Thread by @CarlSchleussner

Accuracy and transparency of science communication on 1.5C matters more than ever. Response to @hausfath @dhofstetter_x
@MichaelEMann
@leahstokes

https://twitter.com/hausfath/status/1385707599757123587?s=20

1/

First: Uncertainty language. The IPCC has assessed pathways "as likely as not" to limit warming to 1.5°C. Peak warming 0.34<P(1.5°C)<0.67. Claim to exceed 1.5C was "very likely" or even "virtually impossible" would require 0.9<P(1.5°C) in IPCC uncertainty language. 2/

This is not just some technical detail. It's not black-white "we can make 1.5C" or "we cannot make 1.5C". There is a huge "as likely as not" range in between due to geophysical uncertainties. A lot of grey. This is where we are in now. 3/

Next point: carbon budgets are way too uncertain to be useful for GMT exceedance. They communicate that stringent action is needed. But lack precision on warming outcome. Non-CO2s matter, etc. See e.g. here: doi.org/10.1038/s43247… 4/

More on limitations on carbon budgets for 1.5 feasibility e.g. here:
climateanalytics.org/briefings/15c-…
climateanalytics.org/publications/2…

There's also a common misunderstanding on scale of net-negative emissions required for 1.5°C pathways. Issues with IAM assumptions widely discussed, but importance of scenario logic often not understood. nature.com/articles/s4158… 5/

Modelling assumptions rather than peak warming determine CO2 removal needs in 1.5C pathways. Assumption of 66% in 2100 alone implies around 400-600 Gt net negative CDR after peak warming. More on this here: 6/ essoar.org/pdfjs/10.1002/…

As most 1.5 pathways in SR.15 (34/53) assume 66% <1.5°C in 2100 after overshoot, one could be quick to conclude that large scale NETs are a key requirement to limit (peak) warming to 1.5°C. But they are not. Achieving zero emissions as soon as possible is. 7/

Side note: "as likely as not" below 1.5C pathways are also "very likely" to hold warming "well below 2C" at all times. Arguably an appropriate interpretation of "well below 2°C". Would question this for a 33% or even 50% chance of exceedance. nature.com/articles/nclim…

Conclusion. Assessment of @IPCC_CH SR.15 still holds. Limiting warming to 1.5C is still possible. Needs rapid decarbonisation in the next decade halving 2030 levels compared to first round of NDCs, net zero CO2 mid century. And a bit of luck. 8/

In fact, not much has changed since SR.15 understanding apart from a pandemic and elections in between, which, if anything have made stringent near-term emission reductions more plausible. @JoeriRogelj @mAndrijevic science.sciencemag.org/content/370/65… 9/

Are we seeing enough of this? No. Progress is being made, but much more needed. esd.copernicus.org/articles/11/69… @climateactiontr climateactiontracker.org/press/new-mome… End.