> I have heard it claimed that 90% would be enough. I don’t know why, as at first glance that seems like it would merely slow down the rate of increase of CO2, not prevent further increases.

CO₂ isn't a fixed quantity in the atmosphere that we add to. If we don't add to it, it gradually gets absorbed by natural processes on the Earth's surface.

The reason the amount in the upper atmosphere is rising is because we're producing more faster than the other processes can absorb it.

The picture is a little more complicated than that though. We are also destroying some of those absorption processes. For example through deforestation of the Amazon.

Also, CO₂ is now a proxy measure for other greenhouse gases produced by human activity. Methane in particular is a stronger greenhouse gas than CO₂, so that matters too and the aborption processes are different. So there are a number of cycles of different things going on, and it's a fairly complicated picture, thus all the models.

However, there are some useful simplifications, and it's too important to let arguing over details be used to avoid dealing with the basic problem. CO₂ is still the main concern and target, because it's produced by a large number of human activities, and they can be changed. Basically anything that involves burning things, whether obviously like in a car, or hidden away in some power station where you can't see it.

All that said, even if we get net CO₂ emissions down to zero now, or even negative (if we find a way), it is generally accepted that we are too late to stop a significant average temperature rise that will cause significant climate changes affecting us, and especially affecting nature across the world.

Because of this, targets these days are the result of negotiating some kind of "realistic" level of harm, which of course involves setting temperature-rise targets, and lots of politics, and plenty of political and industrial leaders trying to see what they can get away with. Nobody is really happy because some people would like less harm than the "realistic" minimum now, and some people would like to carry on with life as before because it was fun, and maybe their wealth depends on it.

> That said, I am optimistic: given the current exponential growth of renewables, I don’t think it matters. Even without major new breakthroughs in storage, all we need is to build the factories to build the solar panels, the batteries, the hydrogen electrolysis plants and the Sabatier machines. We can be carbon neutral and save money/grow our economies at the same time.

I'm completely with you on this. Technology will save the day if... well, the trouble is technology can't be taken for granted, even if there are plenty of great inventions around, and potential future inventions.

Technology can only save the day if politics, behaviour and the economy line up to allow it to happen.

I really don't think we can assume politics and people are collectively all that aligned on these issues or much else at the moment. We can't even agree on whether to protect each other from Covid or not by simple behavioral changes (social distancing, mask use etc), and that's a much more visible, short-term and simpler issue to understand than climate change.