To address your siderant...
****TL;DR Drought. Desertification. The kind that won't stop until the remnants of humanity are only at the Arctic circle by 2100.
My parent post is precisely the kind of mindset I had until sometime last year when I researched this more in-depth. I think this is ultimately why first-world citizens are still ambivalent to climate change: we don't appreciate the potential magnitude of what's going to happen.
So the planet warms a few degrees and some cuddly animals go extinct, maybe even lose some ocean-front property, why should I care?
When I looked at the latest papers myself, I was shocked. The models and predictions have way more confidence than they used to, but scientists simply aren't framing the results at all for the public anymore. (This is understandable due to the fear of stirring up political backlash, but it means the public at large is becoming more and more desensitized to the biggest threat to our species) [Keep in mind that climate change nearly wiped us out 100,000 years ago. Did you know that we all come from roughly 100 surviving humans who holed up in a beach-front cave in South Africa?]
As the Hadley cells expand, the deserts near the equator will grow in size, and the temperate zones will constantly shift North. How much they grow depends fully on what we (and by we I mean China and India) do RIGHT NOW (and by right now, I really mean ten years ago, not ten years from now). If we (again, humanity as a whole, the US makes almost no impact from here on out **except by leading by example**) continue to exponentially grow our coal industry at full steam for the next few decades, then the areas in extreme drought (think worse than the US dust bowl) will basically cover the entire planet by 2100.
Comparing our current CO2 ppm to the dinosaur era isn't accurate enough to spell out our future (I wish it were). Never before has CO2 concentration increased this quickly in such a short period of time. The climate changed gradually over millions of years in the dinosaur era. Biodiversity can easily morph on a planet-wide scale on that time period. Here, we've done that same ppm transition in roughly a century, and with our exponential growth, we're likely to nearly *double* our current concentration in another 50 years. That concentration alone is unprecedented in our planet's entire history, let alone the impossibly short time scale that we'll achieve it in.
Once we appreciate that history has no lessons for our new unchartered territory, we look long and hard at the best simulations we've made to date, which take into account the continents' shapes and locations (which are extremely different from Pangea, I might add), and you find a very grim story: The ocean loses its ability to carbon-sink, the permafrost begins melting and emitting methane, and you have accelerated warming even without anymore added help from us. The result is a disruption of all jet-streams to the point where every year looks different from the previous. Very wet to very dry. Hot to cold. Our existing breadbaskets simply can not handle this level of volatility. Massive crop failure will become the "new normal," and the world's impoverished will be ravaged with famine. Famine is the biggest driver of civil unrest, which probably means many third world countries will simply cease to exist as they degrade into permanent chaos. Almost half of the entire world will become malnourished. This is to say nothing about how the extreme droughts will affect water scarcity, which is already considered the world's biggest risk in terms of likeliness and impact by the World Economic Forum. Water scarcity won't hit the world as hard in more developed regions, because we will burn more energy (like oil, natural gas, and coal...and yes, **eventually** solar power) to desalinate water.
That's the first whammy to humanity. As time marches on, you'll then find the steady desertifcation I mentioned above (Google 'future drought map' and just look at those images until it sinks in). Northern Europe, Canada, and ultimately Russia will, as you alluded to, become wonderful places to grow crops, but the rest of the world will not. Think about the deaths, wars, and sheer magnitude of refugees that will all entail. However, unless we then manage astounding engineering feats requiring resources and coordinated efforts that dwarf World War 2, warming will continue even if we're mostly renewable energy by that point thanks to the long half-life of CO2. That warming will continue the inexorable march of northward desertification until your grandchild is buying oceanfront property in north Canada for a jillion dollars.
What happens after that is a bit speculative, but there is now evidence that mass-desertification of the oceans might explain many of our planet's previous mass-extinction events better than any other phenomenon. (Google 'Canfield ocean') It supposedly is linked to very high CO2 concentrations even beyond that which the dinosaurs enjoyed. Fortunately, all of the work in that field indicates we have at least 150 years to combat the full brunt of that kind of phenomenon. Far longer than the -5 to 15 years we have to fight climate change before it becomes virtually intractable.
To sum up, what the United States does now makes very little difference. We aren't growing anymore in terms of CO2, while developing nations are set to unleash huge exponential growth that will require massive amounts of power. Whatever we do has to take China's view on us into account: that's the only way we can make a difference. But more importantly, the trends on a log plot don't lie. What's coming to hit us like a ton of bricks isn't going to be stopped just like you can't convince 2 billion people to suddenly do what you want. What the world needs now is better climate science and better engineering so that we can quickly and cheaply offset what's coming when the time is right, because right now, we're nowhere close to ready.
If we don't select the outcome for ourselves, nature will do it for us, whether we believe in it or not.