says
As a result, the analysis predicted a steady increase in emissions of carbon dioxide, the greenhouse gas that scientists say threatens a serious warming of theSo a 40% increase according to that news article is going from 29 to 40, that's 40 divided by 29 (= 1.38, so about 38% increase). The emissions increase by just 11 Gt/year. Using our poor estimate of 10ppm/1Gt, that means 10*11= 110ppm!!
Earthlater this century. Between now and 2030, Wednesday's report said, global carbon dioxide pollution is expected to increase by 39%. That translates to 33 billion metric tons in 2015 and 40 billion metric tons by 2030, compared to 29 billion metric tons in 2006, the report said.
UPDATE: This is mistaken. After consultation with somebody who knows about carbon, I think the way I should have done that is to say that there's a 10ppm increase for a 1Gt/year increase in emission rate. The change from 29 to 40 Gt appears to be the cumulative emissions. This, however, suggests a much lower increase in atmospheric CO2. It isn't totally clear how to make the estimate without making a budget for carbon. I'll try to make that estimate and re-update later.