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Glen Peters @Peters_Glen
, 15 tweets, 7 min read Read on Twitter
THREAD (on temperature, overshoot, & afforestation in #SR15)

In this thread I group a few nice (ie, unreadable) Twitter discussions on a few intriguing aspects of the Special Report on 1.5°C:
* What is "no overshoot"?
* What is "temperature"?
* Land impacts of afforestation? 1/
The SPM to the IPCC Special Report on 1.5°C highlighted 4 illustrative scenarios. This is fine. Three scenarios are called "no or limited overshoot". But, why are they grouped as "no or limited overshoot", & not as "no overshoot" or "limited overshoot"? 2/
(Aside: In Chapter 2, the illustrative scenarios are labelled LED, S1, S2, S3, in the Summary for Policy Makers they are labelled P1, P2, P3, P4) /2b
A rather obvious assumption to make is that "no overshoot" is "Below 1.5C", "limited overshoot" is "1.5C-low-OS", & "high overshoot" is "1.5C-high-OS". The sequencing of scenarios from no to high BECCS supports this assumption. 3/
According to the #SR15 scenario database, the illustrative scenarios are 3 in limited overshoot & 1 in high overshoot, with none in below 1.5°C. The P2 (S1) scenario, actually has a lower peak temperature than P1 (LED), that is, it has less overshoot. 4/…
Intriguing point 1: I don't know the background to the labelling of the illustrative scenarios, but it seems a tad misleading to use "no or limited overshoot" when there are actually no "no overshoot" scenarios. Many will make this mistake (as I did)! 5/
#SR15 classifies scenarios into groups (temperature level, overshoot, etc) using the temperature response from the simple climate model MAGICC, though #SR15 also includes results from a new model called FAIR. 6/
There is a rather significant difference between FAIR & MAGICC, say 0.25°C for peak temperatures. Sounds small, but given we are at 1°C & want to stop at 1.5°C, then 0.25°C is halfway to 1.5°C... The figure is for "Below 1.5°C" (no overshoot) scenarios 7/
This is the figure for "lower 2°C" (66% probability of staying below 2°C). All data from…. 8/
Intriguing point 2: The difference is noted in #SR15, rather subtle, but I would imagine if FAIR was actually more realistic than MAGICC, then that would mean a fundamental change to #SR15 (at least to Chapter 2 & questions of feasibility). 9/
Finally, the illustrative scenarios focus on no to high BECCS (it seems), but there is little discussion of the impacts of afforestation (high in all illustrative scenarios). The land-use change impacts of afforestation is actually more significant than bioenergy! 10/
Here are the 1.5°C scenarios that have data on land for afforestation, which is of similar magnitude to the areas for bioenergy. For perspective, India is 330 million hectares, so afforestation is 1-4 India's... 11/…
Intriguing point 3: My impression is that in the SPM there is very little discussion of land areas for afforestration, but the land areas are comparable or even larger than for bioenergy! Not sure why that is? 12/12
CC: @richardabetts @DrSimEvans @mammuthus @civiltalker @Oliver_Geden (who took part in the conversation)
Those that are game, can try and follow the actual thread here . It is actually really hard, but believe me, you will find most of the discussion there (except afforestation, which was a separate thread)
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