I thought I would post this video illustrating the first time that a change in CO2 forcing has been observed from the ground. The paper is an Observational determination of surface radiative forcing by CO2 from 2000 to 2010 by Feldman et al. (2015). I had a quick read and as I understand it, they observed from two different sites and measured the downwelling spectrum in the infrared band. They then had to use radiative transfer models to try and remove things like seasonal variations so as to extract the change in forcing due to changing atmospheric CO2 concentrations. They detect a trend of 0.2 ± 0.06 Wm-2 per decade.
Something to bear in mind, though, that this is not the first time that the radiative influence of increased atmospheric CO2 has been detected. Harries et al. (2001) measured – from space – a change in the outgoing spectrum. This is the first time that it’s been detected on the surface.