"Our greatest responsibility is to be good ancestors."

-Jonas Salk

Thursday, October 6, 2022

How Long Do We Have to Reduce Emissions to Avoid Catastrophe?

Quora question:

How long do we have to reduce emissions to prevent catastrophic temperature rise?

There is both unclarity and uncertainty making it difficult to answer your question.

First of all, what do you mean by “catastrophic”? There have certainly been extreme events recently that would seem very unlikely in the absence of climate change. Here in Canada, in two successive years, we have seen the hottest temperature ever and the lowest surface pressure (associated with sever storms) both by wide margins, and both events straightforwardly much more likely now than in the undisturbed climate. Both of these were locally catastrophic. But as a whole, Canada is still thriving.

If by “catastrophic” you mean bad for the whole world, you also have to say how bad. There’s definitely a case that the damages of increasing CO2 concentrations exceed the benefits already, but this is hotly debated.

Secondly, what do you mean by “reduce” emissions? In fact, things will continue to get worse as long as CO2 emissions exist at all, at least in the net. This is what “net zero” means. This is because natural systems take a very long time to remove excess CO2; the process is geological. So as long as we emit more than we withdraw, we will be driving climate change. The story is more complicated with other greenhouse gases that have shorter lifetimes. The crucial challenge is getting to (or at least so much closer that it hardly matters to) net zero CO2 emissions.

So by when do we need to do that to avoid a global catastrophe? I’d call a global catastrophe a time when human mortality goes up as a direct result of climate disruption. Again this will be hard to call. In an overpopulated world, we have wars and pandemics to deal with too. Neither of these problems is made any easier by climate change, but it may be hard to detect by how much.

Also, there’s some scientific uncertainty. The physics is pretty well constrained but not perfectly. If we pick a particular temperature threshhold (say 2 C warming) to avoid, there’s some uncertainty as to how much greenhouse gas accumulation would cause it, though I’d say this is a relatively minor factor. The bigger uncertainty is what threshhold we could tolerate. There’s long been a rough consensus that exceeding 2 C of warming would be too risky, and so we should put on the brakes before we reach that point. To be really safe, some argue for 1.5 C. The formal international agreement at the Paris conference was “definitely less than 2 C, and aim for 1.5 C”.

And now we get into short time scales. Avoiding crossing the 1.5 C line will be hugely difficult. Some people say it was already impractical at the time the Paris agreement was signed (myself among them). To stay below 2 C requires all nations to take the commitment very seriously, and especially to urgently replace fossil fuels and/or hugely (and probably unrealistically) enhance carbon capture projects.

So we look likely to miss the 2 C target, especially now that we are distracted by war and pandemic. Will that be globally catastrophic? We can’t be sure until we get there, and even if we do have a global failure, it may be hard to sort out how much of it was due to climate stress.

But we really ought to take this problem as seriously as possible even given all this uncertainty, because it's clear that every fraction of a degree makes the future more difficult, possibly for many centuries.

Image: washed out road in Yellowstone Park, US National Park Service (public domain)

1 comment:

...and Then There's Physics said...

I guess my simplistic point is that the best evidence indicates that future warming depends mostly on future emissions. Hence, how much more warming "we" experience largely depends on choices that are made now and in the future, not on some fundamental aspect of the physical climate system, or on past emissions (which determines warming to date). The targets are aspirations that even if they're not met are better to just miss than to miss by a lot.

So, we could choose to give up on some target and move on, and maybe that is the honest thing to do in terms of accepting what is probably inevitable. The problem I have is how we do that while still highlighting that the reason it was missed is mostly because the problem was not taken seriously, rather than because it was always unavoidable. And, if in ~10 years time we get to the point where 2C has now become unavoidable, do we simply move on from that target?