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>I think it’s hard to know what ‘bad’ means here.

Strong disagree. I really dislike this "well, we just don't know what's going to happen!" take just because there is some uncertainty around the edges.

Just take one dead simple example that we know with a quite high degree of certainty - sea level rise. Coastal communities just happen to be where most of humanity lives (here is a nice map from the Map Porn subreddit, showing areas of equal population in the US, where the areas along the coast basically contain the same population as everything else: https://www.reddit.com/media?url=https%3A%2F%2Fi.redd.it%2Fs...).

It's not hard to game out how much devastation there will be with 2, 4, 8 meters of sea level rise, and if anything we've been finding recently that our calculations of ice melt have been too conservative.

>there is some uncertainty around the edges.

I'm not a climate scientist so take this in the curious vein that it is intended, but do we know that the uncertainty is only around the "edges"? From what I know about weather forecasting, relatively small changes in initial conditions can lead to rather dramatic differences in outcomes. It's my understanding this is how they get to the probabilistic estimates like "40% chance of rain"...they run a bunch of different models with slightly different initial conditions and 40% show rain.

Genuine question: if there is relatively large uncertainty at the small geographic and temporal scales, why wouldn't that also propagate to even larger amounts of uncertainty at larger scale models? (FWIW, I'm not trying to wordsmith around the word "bad" but rather pin down the uncertainty aspect of the claim.)

The larger scale models aren't necessarily built on the smaller models, they are independent and generally less specific. They are probably even inputs to the more specific models. So, like rising sea levels and increased temps (on average) is basically guaranteed, the _exact_ change for a particular place is more uncertain.

Neil deGrasse Tyson in his Cosmos show described it like a person walking a dog. We don't know where the dog will be exactly, but we can see the person with the leash is moving in one direction, and the dog isn't going to ever get too far from him (because of the leash).

But the uncertainty in the dog analogy is the length of the leash. If we don't know the length of the leash, we can know the general directionality but can't make very strong conclusions about where the dog will be, right? I don't know that this supports the idea that uncertainty will only be around the edges; I've heard NDT talk about the opposite: even a small shift in in the average causes relatively large changes in the probability of (previous) long-tail events.

A more accurate drill down on the analogy would be that we don't know the exact length of the dog' leash either, but we do know what the limits of it are. We know it's not a 1 km leash, but the "uncertainty around the edges" encompasses not knowing if it's 2 m or 0.5 m. Part of the accuracy of the analogy is that just like when you see the human from a long distance, you don't know exactly how long the leash is, but you do have a solid expectation of the rough upper bound for the dog's distance from the human even still.

With climate change you're talking about having seen the human walk half way down a block already. You can see they're continuing to walk the same direction, and everyone agrees they'll continue walking to the end of the block if nothing changes. Do we know exactly where the dog is during that whole walk? No, but we do know it will cross a line two thirds of the way down the block sometime before the human reaches the end of the block. And the likelihood of the dog crossing is greater and greater the further along the block the human is.

>but we do know it will cross a line two thirds of the way down the block sometime before the human reaches the end of the block.

I'm trying to be careful here because I don't want to come across as if I'm against investing efforts to mitigate climate change. But in the real world, it ultimately comes down to trade-offs. So I agree with the premise that eventually, if unabated, climate change will likely cause "bad" outcomes. Apropos to this thread, though, is the definition of "bad". And from what other commenters have shared, there is large uncertainty of both the "low" and "high" temperature change models out to about 2070 or so.

In other words, even if the circumstances meet the definition for a "high" temperature change it may result in the same outcome as if it were a "low" temperature change, but the mitigation efforts would likely be drastically different. In a world where the solution is trade-offs, mitigating for one scenario over the other comes with real consequences. The point I'm trying to get at here is that I think uncertainty has to be a larger part of the conversation when we're talking about competing risks, especially when that uncertainty is relatively high. While uncertainty is a central part of a scientific discussion, my hesitancy about even bringing this up is that I also recognize that highlighting uncertainty is also a tactic of bad-faith actors to undermine any reasonable conclusions.

