Representative Concentration Pathways (RCPs) used in AR5 are future projections of greenhouse emissions according to different mitigation scenarios. So for example there is a ‘business as usual’ high emission scenario (RCP8.5) and a midrange mitigation emissions scenario (RCP4.5). CMIP5 model projections are then based on runs with each scenario.
Here is the fallacy. The number 8.5 refers to the resultant climate forcing in watts/m^2 at the top of the atmosphere by 2100. Yet to know what this forcing will be in 2100 you have to use a climate model! This is a circular argument as it predetermines the answer before any ‘testing’ of the models even starts. So for example RCP8.5 gives a warming of
(with no feedback)
(with feedback) for f=0.4
How can you test the models with an emission scenario based precisely on those same model results? These ‘scenarios’ must include highly uncertain assumptions about carbon cycle feedbacks, cloud forcing feedbacks that are built into the very models you are supposed to be testing.
The SRES scenarios used in CMIP3 predicted atmospheric CO2 concentration NOT the final forcing. Why was this changed ?
Reading Moss et al. one sees that RCPs were a deliberate choice to define an end goal for radiative forcing rather than CO2 levels, apparently to speed up impact assessments by the sociologists. In fact these are just guesses for radiative forcing in 2100 and there are no emission scenario behind them. Instead the modelers can chose any plausible emission ‘path’ they like that reaches the required forcing. Each emission path is just one of many alternative pathways. This is like saying: We know that the world is going to fry by 2100 if the sun’s output were to increase were by 8.5 watts/m2. Lets simply use that figure for the enhanced GHE by 2100 and call that our ‘business as usual’ scenario RCP8.5. That way we can convince the world to take drastic mitigation action now, by scaring everyone just how bad things will be.
It is interesting also to look again at Fig 10 in the AR5 Summary for Policymakers. This figure had and continues to have a huge political impact.
Firstly note RCP8.5 seems to imply that the carbon content of the atmosphere will triple by 2100. That means that in the next 85 years we will emit twice as much CO2 as emitted so far since the industrial revolution. Current CO2 levels are 400ppm – an increase of 120ppm. So by 2100 the graph seems to imply that levels would be 640ppm. Yet that is only a total increase in CO2 forcing of 4.4 W/m2. To reach a forcing of 8.5 W/m2 the atmosphere must reach a CO2 concentration of 1380 ppm
What is going on ? There is a subtle assumption behind Fig 10.3 namely that natural CO2 sinks will saturate and instead of just half emitted CO2 remaining in the atmosphere eventually all of it will remain. There is no real evidence whatsoever that this saturation is occurring.
Could we actually reach 1380 ppm even if all emitted CO2 remained in the atmosphere?
|date||50% retention||100% retention|
We still can’t reach 1380ppm even if all emissions remained for ever in the atmosphere. Can positive feedbacks save RCP8.5 by boosting external forcing? Perhaps, but my problem with that is the incestuous relation between the assumptions made in the models with the scenarios. You need the high sensitive models to justify the target forcing of 8.5 w/m2, so is it any surprise that those same models then predict 4.5C of warming? The same argument applies to emissions limit to meet the 2C limit to be discussed in Paris.
Arguments that other anthropogenic greenhouse gases like methane or SO4 make the difference seem rather dubious since CH4 and SO4 have a lifetime of just 4y in the atmosphere, as do aerosols.
Fig 10. is highly misleading for policy makers (IMHO)!