Thread by @CarlSchleussner

I’ve been seeing a lot of talk recently that it’s certain that we will fail to limit global warming to #1o5C. I don’t think the scientific evidence supports such claims and I would urge caution before drawing absolute conclusions. A thread 1/

For a start, it seems such claims are linked to the question of whether the world will half emissions by 2030. This is deemed unrealistic by some, therefore 1.5°C must be lost. Lots to say about this, but first and foremost there’s an important logical flaw here. 2/

The science on pathways to 1.5°C is not a simplistic yes/no black/white question. It’s not ‘we can achieve it’ or ‘it’s lost’ - there is a lot of grey. It’s a question of probabilities, and we are deep in the grey zone already. 3/

Pathways (or carbon budgets) linked to limiting warming to 1.5°C without overshoot commonly have a 50% chance to do so. So 1.5°C could well be exceeded even in such a pathway, as the uncertainty range is very substantial. More on this: constrain-eu.org/wp-content/upl… @CONSTRAIN_EU 4/

But uncertainty cuts both ways. If we call a 1-in-2 chance ‘achievable’ I don’t think it makes sense to say, that e.g. a 1-in-3 chance is ‘certainly lost’. A high probability would be required to have ‘certainty’ that we would exceed 1.5°C (IPCC language would require >=99%.) 5/

Clearly, only stringent reductions by 2030 will keep 1.5°C “within reach”. Without them, it’s slipping away, it’s increasingly unlikely etc. All phrases describing the ‘grey zone’. But being certain it’s lost requires a higher burden of proof. 6/

There might come a point where warming reaches 1.5°C. But we would only really be able to say this in hindsight - probably only up to a decade after. Why is that? 7/

First, the PA temperature goal refers to human-made warming. We measure this + natural variability. A single year (even a couple of years) above 1.5°C doesn’t mean the limit is breached... 8/

...It needs to be the long-term multi-decadal average or a statistical method to identify the human-made component. carbonbrief.org/guest-post-int… 9/

And the assessment of human-made warming from an ongoing temperature record is subject to uncertainties. Here is from the AR6 WG1 - uncertainty on decadal timescales of about ± 0.25°C. 10/

We’ve looked into this in a paper led by @kasia_tokarska and shows that we will only be able to say in hindsight that we have passed a temperature level. 11/

iopscience.iop.org/article/10.108…

Taken together, this means that while 1.5°C will become continuously less likely with every year we fail to bend the emissions curve, and it might take well into the 2030s before we would have scientific certainty that we will or have passed that level. 12/

Communication on this matter should take this into account. 13/

This brings me to my last point, leaving the realms of climate physics: it seems to me that the judgement of whether 1.5°C is still within reach is right now more based on a personal opinion than on hard science. 14/

Everyone’s of course entitled to their opinion on whether they think halving emissions by 2030 is a realistic or plausible scenario. But it should be clear what is a personal opinion and what is scientific evidence. In particular when scientists are providing their views. 15/

And it should also be clear that it is a matter of values (desirability) as much as it is about feasible transformations towards the goal. We generally focus on feasibility, but when we decide something is desirable, the feasibility frontier can shift dramatically. 16/

If anything, the accumulating evidence on the scale and severity of climate impacts >1.5°C, including crossing potentially irreversible thresholds, make limiting warming to 1.5°C ever more ‘desirable’ - if that’s the right word to use in this context. 17/

science.org/doi/10.1126/sc…

Full disclaimer: I share everyone’s frustration about the state of global climate action to date. But this critical decade has 8 years more to go. Walking away from this ambition when we will never have a better shot at averting the worst would be an enormous moral failure. 18/

Every action that brings emissions down now, brings us closer to ‘keeping 1.5°C within reach’. end/