Rougly said, the chaos you are talking about is averaging out over larger spatial and temporal scales. The errors in climate models are coming from

A) necessary simplifications (we can only resolve the earth to a scale of a few kilometers or so and need to use subgrid models to approximate processes that happen on smaller scales such as cloud formation) and

B) unknown or not well enough known parameters and model behavior and

C) Unknown input quantities (most prominent: future man-made CO2 emissions)

There is thus quite some disagreement between different climate models[1] and uncertainty is sometimes reported based on the amount of disagreement between these models.

[1] See https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6..., page 17, in the high emissions scenario the uncertainty range lies between 3 and 5 °C.

This is a great explanation, thank you. If I'm reading that chart correctly, out to 2060 or so, the uncertainty is high enough that both high and low temperature change scenarios overlap, but the overall risk is still moderate to high, is that right?

A lot of the more immediate impact of sea level rise, though, is increased frequency of hard to predict storm surge. And while we know there's sea level rise, we don't know how much; if we knew sea level rise would stop at +4.2 feet we could build infrastructure to deal with it (at least in the developed world) ... it's much harder to build infrastructure to deal with these challenges when rise might be +2 feet or +10 feet.

> I think it’s hard to know what ‘bad’ means here

I think with how slow evolution happens, bad is anything that isn't the status quo. Yes, some species that are more adaptable will survive and could even thrive as competitors die off, but many more species will go extinct. It's hard to imagine any path here (at least on a short to medium term timescale) where biodiversity increases. And given that biodiversity is what got us to where we are, I think it's easy to agree that it's a good thing, so anything that threatens it is bad.

> All that said, I don’t think we have the computational models to predict what happens when you increase the temperature on earth for a century.

This definitely seems true. As a layman it seems like this is a classic case where chaos theory comes into play. Even tiny errors in our projections will lead to wildly different outcomes, so the only thing that we can say for sure is that we don't know. That's scary.

> And given that biodiversity is what got us to where we are

Mass extinctions also is what got us to where we are. If dinosaurs were still around when humans started to evolve, it might have been a Bad Thing.

Many species are supposed to go extinct, all the time. That's is how nature and evolution works. If we become extinct, then it paves the way for human++. This is neither good nor bad, it's just how life has evolved for billions of years. I think if we try to stop this and maintain status quo, that is ethically and morally wrong.

> Many species are supposed to go extinct, all the time.

True, but what are called "mass extinction events" actually took place over tens of thousands of years, which is "quick" when talking about geological time. We're potentially talking about a mass extinction event taking place over hundreds of years. The problem is that's a long time compared to a human lifespan, but an extremely short time from a geological standpoint. We have nothing from the past to analyze what the effects of this exceedingly fast extinction event will be - including whether it will lead to such ecological collapse that complex land animals such as humans can continue to exist. We simply don't know. One would think we should proceed with caution, but instead we're hitting the accelerator.

There are several effects of climate change that are undeniably "bad" for human civilization. Rising sea levels that wipe out coastal communities. Increased intensity of the biggest storms. Temperatures in some areas of the globe exceeding human tolerance for ever lengthening periods in the summer. Coral reefs dying off.

Even with a few silver linings here and there we should be doing what we can to reduce and even reverse climate change.

> I think it’s hard to know what ‘bad’ means here.

We have an entire civilization completely optimized for the climate of the late 20th century.

It's hard to know exactly how it will be bad, but it's an incredibly safe bet that global warming is bad.

> I think it’s hard to know what ‘bad’ means here

* Rising sea levels

* Rising water temperatures decimating marine life

* increased temperature swings, including higher highs. The ways things are going, there are going to be vast areas which become uninhabitable by humans as they are today in places with tens of millions of inhabitants like Iraq, Pakistan, India.

Those are prety obvious, already in progress, and universally bad.