Imagine we see a 4-5 (or higher) degree increase in temperatures by the end of the century. Obviously, the impact on humanity would be catastrophic. At some point, perhaps just by so many people being dead, the levels of CO2 and other emissions would be drastically reduced. How long after that would it take the earth to return to a reasonably habitable place? Would there be any large portions of the world that remain reasonably suitable for modern society?
It is complicated by the fact we don’t know and can’t predict what kind of conflict will be set off as the coasts flood. Coastal and tidal river cities and towns represent over a billion people. It might be as high as 2.2 billion. This will be bad enough.
Will we be able to collectively keep sane and not release Chemical, Biological or Nuclear attacks?
Will the temp by 2100 be 2° higher or 5° higher. A 10’ rise wipes out the coasts, states like Florida and NJ will be half gone. The economic engines of the US, NY, Boston, LA, SF, Seattle and Portland will be largely gone. Not to mention DC, San Diego, New Orleans, etc. The only significant Florida city that will be left will be Orlando.
The recovery is largely predicted to be slow, very slow. But with a lower population, the human race can adjust. There is a really good chance this planet just isn’t meant to support more than maybe 5 billion of us. It would probably do a lot better with only a billion. As we near 8 billion, it is getting crazy.
Here is a pretty good report on What happens after …
https://www.nature.com/scitable/knowledge/library/what-happens-after-global-warming-25887608/
Any answers anyone gives you to your questions will be entirely speculative because the condition we are creating and the speed at which the climate is shifting is unprecedented even in the context of the fossil record going back to the last global extinction, the Middle Miocene disruption (~14 Mya), and in terms of similiarity of effects the Cenomanian-Turonian boundary event (92 to 96 Mya). You’ll hear widely discussed in climate change the concept of “tipping points” in climate, where a shift of certain conditions will cause a transition from one regime of stability to a different regime of equal stability at the new conditions. This is often accompanied by colorful 3D projections showing saddle points of global temperature versus CO2 concentrations and so forth, but it should be understood that all of this is based upon models that are not only outside of experience but because of the enormous complexities of various feedbacks cannot ever really be validated using existing data. Of course, the climate is non-equilibrium thermodynamic (NET) system and so any discussion of “stability” has to be caveated as some relative measure of variance about some mean rather than a constant.
What is quite apparent is that the current climate regime and especially the concentration of natural greenhouse gases in the atmosphere (primarily carbon dioxide and methane) is regulated by the biosphere and sits at an equilibrium point that the biosphere itself is evolved to tolerate. Industrial human society has not only released massive amounts of long-stored carbon and methane (as well as produced much more) but also potent artificial greenhouse gases such as nitrous oxide, carbon black perfluorocarbons, halogenated gases, et cetera. Most of the more complex products will break down on the span of tens to hundreds of years, but for the most part they break down into CO2, which will persist as long as it is not absorbed and mineralized. Given the ongoing destruction of many of the individual biosystems that absorb atmospheric carbon such as wetlands, coral reefs, boreal forests, savannas, et cetera, these would have to first recover before the (slow) reabsorption of atmospheric carbon would begin, and the rate at which they can absorb carbon is a tiny fraction of the speed at which carbon dioxide produced from combustion of oil, coal, and gas was released.
In the best case, even assuming a rapid halt to the release of further carbon dioxide, we should expect a new equilibrium point that is traps considerably more radiant energy from the Sun for tens if not hundreds of thousands of years, and potentially a new global thermal “set point” at which the biosphere will evolve to stabilize at, likely at several degrees Celsius above the pre-industrial level. That this is not conducive to human industrial civilization as we know it may actually be a boon to life adapted to such conditions as it will be able to thrive without routine interference and contamination from ‘development’, pollution, and agricultural runoff.
A population of 5 billion people, living even at basic sustenance level, is definitely greater than the sustainable carrying capacity of the planet. Paul and Anne Ehrlich have gotten a lot of grief over the decades for being “wrong” about predictions made in The Population Bomb, but those predictions explicitly did not account for improvements in agricultural yields made by the “Green Revolution”, and as has been noted elsewhere, those supposed gains were made at the expense of overuse of non-sustainable resources and have resulted in the contamination and destruction of natural resources such as wetlands drained for agricultural use that are potentially as much a contributor to destabilization of the climate as all of the carbon being pumped into the atmosphere. A population of 1 billion is probably about the limit for a sustainable human presence at something like current First World lifestyles, albeit still requiring more efficient and considered use of resources.
Stranger
Malthus is calling and says you are cribbing all his lines. 
No idea where you are getting that the planet can’t support X number of people from. The planet can support, with technology, a hell of a lot more people. We could be doing it today without impacting the climate if we were able to switch to alternative power systems that didn’t use so much fossil fuels. The issue is we are supporting all of these people with an ever-rising standard of living and wealth for more people (who in the past would have been subsistence level farmers with very little carbon footprint) and doing it with technology that is causing serious impact, not just with respect to CO2 but water and everything else. It doesn’t have to be that way, just like we don’t live in cities that periodically have a third to half to their population dead from typhoid or some other rampant disease today. Because it’s the way it is right now doesn’t mean it always has to be that way.
Anyway, as to the OP, it would be pure speculation. I don’t even know of a historical period where we had such a rapid climate shift was what the OP is proposing (4-5 degrees C in less than 100 years??)…maybe the Chicxulub impact caused a similar and sudden climate shift as we are seeing, or maybe the last major Yellowstone eruption or one of the other supervolcanoes, but it would be very, very bad.
A reduction in population due to disasters won’t “save us”. There will still be plenty of people, and the richest people with the highest consumption will generally not be the ones who suffer loss of life.
