5.5% of GDP

The NZ Herald reports on the latest IPCC missive.  As always I am interested in the hard numbers so one can do meaningfull cost vs benefit analysis.

We have the already known prediction that by 2100 sea levels will rise by between 18cm and 59cm – an average rate of 1.8mm – 5.9 mm per year.

We then have the statement that stabilising CO2 levels by 2050 will mean decreasing global GDP by 5.5%.  Now I have not yet gone to the source data, but maybe someone could clarify if it means have GDP in 2050 5.5% less than it is today, or have the total GDP growth to 2050 5.5% less than it would have been otherwise.  For now I am assuming the latter.

Now what is 5.5% of global GDP? It’s US$2.65 trillion.  Not exactly spare change. And this is an annual amount of reduced wealth.  Now I’m not against this – just saying we need to be clear about the cost – reducing the world’s wealth by $2.65 trillion a year.
But hey if that is the price to keep temperatures stable, then that may be fine.  But is it?  You see that is the price to keep CO2 levels stable by 2050.  Now again I have yet to read the source report but maybe someone who has could explain whether that means keep them from increasing after 2050, or have them at 1990 levels by 2050?

And then once that is clarified can someone please find what either of those scenarios means in terms of average temperature by 2100 and average sea level rise?

Because if parties in NZ are talking about reducing the 2050 levels to 30% to 50% below 1990 levels, I guess that $2.65 trillion isn’t enough.  You see that’s the figures I really want to know.  How much money has to be spent to stop global warming due to CO2 totally by say 2050 or 2100?

Please note I am accepting everything the IPCC says as correct.  I am assuming they are experts in both climate science and economic forecasting and costing.  I just want to use their own figures to get clarity over the costs and the benefits.  Because at the moment they are expressed (at least in the NZ Herald summary) in a way which is incomparable. If we want the benefit to be stopping at 18 to 59 cm increase in sea levels, what is the cost of doing that?  Is is stabilising CO2 levels by 2050 or is it much more than that?