Ed Hoskins gives a solid presentation (at the link below), with good graphs, on the logarithmic relationship between CO2 concentrations and its greenhouse effect. One must be aware that, in general, the experimental research was done with dry air, which does not include the dominant greenhouse gas, water vapor. High humidity of a particular region will reduce the influence of CO2 for that region. The vexing problems are feedbacks. The alarmists, and their models, predict net positive feedbacks. The main assertion is that a warming from CO2 will be amplified by an increase in water vapor. The skeptics assert that the net feedbacks are negative – increase in cloud cover. The feedback issue makes it difficult to determine the temperature effect of a doubling of CO2. With the current trend of no significant warming or cooling as CO2 emissions are increasing, nature seems to be supporting the skeptics. ~ summary by Ken Haapala, Exec Director at Science and Environment Policy Project.

http://wattsupwiththat.com/2014/08/10/the-diminishing-influence-of-increasing-carbon-dioxide-on-temperature/