[cconroy]

Is it known what are, perhaps narrow, the climate conditions for humans? Humans evolved during the Pliocene which was 2-3 degrees warmer than today.

My gut feeling is not good, especially on geologic time, of which most spans humans would most certainly not survive. It feels like we are tossing 100s of thousands of years of stable climate and pushing Earth to points in its history were fauna/flora were very different from today.

[shalmanese]

Are you asking about humans or civilization? If you look at the habitable range of humans today, obviously a change in a few degrees is not going to do much, some previously habitable land will become uninhabitable and vice versa but large swathes will remain.

Civilization is a lot more fragile and dependant on the pace of change, not the absolute magnitude. The problem with civilization is it's like a Jenga tower, you can continue building higher and higher up the complexity ladder and gradually knocking out supporting pegs for a little bit of time and things will seem fine, but keep knocking out just a few of the wrong pegs and there's no halfway state of falling, it's either intact or severely collapsed.

[cconroy]

Humans, on time scale of 100s of thousands of years. You wouldn't expect Earth to be too dissimilar from today, if not for fossil fuel consumption.

[RupertEisenhart]

See my post below, but here is one paper: https://www.pnas.org/content/117/21/11350

[tzs]

A few degrees either way would not change whether or not humans can survive in most places. What it would change for many places is (1) how much effort it requires to live there, and (2) how many people can live there.

E.g., if climate change greatly reduces the water supply in a region, that can be address with effort (transporting water in from somewhere else) or by cutting the population in the region down to what the reduced supply can support.

The problem is finding a way to fund that effort in such areas or to reduce their populations without it degenerating into wars and terrorism.

As an example, consider India and Pakistan. They both greatly depend on water from Himalayan glaciers that are rapidly shrinking. It is not at all hard to imagine them going to war with each other over how to allocate that water between them.

They both have nukes. You might think a nuclear war between them would not be a threat to the rest of the world because neither of them have big nukes. But it is not the nukes that would hurt the rest of the world. It is the firestorms.

Here's a paper [1] and article [2] that looks at a hypothetical nuclear war between them involving about 100 nukes the size of the Hiroshima bomb from WW II, directed at the major population centers. That's about 1/3 of the nukes they have available.

Based on the amount of combustible material in the target areas, they estimate 1.5 Tg of soot aerosol would end up in the upper atmosphere. They then use climate models to predict what that would do to temperature and precipitation, and applied that to crop models.

What they get is several years of temperature reduction and precipitation reduction. The strongest effects would be in the temperate regions of the US, Europe, and China, and last 10-15 years. That should result in serious worldwide food shortages for quite a while.

Note that the shortages would not just be in the poor, undeveloped countries. This would be food shortages even in the rich, developed world. It is not unlikely that this would lead to more wars.

[1] https://www.pnas.org/content/117/13/7071

[2] https://blogs.ei.columbia.edu/2020/03/16/even-limited-india-...