Because there aren't going to be any two degrees of warming by 2020. Not going to happen. Period. And I bet those douchebags know it.
Look: the globe warmed up by 0.6° in the 20th century, and most of that warming occurred before 1940. And it seems likely that most of that warming is also the result of bad data collection--badly sited sensors--and about an order of magnitude too high.
So the globe warmed 0.1° in the 20th century--that's being generous--and most of that warming took place in the first 40% of that timeframe. 0.1° isn't warming; it's noise--and the science which supports that figure is at least as good as the science which says we'll see "only" two degrees of warming by 2020. (Probably better.)
But even if we accept that the Earth warmed by 0.6° in the 20th century--and I am willing to accept that figure as long as we also accept that it cooled 0.0775° in the past two years to about 0.53°--even accepting that, it means that warming would have to suddenly and catastrophically jump 1.4° in the next eleven years, and there is no observed mechanism by which that can happen.
Ice ages, we have learned, come on suddenly: in a matter of decades, the place I'm sitting right now could go from temperate to arctic, given the right circumstances. The ice comes suddenly, in geological terms; but even so, it would take more than eleven years, and the cooling required to do that would utterly swamp the 0.6° rise of the 20th century. (By how much? I figure a factor of five would certainly do it but it probably doesn't even require half that much cooling for it to happen, as long as the cooling is consistent and ongoing.)
The only place we see that kind of warming--1.4° in eleven years--is in the computer models. And we know those are faulty. 1.4° of warming in the next eleven years means 0.12° per year, which is twenty times the average rate of warming of the 20th century.
If human carbon dioxide emissions were on the verge of causing that kind of warming we would see some kind of indication of it, and we don't. We're told that the upper atmosphere is cooling because of warming in the lower atmosphere, which is thermodynamically impossible, and we're told that the oceans are cooling because of atmospheric warming, which is thermodynamically impossible. We're seeing all kinds of cooling everywhere--including the lower atmosphere (remember that 0.0775° figure I mentioned above?)--and yet the bozos at Carbonhagen are telling us they've managed to hammer out a deal which will restrict global warming to two degrees by 2020?
It's all smoke and mirrors.