I suppose it could lead to a Venus like atmosphere if things got really crazy. Realistically the worst case scenario would just lead to coastal areas being uninhabitable and widespread famine and unrest while the worlds farming and population gets redistributed to more habitable latitudes. I don't think it would ever wipe out EVERYONE even if it's a sizable percentage.
There is no "redistribution to more habitable latitudes" (a breathtakingly vast understatement of human suffering) without all out war. There is no all out war without nuclear destruction at our own hand.
If your plan involves moving the entirety of food production to areas of the globe that are currently permafrost and have no comparable ecosystems to where we are currently farming, you are going to have a bad time. You can't just bootstrap a healthy, thriving set of soil conditions, food webs, etc in a couple years. These things normally take thousands or even tens of thousands of years on geological timescales. Just look at how long it takes deciduous forests to reach their apex state: thousands of years.
I agree with you. But you don't have to go quite all the way to Venus for a plausible scenario that would, probably, wipe out humanity. See page 6 of this link for a scary infographic.
Intuitively it seems very difficult to see how things could go all the way towards being Venus-like if all the positive feedback mechanisms kick off. We're really, really similar to Venus, but Earth's crust wouldn't appear to have that many greenhouse chemicals stored up. (Unless geologists have failed to account for something)
But there's a long distance between Venusian hellscape (hot enough to melt lead and raining sulfuric acid) and what we have today, and most of it is pretty awful for us. "Kill everybody" isn't crazy.
Sorry, we don't even understand the 'realistic' worst case scenario. We've blown past previous estimates of temperature rise and atmospheric co2 by decades. It's a nonlinear system with many unknowns, this is why its critical we operate on the precautionary principle.
No climate forecast that I'm aware of predicts human extinction; much of Earth would still be habitable by humans even at +20 C (although that would trigger a huge mass extinction among other species). There's a big gap between "pretty bad" and "literally no survivors".
+2c would make plenty of inhabited areas uninhabitable, ive never seen predictions of +20c, what would that even look like?
The crop failures and localised starvation of +2c would become global at +20c.
The migration of people, something that humans are historically absolutely terrible at handling, would be on an unimaginable level. Imagine the bloodshed.
20c being habitable seems like a stretch to me, and my Google Fu only turned up mentions of Antarctica hitting 20c. Have any references for that number? Most reports I've read say a +2c will cause significant problems globally.
+20C global average warming would correspond to roughly +40C land average warming. With polar regions seeing significantly more warming, at least +60C. That would see the coast of Antarctica averaging Death Valley's summer highs. Suffice to say, humans are not going to survive +20C global average warming.
The risks involved with rapidly changing the planet's climate are huge and largely unknowable. We're running a very dangerous experiment where we can't know what all the results will be. Hence, you don't run the experiment.
Europe has struggled "pretty badly" with migrant crisis on the order of a few million people. Fairly soon, estimated climate change refugees in one of the most prosperous and advanced areas of the world is going to be at one order of magnitude more[0], i.e. about 1 person in 10 in Europe will have been fleeing death. This isn't +20C, this is within the next few decades.
At the scale of the world, we're talking about predictions of 150 million refugees by 2050[1].
Sea level rise is one thing, but climate change is also desertification, tornadoes, death of the seabed[2], melting glaciers and re-arranging of maritime currents therefore weather patterns. The decline of structures of agriculture can also lead to radicalization of people in some areas of the world (see [1] again). The collapse of food chains can also be global, and just for the US alone for example a rise of 4 degrees would cut corn production in half[3], while precipitating what is already in motion (again, just for the US as an example, migrant crises from Central and South America[3]).
Uprooting not only entire industries but entire cities, states, and lives on an order of magnitude significant for any major country of union in the world, is not something that will be easily done without significant loss of quality of life, rise in tensions and potential wars, and widespread shortages of resources.
I imagine your reference to +20C is wildly exaggerated, because there is so little research about that since predictions about compound effects become exponentially more difficult. Even just an estimate that's over a decade old of up to +5C change already presents risks to India, China, ocean life, and major cities like NYC, London and Tokyo[4]. At +3C in the world, due to differences between land and sea it is possible that on average people (who are on land) would experience rises of averages closer to +7C already.[5]
Needless to say that +20C is something else entirely, and it's likely that no possible study would do justice to the idea due to catastrophic cascades of effects. The claim that humanity would still be inhabiting Earth would probably require some redefinition of "humanity" and "inhabiting" if survival/adaptation proved to be possible, which are all far-reaching thought experiments with the extent of current knowledge.
> Fairly soon, estimated climate change refugees in one of the most prosperous and advanced areas of the world is going to be at one order of magnitude more[0], i.e. about 1 person in 10 in Europe will have been fleeing death.
I initially read this wrong and tried to find where in the report they said that 1 in 10 Europeans would be fleeing due to climate change.
The claim is that 10-20 million will be fleeing draught or extreme weather events in Africa and they will tend to flee North not South. I’m seeing a quote from a US General but not a link to an actual study.
Longer term, over the next 40 years, they have this to say;
> There is no clear global dataset on displacement by slow-onset climate extremes such as sea level rise and desertification; often this migration is classed as economic or other planned migration, failing to acknowledge fully the ‘push’ resulting from climate change impacts. This leaves the full human impact of climate change unknown and depends not only on the magnitude of the event, but also on the vulnerability of the area and the society it impacts. Communities from Alaska to Fiji and Kiribati have already been relocated, or are making plans to do so
because rising sea levels threaten their lands. Developing countries - that have contributed least to climate change - are experiencing the strongest negative impacts, with increasing frequency and magnitude of extreme weather events that pose potentially disastrous consequences for agriculture and food security.
> According to a recent study, 1.4 billion people could be forced to leave their homes by 2060 and this number could rise to two billion by 2100. This estimate is based on combined projections of population growth, submerging coastal zones, exhausted natural resources, declining net primary production, desertification and urban sprawl.
It’s not much of an analysis, but it’s rooted in something real. Coastal populations number about 600 million, maybe up to 1 billion this century. SLR (sea level rise) has been 0.4m since 1900 and estimated to be between an additional 0.4-2.5m this century. “As a result, SLR is anticipated to be one of the most expensive and irreversible future consequences of global climate change.”
I liked this Nature article for a more balanced review of the literature;
It looks at various estimates ranging from 80 million to 1.4 billion people displaced by the end of this century based on SLR and the various assumptions being made by the respective models. For example, looking at areas which lie in 100-year floodplains assuming they’re were all inhabitable, that would impact an estimated 444 million people by 2100 factoring in population growth.
> However, ...residence in the 100-year floodplain may not necessarily result in migration responses to SLR. Indeed, many low-lying areas in the 100-year floodplain, such as Asia’s densely populated ‘mega-deltas’, possess fertile soil and ample water, which is ideal for farming and fishing. Floodplains thus attract large numbers of migrants from other areas, notwithstanding the presence of coastal hazards. Simple residency in the 100-year floodplain does not, therefore, result in migration; it is only when the costs of increasing exposure to SLR hazards exceed the benefits of coastal environments that migration may occur.
If you look at just areas that would become permanently inundated, it is a much smaller land area and estimated to impact 88 million people by 2100.
SLR may be among the greatest/costliest threat of climate change, but SLR models have a huge variance of outcomes, and fairly long time horizons.
