Thread by @Peters_Glen

THREAD "Limiting climate change to 1.5°C is now virtually impossible"

Therefore, a report that focuses on 3°C temperature rise by 2100 (2.7–3.1°C based on current climate policies).

While noting "acting early & urgently reduces the scale of the impacts"

science.org.au/supporting-sci…

2. I am not sure what the fuss is about "virtually impossible"? Has anyone read the 'consensus' #IPCC #SR15?

The SPM writes 1.5°C pathways "require rapid & far-reaching transitions in energy, land, urban & infrastructure and industrial systems (𝒉𝒊𝒈𝒉 𝒄𝒐𝒏𝒇𝒊𝒅𝒆𝒏𝒄𝒆)"

3. Current "ambitions would not limit global warming to 1.5°C (𝒉𝒊𝒈𝒉 𝒄𝒐𝒏𝒇𝒊𝒅𝒆𝒏𝒄𝒆)"

Not even 'virtually', just "not" possible!

Noting, that even updated pledges so far lead to a 1% decrease in global emissions, not the required 45% reduction!

4. Even if the world follows the oft-cited 50% by 2030, net-zero CO₂ by 2050, then CDR on massive scale in scenarios (100–1000 GtCO₂).

"CDR deployment of several hundreds of GtCO₂ is subject to multiple feasibility & sustainability constraints (𝒉𝒊𝒈𝒉 𝒄𝒐𝒏𝒇𝒊𝒅𝒆𝒏𝒄𝒆)"

5. Yes, the 'carbon budget' is uncertain. Yet, everyone uses the smaller 66% budget for 1.5°C, not the 33% the-budget-is-uncertain bigger budget.

(Deduct 3*40=120GtCO₂ from these budgets to be relevant from today).

Of course, IPCC AR6 will update these budgets. Maybe bigger?

6. This paper really shows the uncertainty in a 1.5°C remaining carbon budget. It could be negative or it could be large.

An uncertain carbon budget could change 1.5°C from "virtually impossible" to anything from "impossible" to just "challenging".

nature.com/articles/s4324…

7. I am going to get @'ed this thread from @JoeriRogelj a billion times today.

I understand Joeri thinks "virtually impossible" is overplaying, but saying 1.5°C is "challenging" is underplaying (IMHO).

Note: "virtually impossible" ≠ "impossible"

8. With "virtually impossible", it is important to note there is still a huge role for deep mitigation.

The IPCC did a good job at trying to reframe from a 1.5°C 'deadline'.

“Every bit of warming matters, every year matters, every choice matters”

9. It seems 1.5°C has come down to semantics, "virtually impossible" versus "challenging"?

I see my job as a scientist to be frank on the challenges, but to doomsdayers, noting the challenges is not the same as giving up. Still plenty to fight for.
cicero.oslo.no/no/posts/klima…

/end

Bonus tweet 1: I think the scientific community has done itself a disservice with framing carbon budgets as single numbers with a probability (eg 475 GtCO₂ for a 66% chance).

Better to use ranges (eg 760 GtCO₂ with 33–66% range 475–930 GtCO₂). rdcu.be/bHT2C

Bonus tweet 2: Using a single number gives a false precision, & leads to the "12 years to..." framing.

A range explicitly brings in the uncertainty, & allows more nuance to be given on the feasibility of 1.5°C. This would avoid much of this thread...

Bonus tweet 3: Here is the 5-95% temperature uncertainty for 1.5°C scenarios from two simple climate models (FAIR, MAGICC) used in SR15. The peak temperature ranges from 1.1°C to >2°C.

If FAIR is correct, 1.5°C is not 'virtually impossible'...