What makes anyone so sure that we need cooling? What if they go over the edge and trigger irreversible Ice Age conditions? I should think we'd find that a more difficult problem to rectify! I don't think Earth can warm faster than our technology to stop it, at current rates.
We could stand a warmer climate -- dare I say it, a much warmer climate if you ask North Dakotans -- so long as we address the issue of desertification.
I promise it's much more complicated, but it should be sufficient to say that more ground cover planetwide will bring more atmospheric moisture, which leads to more ground cover, which, yadda yadda, prevents things from getting too warm. I'm told there is a period of history in which the global climate was much warmer -- and more importantly, wetter -- than it is now, and remained stably so for several hundred million years. I don't think the matter of stability is being addressed by these folks.
It's not really warming that we should be going all Chicken Little for; it's desertification. