I can see all of these happening but billions dead is a really high number so my question is more about the quantitive aspect. Are there estimates that land on this number, or is it just a high number more or less guessed
Well, the Black Death killed ~15% of the world's population in the 14th century[1]. 15% today would be about 1.17 billion people. It's horrific but not implausible.
Yes but that was during a time before we had even entered Enlightenment. I don't see how it says anything about consequences of a 1.5 degree average warming.
I don't know about how many deaths this would cause, but there was an interesting paper that looked at the effects of a limited nuclear war between India and Pakistan [1]. Here's a blog post about it [2].
India and Pakistan both depend on water from Himalayan glaciers that are shrinking due to warming, so the idea of them at some point going to war over control of that shrinking resource is not too far fetched.
What's interesting is how much of the rest of the world could be affected.
The paper looks at a war where the two counties exchange 100 nukes each about the size of the bomb used on Hiroshima, directed at each other's major population centers. That's about 1/3 of their arsenals. Besides killing a lot of people in those population centers, this would set of firestorms that would release a lot of soot into the upper atmosphere.
The paper used state of the art atmospheric models to predict what would happen to that soot, and state of the art crop models and food distribution models to predict what that could do.
Here's the abstract:
> A limited nuclear war between India and Pakistan could ignite fires large enough to emit more than 5 Tg of soot into the stratosphere. Climate model simulations have shown severe resulting climate perturbations with declines in global mean temperature by 1.8 °C and precipitation by 8%, for at least 5 y. Here we evaluate impacts for the global food system. Six harmonized state-of-the-art crop models show that global caloric production from maize, wheat, rice, and soybean falls by 13 (±1)%, 11 (±8)%, 3 (±5)%, and 17 (±2)% over 5 y. Total single-year losses of 12 (±4)% quadruple the largest observed historical anomaly and exceed impacts caused by historic droughts and volcanic eruptions. Colder temperatures drive losses more than changes in precipitation and solar radiation, leading to strongest impacts in temperate regions poleward of 30°N, including the United States, Europe, and China for 10 to 15 y. Integrated food trade network analyses show that domestic reserves and global trade can largely buffer the production anomaly in the first year. Persistent multiyear losses, however, would constrain domestic food availability and propagate to the Global South, especially to food-insecure countries. By year 5, maize and wheat availability would decrease by 13% globally and by more than 20% in 71 countries with a cumulative population of 1.3 billion people. In view of increasing instability in South Asia, this study shows that a regional conflict using <1% of the worldwide nuclear arsenal could have adverse consequences for global food security unmatched in modern history.
I bet that would be very destabilizing in the US and Europe. Think about how crazy some people got during COVID in the US with the relatively minor (but annoying) shortages during that. This would be worse.
Famines due to changing weather patterns causing widespread harvest failures, wars due to reduces resources and changing liveability of regions, far bigger refugee crises than we've seen so far, etc.
We're in general really dependent on our climate for global stability.
