Thread by @richardabetts

.@hausfath & @Peters_Glen correctly highlight that RCP8.5 *emissions* are not "business-as-usual"

BUT it's important to note that in Earth System models, the RCP8.5 CO2 *concentration pathway* can still arise from a lower emissions scenario (eg. SRES A1B) if feedbacks are strong

@hausfath @Peters_Glen This is a very important point (which Zeke and Glen do make, but it seems to get overlooked)

Any particular emissions scenario can give rise to many different future concentration pathways, some rising faster and some slower, depending on the strength of carbon cycle feedbacks

@hausfath @Peters_Glen Most climate modelling studies that use RCP8.5 are using the scenario in the form defined in terms of concentrations (the amount of CO2 & other GHGs bulding up in the atmosphere), not the form defined in terms of emissions (the amount humans are releasing). This is a key point.

@hausfath @Peters_Glen RCP stands for Representative Concentration Pathway. These scenarios were originally defined in terms of concentrations, for use as input to climate models. Some standard emissions scenarios were developed to go along with them for studies which required this instead / as well

@hausfath @Peters_Glen The original RCP8.5 concentration pathway has CO2 reaching 939 ppm by 2100

Variants of the HadCM3C model can project a CO2 rise above that when driven by the lower emissions scenario SRES A1B

journals.ametsoc.org/doi/pdf/10.117…

(box: 25%-75% interquartile range, whiskers: full range)

@hausfath @Peters_Glen That paper (Booth et al 2017) used a perturbed-parameter ensemble of HadCM3C, exploring the effects on climate-carbon cycle feedbacks of varying the values of key uncertain parameters in the model

This still suggests RCP8.5 concentrations are less likely, but can't be ruled out

@hausfath @Peters_Glen This highlights how problems with communicating scenarios go beyond incorrectly calling RCP8.5 "business-as-usual"

Unfortunately many papers discussing RCP-driven climate projections say they're using an "emissions scenario" when in fact they're using a *concentrations scenario*