1. Does the continued use of RCP8.5 in climate research represent "something of a breakdown in communication between energy systems modellers & the climate modelling community"?
2. Why do we have Representative Concentration Pathways (RCPs)?
It all started as a pragmatic solution to get new scenarios into AR5, which meant climate & energy system modellers worked in parallel.
3. The RCPs were essentially stripped of their socio-economics, deliberately, as in the "integration" phase would bring it all back together. link.springer.com/article/10.100…
4. If I freelance a little...
I recall energy & climate modellers trying to find the socioeconomics of the RCPs: what population, how much BECCS, etc. And would essentially try & read numbers direct of figures, etc.
5. The intention was that the RCPs were essentially naked, with the clothes returned via the integration phase.
The problem is that the RCPs are not comparable, they are from different models & socioeconomics. That message never really got out...
6. “RCP8.5 cannot be used as a no-climate-policy reference scenario for the other RCPs because RCP8.5’s socioeconomic, technology and biophysical assumptions differ from those of the other RCPs.”
7. AR5 is full of figures that compare RCP8.5 & RCP2.6, though the text is a little more nuanced.
These figures will obviously take people down the path that RCP8.5 is no policy & RCP2.6 is strong policy.
Comparing 4.5 & 6.0 will imply policy has little or no effect!
8. “RCP8.5 is, because of its assumptions of high population & slow technological progress, on the higher end of the range of possible baseline scenarios…I wished I would have been clearer with what I meant by business as usual in that paragraph.”
9. The new Shared Socioeconomic Pathway (SSP) framework is more explicit that there are a range of baselines (grey region), depending on socioeconomics (& IAM). Some baselines have declining emissions...
10. The energy system of each baseline is very different across SSPs, but also across IAM. In my view, the variation across IAM should receive as much attention as variation across SSPs...
11. Some baselines have pretty crazy coal use, something @jritch & others have argued is based on bad modelling assumptions.
Today, even without climate policy, it is likely solar & wind will out-compete coal in the future.
12. A fundamental issue, in my view, is that there is no longer a no policy world. We have climate policy, albeit weak, and that should be embedded in the baseline.
13. For balance, climate modellers like RCP8.5 for continuity with previous modelling, signal-to-noise, illustrate potential avoided impacts, potentially capturing unrepresented feedbacks, etc.
Sure, but eventually you have to let go of the past...
14. There is also a view that we are following high-end pathways. I wrote a paper on it (whoops) rdcu.be/brDGx.
Though, things have changed somewhat in the last 5 years...
15. Scenarios are often reported with 10 year time steps, scenarios are updated regularly, & in a sense, we are always on track...
This figure shows the latest SSPs (with the CMIP6 markers in bold)
16. It is best to look at progress relative to the underlying energy system.
In my view, if you push up baselines, imply lack of progress, then it makes 1.5°C or 2°C look harder. We should truly reflect how we are going, & how easy some gains are (even if 1.5°C or 2°C is out of reach)
18. A key challenge is that it takes decades for climate signals to emerge if two scenarios are close (bottom), even though, the mitigation pathways can be dramatically different (top).
Maybe that means we will forever have tensions between mitigation & impact research?
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I am still pondering over 2023 & El Nino. Is 2023 an (unusual) outlier or not?
Looking at anomaly in 2023 relative to the trendline (loess 50 year window), without (left) & with (right) annualised ENSO lags, then 2023 is rather mundane.
1/
When looking at the temperature change relative to the previous year, without (left) & with (right) annualised ENSO lags, then 2023 is more unusual depending on the lag.
If 2023 is unusual, then it could be equally explained by 2022 being low (rather than 2023 being high).
2/
There are numerous ways to consider ENSO. I have used annualised indexes, & various lags can be included. It is also possible to take sub-annual indexes (eg, several months), & again, various lags.
What is statistically best? I presume there is a paper on this.
I started to take an interest in the 2023 temperature increase...
The first plot I did, to my surprise, seems to suggest that 2023 is not unusual at all (given El Nino).
Why?
1/
It all depends on how you slice the data. The previous figure was the anomaly relative to a trend (loess with 50 year window).
If I plot the change from the previous year (delta T), then 2023 is more unusual. Though, still, is it 2023 that is unusual, or 2022, or 2016, or?
2/
The loess trend changes shape with the data, making the 2023 anomaly smaller. It is also possible to use a linear trend, making the 2023 anomaly larger.
Comparing the anomaly to a linear trend will make 2023 more important (than if loess is used).
I am not so convinced. The land sink has a lot of variability, mainly due to El Nino, and an El Nino overlapped 2023. So we expect a lower land sink in 2023.
(My estimate assumes the ocean sink was average).
1/
Was 2023 an El Nino year? That is not so obvious...
How does one average the monthly sea surface data to an annual value El Nino index? How does one account for the lag between El Nino and the change in atmospheric CO2 growth?
There is no unique answer to this.
2/
This figure shows the monthly El Nino index annualised with different time lags. 2023 is an El Nino or La Nina, depending on how you average!
@richardabetts & @chrisd_jones use a 9 month lag in their work (which means 2023 was a La Nina)!
Record high emissions means record high radiative forcing.
We have you covered, we also include aerosols (SO2, etc) & have done so for decades. Also shipping!
Short-lived aerosols are important, but should not distract from the drivers of change: greenhouse gas emissions!
2/
Most of the energy put into the system ends in the ocean (90%), so the Ocean Heat Content (OHC) has been increasing along with emissions and radiative forcing.
This also means the Earth Energy Imbalance is also increasing.