No idea if or how it might be calculated, but "Percent of humanity who will need to move elsewhere. Either because their old homes are now too besieged by flood / fire / drought / extreme weather to be viable any more, or because the agricultural areas previously providing them with food are similarly besieged."
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That kind of figure is isn't just determined by climate science but also by economics. Relatively well-off people in rich countries can hang on for a lot longer in the face of unpleasant climate trends than world's poor can. The better-off can afford air conditioning systems, rooftop solar, etc. First world governments will often pay for sea walls, flood mitigation systems, desalination plants (indeed in many places they already are).
True - but as a recent condo tower collapse in Florida illustrated, "can pay" !== "pay", even when there are no NIMBY, legal, environmental, etc. barriers at all. Thousands of communities in California, Oregon, etc. could be far more resistant to wildfires...but aren't. If New York City gets flooded by another hurricane or few, will angry culture-war America rush to spend enough $billions to build a huge sea wall system? Or will the rich start leaving NYC for higher ground, similar to how so many of them left during NYC's worst COVID spells? Large sea wall and flood-control systems can take decades to agree upon, design, and actually construct.
At the other end of the spectrum - ask a European politician how great it is, when things are just fine in your relatively rich country...but an endless tide of desperate refugees from broken countries are washing up on your shores. (I had that in mind when I originally said "Percent of humanity who...".)
To try to put numbers on it - at least in America, rising sea levels, shoreline erosion, etc. have been chewing away at well-to-do coastal communities for decades now. Getting enough real-world data to say "If it would cost $X to seemingly protect a $Y property for the next Z years, then there is a W% chance that the $X will be spent" would not be all that difficult.
BUT - historical $X / $Y / Z / W% numbers could badly under-estimate the rich-country problems, if (say) public perception was that Greenland's ice sheet was doomed. (Melting that would add ~7 meters to global sea levels.) Perceived Z's would fall, and folks figuring that the deal was "after Z years at best, we'd lose both $X and $Y" might cause W% to plunge.
> If New York City gets flooded by another hurricane or few, will angry culture-war America rush to spend enough $billions to build a huge sea wall system? Or will the rich start leaving NYC for higher ground, similar to how so many of them left during NYC's worst COVID spells?
NYC is the media, financial and cultural capital of the US, and the diplomatic capital of the world. The US cannot allow flooding to ruin it; that would cause immense damage to both the US national economy and national prestige. The US is legally obligated to protect the UN Headquarters from flooding; if it fails to do so, the UN may respond by moving their headquarters somewhere else, which would be a huge loss of national prestige and diplomatic influence. So if the city or state governments don't do something about it, federal intervention would be very likely.
> Large sea wall and flood-control systems can take decades to agree upon, design, and actually construct.
In a national emergency things can happen very fast. Special funding becomes available, ordinary regulations can be suspended, special legislation can be passed. COVID is a good example of this. Normally it takes years to get new vaccines approved by the FDA. But the FDA suspended its normal approval process. Similarly, if coastal flooding starts to get a lot worse, don't be surprised if Congress establishes a special multi-trillion dollar "Coastal Defense Fund", suspends all environmental and planning regulations, and the President sends in the Army Corps of Engineers to build it. Major metropolises such as NYC will be a priority.
> At the other end of the spectrum - ask a European politician how great it is, when things are just fine in your relatively rich country...but an endless tide of desperate refugees from broken countries are washing up on your shores.
Australia basically eliminated the problem of people arriving on boats by taking an extremely hard line against them. The Royal Australian Navy would intercept them at sea and take them to camps in a third country (such as Papua New Guinea or Nauru). PNG and Nauru were happy to host these camps because the Australian government paid them untold millions for the privilege. Many people argue that the Australian government's actions are immoral and even illegal (under human rights laws and the Refugee Convention). But the Australian government has gotten away with it, and the majority of Australian voters rewarded them. Could European countries take a similar approach? I think they could, and maybe in the future some of them will. Europe accepts millions of refugees out of a moral belief that it is the right thing to do, not out of a practical impossibility of refusing them. The anti-refugee measures of Viktor Orban's right-wing government in Hungary are an example of the practical possibility of refusal. Far-right political parties are growing in Europe, and stand a real chance of winning elections, and many of them admire Australia's approach and hope to emulate aspects of it.