As to this question:
The Earth will still be a habitable place. There might be a meteor-wiping-out-the-dinosaurs level extinction event, but even that didn’t kill all medium/small species. And depending on the speed of changes modern society can adapt to changes in growth zones and swap out staple foods for ones that will survive in the (temporary) new climate.
Areas that are harsh or marginal today with regards to heat and risk of flooding will become uninhabitable, and our change in use of the remaining land will help accelerate the demise of additional species. But unless the fabric of society completely unravels, rather than “just” fray at the edges, there will still be human society. Now it it completely unravels I don’t see the human species survive except by luck in isolated pockets that just happen to make it through somewhat okay.
I had understood two of the major concerns with climate change is the sea level rising, causing major flooding for the coastal zones (where an overwhelming portion of the population lives), and severe crop failures. It’s hard to imagine current society surviving with those two conditions, but perhaps I am just too pessimistic?
The concept of the carrying capacity of a system is quite familiar to population ecologists and in the study of system ecology in general. Invoking “technology” as a kind of magical ward against resource overuse or depletion is not a persuasive argument because although technology improves the efficiency of processes it generally doesn’t create new resources, or if it does (in the case of the Haber-Bosch process of artificial nitrogen fixation) it does so at the expense of using up other resources and producing excessive waste. In population ecology, there are always limiting critical resources or factors that dictate the maximum level of sustainment for a population essentially regardless of the rate that the population uses them because the rate of renewal is so much slower than use.
In fact, it is our application of technology that has brought such an abrupt crisis with regard to global climate change (and other issues that are masked by the severity of climate change); it we were still living at pre-WWII population levels and pollution outputs, we would still be well beyond the sustainable carrying capacity of the planet, but it would take several centuries to get to a point of ecological crisis or resource depletion. Instead, it is the very technology that has allowed civilization to grow (and be incredibly wasteful in the developed world) which has masked depletion through its effectiveness and brought along such dramatic changes that are well and beyond the ability of any technology, existing or proposed, to materially affect.
I like having technology like computers and electricity and alloy steels, and use it in both my occupation and entertainment, but I’m not under any illusion that some magic hat trick is going to come up with a way to stop extreme weather events, prevent permafrost from thawing and topsoil from being washed away, or grind out nutrition for the billions of people affected by crop failures. The developed world has thus far been able to insulate itself from the worst effects, which are mostly limited to complaining how expensive avocados have become and debates over whether it is more of an ethical conundrum to eat hamburger or drink almond milk, but for a large portion of the global population food insecurity is a real and persistent threat that no amount of ‘technology’ is going to address.
Stranger
In the affluent parts of the word we’ll be able to move inland. We’ll also have more resources available to predict and mitigate climate impact on agriculture. It’ll be costly, but not deadly.
One thing I will add to some of the good information already posted is that a lot of our information about climate behaviour comes from empirical observations of paleoclimate reconstructions. One such chronology of particular interest is from the Paleocene-Eocene Thermal Maximum (PETM), a period of massive carbon release into the atmosphere that happened around 55.5 million years ago.
The period of carbon release is estimated to have lasted between 20,000 and 50,000 years, likely from undersea biomass and/or methane clathrates or the like, with the release rates varying over time but perhaps similar at some of their peaks to the incredible rapidity of modern GHG emissions. The PETM drove up global temperatures by between 5 and 8°C, similar to the upper end of the worst-case scenario that we may face in as little as another 100 years. The entire PETM lasted on the order of about 200,000 years, with most of that time taken up by slow and gradual cooling and re-sequestration of the carbon.
Another point of interest, in recent geological history and hence much more accurate, is the behaviour of semi-regular ice age cycles. The warming period as CO2 levels build is relatively rapid, taking on the order of about 10,000 years, but the cooling period is much slower, and is on the order of close to 100,000 years. During the transition to an interglacial, CO2 rises from a low of about 180 ppm at the peak of glaciation to a high of around 280 ppm at the peak of the interglacial, and global temperature rises between around 7 to 9°C. It then typically takes nearly 100,000 years to re-sequester that 100 ppm or so of CO2. Note that pre-industrial CO2 levels were around 285 ppm, so at more than 400 ppm (and rising fast) we currently have a significantly greater excess of CO2 in the atmosphere than the differential between an ice age and a warm interglacial, and the oceans hold more CO2 than at any time in at least the past 3 million years.
Every climate regime is different and that’s why it’s important for modeling studies like the ones described in the above-cited Nature article to be carried out, but both tentative modeling and paleoclimate data suggest that the time period for some nominal amount of climate recovery should be measured, not in hundreds of years, but in thousands, and likely many tens of thousands of years or more.
Beyond this, many important questions remain. If we do hit a tipping point – and tipping points are a common feature of historical climate behaviour – recovery time may be much longer or in a practical sense may never happen. Tipping points would be more likely if we continue the climate madness long enough to effect long-lasting significant changes in polar ice cover, for example. Either way, with or without triggering a tipping point, it’s virtually certain that a complete “return to normal” will never happen; in terms of global average temperatures, global circulation system changes, major ecosystem changes and large-scale extinctions – those things we’re just going to have to live with as we stabilize to a “new normal”.
Perhaps one of the most important questions, then, is not how long it will take temperatures to fully recover to pre-industrial levels, or for our terminally damaged ecosystem to ever recover, but how long will we be faced with extreme weather events as the global climate system reacts to the strong forcings of unprecedentedly rapid GHG emissions. That question, at least, may have a somewhat more optimistic answer than the others. But the answers to all of them hinge critically on how effectively we are able to react to mitigate the climate crisis.