What these naysayers forget is that the effects won't stop at 2100. They are a legacy that will get worse and worse as time goes on, long after we have stopped burning fossil fuels.

The risks that have not been fully quantified include the sudden release of methane from clathrate deposits located under ocean sediments. 

Then there is the progressive reduction of ocean pH and the implications for reduced calcification of many marine organisms, including otoliths in many important food catches, together with the differential migration rates of benthic and pelagic components of whole marine ecosystems.

Many countries rely on seasonal melt water for clean water supplies.  As temperatures rise, these supplies will eventually dry up.

it isn't just a case of rising sea levels, and even progs might be able to outrun  that. However, rising sea levels bring numerous secondary effects, including the increased salinification of coastal aquifers, and, particularly topical for Queensland, WA and Northern NSW, increased erosion rates of coastal acid sulphate  soils that are found around mangrove ecosystems.  

This is serous case.