This is yet another myth I forgot, that GW might be due not to GHG's but to waste heat. This is not a totally stupid idea: back in the 1970's people considered it.
From this, global energy use in 1990 was 13.2 TW. It'll be a bit bigger now I suppose. So what is a terawatt? Its a billion kilowatts, err, so that makes it 1e12 watts. All of that ends up as waste heat somehow. I think the area of the earth is 4*1.3e14 m2. So the average global forcing is... 13.2e12/(4*1.3e14) = 0.025 W/m2. Compared to which, the well mixed GHG's are about 2 W/m2 TAR spm fig 3. So waste heat is about 1/80th of the GHG forcing. Which is clearly small globally, and probably so locally too.
[Note: this calculation is wrong, at least in the long term. See here.]