RealClimate: Unforced variations: Feb 2024

Julian: – “Since the last comment I made back in the November last year, I’ve decided to learn more about paleobiology.”

I presume this was you – see link? There’s an extensive comment thread that follows…
https://www.realclimate.org/index.php/archives/2023/11/unforced-variations-nov-2023/#comment-815434

Julian: – “1. How good are the models? I know the “all models are bad, but some are useful” mantra, but given the amount of effort that went into CMIP, they must be at the very least somewhat accurate, right?”

Reality is happening faster than most models have forecast.
Models don’t do well with non-linear phenomena.

The 2023 extremes were a shock. Prof. Katharine Hayhoe told the Guardian that: “We have strongly suspected for a while that our projections are underestimating extremes, a suspicion that recent extremes have proven likely to be true… We are truly in uncharted territory in terms of the history of human civilisation on this planet.”

https://johnmenadue.com/humanitys-new-era-of-global-boiling-climates-2023-annus-horribilis/

Julian: – “2. How likely are temperatures to decline in the future (several centuries) if we stopped emitting by mid-century?”

I think one cannot dismiss what Professor H. J. Schellnhuber CBE said on 17 Dec 2018 in delivering his Aurelio Peccei Lecture, titled Climate, Complexity, Conversion, which can be seen/heard in the YouTube video titled Keynote Debate Can the Climate Emergency Action Plan lead to Collective Action? (50 Years CoR), duration 2:23:08. I’d suggest the relevant statements by Prof Shellnhuber that may answer your question begin from time interval (bold text my emphasis):

0:20:56: Schellnhuber: “So, some people have speculated the next ice age will be next week. I can tell you: It’s not true! Don’t believe that! [audience chuckles] It will happen… I blow it up… Actually, never again! That’s why we are in the Anthropocene. Remember, if the blue line is crossing or cutting the black line, from the left, there will be another glacial inception. Now this is a hundred-thousand years into the future, and if you look where, in fifty-thousand years, there would be another ice age, but only if the CO₂ would not be influenced by human intervention. Actually now, the atmospheric content is, according to the orange line, and you see, the lines are not crossing anymore, but we will add another billion, and hundred-billions of tonnes CO₂, where rather we will have to use the brown line, so there will be no ice age anymore. The human impact is so powerful already – that’s why we talk about the Anthropocene – that we have suppressed the Quaternary planetary dynamics already.”

0:22:15: Schellnhuber: “This is a fact… but let’s see what will happen in the future beyond that. So, just for you to remember, the Holocene… Holocene mode of operation, the last twelve-thousand years where human civilisation was created, will not come back, not for the next millions of years. It’s just… done!”

0:46:12: Schellnhuber: “Ja. OK, let me answer it directly, because it is such a rich question, ja? So I will not take others for the time being, but of course later. Now first of all, we are not mixing-up timescales. We have to consider all of them in parallel, unfortunately, ja? And I just introduced the Pliocene and the Miocene and all these, ah… stupid names, er… geologists have developed, ja, simply because this is our reality lab, ja? I mean, if I cannot see under comparable conditions, a major shift in the state of the planet, in the back, er… in the… in the… back in fifteen-million years, when I have no evidence, actually. So, this is just in order to underpin some of the things. And looking forward, I mean, I excuse for… I apologise for that, but… we have actually ended the ice age cycle, the, er… the glacial dynamics for good, or for bad, or for whatever – that’s how it is. But your question is of course extremely important, because… I… I once coined… We had a meeting at the Belgian Academy of Sciences and I coined this expression, which became quite… quite, er… sort of seminal, actually: ‘Avoiding the unmanageable and managing the unavoidable.’ So you see, avoiding the unmanageable would be three, four, five, six degrees. I’m, I’m pretty sure we cannot adapt to that. But if the world warms by one… it has warmed already by one degree, and actually half of a degree is masked by air pollution. So if you would clean the air over China and India and so on, you immediately would… you get another half degree. So, one-and-a-half degree – we are there already, ja? But if we stop it at two, er… two-point-five degrees maybe… and actually CO₂ stays within the carbon cycle for more than twenty-thousand years. People think this is a matter of a hundred years. Yes, it goes into the sediment, but it’s re-mineralised and goes back into the air, and so on. So it’s longer lived than plutonium, actually, ja? Atmospheric CO₂!”

0:48:29: Schellnhuber: “So, yes… can we survive those five-hundred years, if we… hold the two-point-five- or two-degrees line? Yes, we could – not everybody. But we would need to introduce real instruments for adaptation.”
https://www.youtube.com/watch?v=QK2XLeGmHtE

I’d suggest you see/hear the full Schellnhuber address and Q&A that follows.

What I find astounding is this session occurred more than 5¼ years ago, and it seems to me the mainstream media still haven’t reported what was known then. IMO, the media has a lot to answer for…

Julian: – “4. Lastly, what do?”

Reduce. Remove. Repair.

Paul Beckwith uses the analogy of the three-legged bar stool – without all three legs in place (i.e. “slashing fossil fuel emission, carbon dioxide removal, that includes methane removal; and solar radiation management“) the bar stool (i.e. a habitable planet for civilisation) will fall over.
https://www.realclimate.org/index.php/archives/2024/01/unforced-variations-jan-2024/comment-page-2/#comment-818785