Warming has not been lower than forecast (what stinking place did you pull that from?)
I pulled them from a VERY stinking place, some place most people never go, the actual data. Take a look at the IPCC forecasts from 1999 IPCC now take a look at actual data from 1999 to 2012 at NOAA (or Hadley CRUT).
It clearly shows that while there has been warming it has been lower than the low forecast.
If you don't want to sift through the data (although I encourage you to do so and see for yourself), here's an article from an anti-denier site showing Hansen's 1988 predictions similarly being low. Note that this site is in the business of proving that global warming is real, their bias is strong and their data is suspect but even they clearly admit that actual temperatures are below the forecast.
These aren't cherry picked examples either, take most past temperature predictions and chart them against actual and you'll see that the rise is less than predicted. Or check the IPCC predictions from edition to edition and you'll see that they are slowly moving down in the near term (although often have global warming shift into high gear a few decades hence).
To be clear, I'm not a denialist. I do think global warming is real and a problem. But I think Climate Science is a lot like economics, they have a pretty good idea what's going on and you'd be foolish to ignore them, but you'd also be foolish to think that they have everything fully figured out or that they aren't missing some really big and important factors in their analysis.