I feel like their argument against it probably needs a little unpacking.

It seems a little disingenuous to me to only examine a model where carbon emissions don't decrease and then attribute the result to the shield. If the shield is used in addition to reducing our carbon output we'd presumably be cooling things off in both the short and long term.

The result of failing to reduce our carbon emissions is already projected to be essentially apocalyptic in scope. The rest of the planet might luck out if our own actions reduce us to a population that we're physically incapable of continuing to output enough carbon to keep warming it, but human civilization certainly doesn't seem like it can survive keeping it up at the very least.

If we do get it together push against the wishes of the greediest humans and act as responsible stewards of the planet, it would be smart to try to save as much as possible. If a solar shield can help protect our biodiversity and the stability of our civilization while we get our collective shit together, that's fantastic. It may even bring with it a sense of urgency and collective responsibility to fix the problem before anything happens to our buffer.

I get the arguments about the rapidity of change if the shield fails and the difficulty of animals migrating much more quickly, but if something doesn't give soon they're not going to have much of anywhere to migrate anyway.

At what point does the potential benefit of the shield outweigh the risk?

If I'm falling out of an airplane and my chute is kinda lopsided or whatever in a way that might strangle me if i don't get my head out of the way, am I just going to let myself hit the ground instead? Or am I going to take the shot I've got and make the most of it?

We're in freefall and the ground is down there somewhere, rapidly coming up to make friends. We need something now.

If this is it I say we take it. And we let it be the act that prompts us to be responsible with our planet.

The idea is to enjoy yourself without consequences.