@charlie2009 has got the method about right.
But his facts/data are totally wrong.
Presumably he's reading doctored and false blogs that have given him a false understanding.
Here's some validation of climate forecasts, including those made as far back as 1973. Hope that helps @charlie2009, and maybe others.
https://www.carbonbrief.org/analysis-how-well-have-climate-models-projected-global-warming
http://www.realclimate.org/index.php/climate-model-projections-compared-to-observations/
