I haven't read the report, but I assume the process goes something like this:
1. Run several climate models, to get estimates of how much warming will be caused by what concentration of atmospheric CO2 in a given time frame (presumably, somewhere between a year and some number of decades)
2. Analyze the results, and figure out what concentration of atmospheric CO2 has at least a 2/3 chance of staying below a 2 degree rise in the desired time frame.
3. Figure out the current atmospheric concentration of CO2, and what fraction of emitted CO2 is likely to be absorbed by natural processes.
4. Subtract. End up with a figure for CO2 emissions that will keep the atmosphere at or below the calculated target CO2 concentration.
Read the report or at least the summary. Its all explained. The methodology, the reasoning and the assumptions. There are also references to the evidence should you want to analyze it yourself. Plus all other inputs to the IPCC report are provided for you. Go for it. This is hardly the place to look for detailed answers of that sort.
Your reason for asking is clearly not a genuine search for the answer.
The ipcc is notorious for simply making up statements to serve a political goal. Any thing they say should not be considered truthfull but more likely designed to keep their self appointed positions intact and the goals of the united nations world government moving along as planned many years ago.
This is the number required to reduce the west to third world status.
The IPCC cannot force or enforce it anyway .
Lame ducks or just lame.
It makes me depressed to see science treated this way by politics.
I mean they are 95% certain that at least half of warming is by man, but they are so uncertain of our climate sensitivity they cannot come up with an average figure, it doesn't make sense.
They get this out of a hole in their backside. There is no credible scientific evidence of this.
The IPCC has set what is usually called a carbon budget. To have a two-thirds chance of keeping global warming below 2oC, it says, “will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1,000 [trillion tonnes]”.
Where do they get this figure?
