I’m not so sure I’d want to go down this path that Obama’s science advisor wants to take. Not that I’m a Nobel prize winner, nor is my fascination in the topic as in depth as any of these experts, but it’s based on a few common sense methodology.
Here’s the issue I see with geoengineering which is the application of planetary engineering specifically to Earth. Ever watch a lot of science fiction? Know the term terraforming? It’s basically the process of turning another planet into one suitable for living for human beings. The scale and technology that you would need to do such a maneuver is so enormous that it’s mainly theoretical and inconceivable at where are scientific research is currently. On top of that, we’re lacking in the tools and technology, but the ones we have for planetary terraforming are crude at best. Mimicking volcanoes by shooting sulfur into the atmosphere? What guarantees would you have that you wouldn’t bring the apocalypse as we know it upon ourselves because we don’t understand how the weather systems work?
I think we know too little to be delving into trying to cool the Earth outside of enforcing what it sent out into the atmosphere or perhaps create some artificial means of doing what we’re currently lacking (like reproducing the types of effects the rain forest has on the Earth by artificial means). The issue is that the delicate nature of how weather systems are make it impossible with current mathematics to understand if we’re overdoing something that might upset the balance the other way.
So terraform Mars? Sure, why not. There’s nothing to lose there. But we’d have to get there, understand how gravity functions on a human body, how to change a planet from the inside to out, understand gravity itself, how to do massive experimentation on planetary scales, and so on so forth. To try to do it on Earth, where we have issues predicting the weather a week in advance is almost absolute insanity.