They add that , by the year 2100 , global average surface temperatures could rise by between 1.1 C ( 1.98 F ) and 6.4 C ( 11.52 F ) compared to 1980-99 levels , while sea levels will rise by between 18 and 59 centimetres ( 7.2 and 23.2 inches ) , according to the IPCC 's forecast .