but on what is this pessimism based?

In my case, I am pessimistic that we, as a species, will be able to act with sufficient alacrity to avert ambient temperature increases of 2-4C, or possibly more.  The collective reporting I have seen on the Greenland and West Antarctic Ice Sheets, combined with what I have read about the probable rapid rate of melting of the North American Ice Sheet at the beginning of our current epoch, leads me to doubt that extrapolations of melt rates for these ice sheets will follow current linear models.  

I fear current models assume that the ice sheets will melt in place and perhaps the rate of calving of icebergs into the ocean will increase at about the rate of increase we have recently observed.  Were you able to inform me otherwise I would be relieved.  I am concerned that recent reports of melt water toward the center of these ice sheets creating channels to bedrock will increase and will increase the rate of motion in second order or higher ways.

I think it is more likely that we will document accelerating  processes at work within a decade or so than that we will confirm existing models.  I have no way of knowing how fast melting will occur, but a one meter rise seems much more likely to me than a 10cm rise by mid-century.  I am also concerned that responsible and competent authorities are not creating models that are based on accelerating melt rates and ice movement and using them to evaluate which model best fits observation.  If they are and if they have published results, I have missed the report.

We have all deplored the extent to which Bush era science was subjected to political censorship.  But I do not think that this problem has vanished with the change of one administration in one country.  Nor do I think that current estimates err on the side of pessimism.  To me the question is how much less optimistic should we be.  

"It is not necessary to have hope in order to persevere."