If we do this enormous amount of work, globally, to solve the political coordination problem of cutting emissions, if we spend several percent of GDP for the whole century to cut emissions to zero, which we should do no matter what, when we do that, all we do is stop making the climate problem worse. You don't go backwards, you just stop making it worse. That's because of the long lifetime of CO2 in the atmosphere.

That's still better than not doing it, but it's not environmental success. And doing a combination of solar geoengineering and then carbon geoengineering allows us to talk about actually bringing the climate back to pre-industrial over, say, two centuries. Not perfect, there will be problems, but it allows us to talk about getting in two lifetimes to the pre-industrial climate.

Reducing emissions to zero and then gradually taking CO2 out of the air, and to doing solar geoengineering for a while to really quickly peak and then reverse the growth of what we call radio forcing of climate, and then get back to a place, say 150 years or 200 years out, where you're not doing solar geoengineering or carbon geoengineering

because you're back to pre-industrial. That's a possible outcome. I can't say whether it's the right outcome, but it's an outcome at which humanity could aim, that is fundamentally, in my view, a more environmental outcome than just cutting emissions.