According to the IPCC there is a 2.5 degree C rise for every doubling of CO2.
So our base is 280 ppm.
Double 560 ppm - 2.5 C
Double 1120 ppm - 5 C
Providing the ECS is at the mid-range of IPCC estimates.
However there are 14 papers with lower ECS estimates and a new paper from Penn University (home of M Mann), which says M Mann's tree-ring calculation on temperature increase is wrong.
RCP 8.5 is the only scenario suggesting such a rate, and that is considered unlikely.
The scenario for RCP 8.5 is -
'This future is consistent with:
Three times today’s CO2 emissions by 2100
Rapid increase in methane emissions
Increased use of croplands and grassland which is driven by
an increase in population
A world population of 12 billion by 2100
Lower rate of technology development
Heavy reliance on fossil fuels
High energy intensity
No implementation of climate policies'
http://www.sei-international.org/mediamanager/documents/A-guide-to-RCPs.pdf'World population to keep growing this century, hit 11 billion by 2100'
http://www.washington.edu/news/2014/09/18/world-population-to-keep-growing-this-...Other forecasts for population are lower.
Guidlines to RPC
'The RCPs are not forecasts or boundaries for potential emissions, land-use, or climate change. They are also not policy prescriptive in that they were chosen for scientific purposes to represent the span of the radiative forcing literature at the time of their selection and thus facilitate the mapping of a broad climate space. They therefore do not represent specific futures with respect to climate policy action (or no action) or technological, economic, or political viability of specific future pathways or climates. '
http://tntcat.iiasa.ac.at:8787/RcpDb/dsd?Action=htmlpage&page=welcome#descriptA very scary scenario only if
all parameters are met.