This is the easiest parameter to measure directly with physics, but it won't be done, since they know it will be zero. The warmies are using 1850 absorption values, combined with curve-fitting. The assumption is that the co2 absorbs the heat and doesn't do anything with it.
However, these guys claimed to have increased the parameter, thus bringing the end of the world closer. I was astonished, so I assume they are curve-fitting just the 2016 El Nino which was a huge spike. With this new value, there is no hope for anybody. Just relax and have the 'beaujolais nouveau' cannabis from your garden.