This is a useful post and updated my estimate of the chance of lots of warming (>5 degrees) downwards.
Quick question: Do you have a rough sense of how the different emission scenarios translate into concentration of CO2 in the atmosphere?
The reason I ask is that I had thought there’s a pretty good chance that concentrations double compared to preindustrial, which would suggest the long-term temperature rise will be roughly 2 − 5 centigrade with 95% confidence – using the latest estimate of ECS.
However, the estimates in the table are mostly lower than this. Are they lower because:
Concentrations won’t double on these emission scenarios?
The world will still be warming in 2100, and won’t have yet reached equilibrium?
CO2 concentrations on the different shared socioeconomic pathways are shown in Table 5 here. On the most likely scenario—RCP4.5 - CO2 concentrations would double relative to pre-industrial by around 2060.
I think this comes down to the difference between the transient climate response to cumulative emissions and equilibrium climate sensitivity. On the assumption that CO2 concentrations stabilise, ECS tells you the warming you get eventually once the climate system has reached equilibrium (not including ice sheet feedbacks). If CO2 concentrations stabilise, then it would take decades to centuries for the system to reach equilibrium. Whereas the warming figures in the table is the warming you get at 2100. I have wrestled with trying to convert things to CO2 concentrations and then trying to infer warming from ECS, but it is unnecessary. CO2 concentrations will not stabilise, so the system will never truly be in equilibrium. The TCRE is much more informative.
How cumulative emissions translate into CO2 concentrations is model-dependent. 1 ppm of atmospheric CO2 is equivalent to 2.13 gigatonnes of airborne carbon. However, the amount of carbon that we burn that remains in the atmosphere (the airborne fraction) changes with emissions—the airborne fraction increases the more we emit because land and ocean carbon sinks get exhausted, which you can see in the doughnut charts below.
This is a useful post and updated my estimate of the chance of lots of warming (>5 degrees) downwards.
Quick question: Do you have a rough sense of how the different emission scenarios translate into concentration of CO2 in the atmosphere?
The reason I ask is that I had thought there’s a pretty good chance that concentrations double compared to preindustrial, which would suggest the long-term temperature rise will be roughly 2 − 5 centigrade with 95% confidence – using the latest estimate of ECS.
However, the estimates in the table are mostly lower than this. Are they lower because:
Concentrations won’t double on these emission scenarios?
The world will still be warming in 2100, and won’t have yet reached equilibrium?
Something else I’m not understanding?
Hi Ben,
CO2 concentrations on the different shared socioeconomic pathways are shown in Table 5 here. On the most likely scenario—RCP4.5 - CO2 concentrations would double relative to pre-industrial by around 2060.
I think this comes down to the difference between the transient climate response to cumulative emissions and equilibrium climate sensitivity. On the assumption that CO2 concentrations stabilise, ECS tells you the warming you get eventually once the climate system has reached equilibrium (not including ice sheet feedbacks). If CO2 concentrations stabilise, then it would take decades to centuries for the system to reach equilibrium. Whereas the warming figures in the table is the warming you get at 2100. I have wrestled with trying to convert things to CO2 concentrations and then trying to infer warming from ECS, but it is unnecessary. CO2 concentrations will not stabilise, so the system will never truly be in equilibrium. The TCRE is much more informative.
How cumulative emissions translate into CO2 concentrations is model-dependent. 1 ppm of atmospheric CO2 is equivalent to 2.13 gigatonnes of airborne carbon. However, the amount of carbon that we burn that remains in the atmosphere (the airborne fraction) changes with emissions—the airborne fraction increases the more we emit because land and ocean carbon sinks get exhausted, which you can see in the doughnut charts below.
That all makes sense, thank you!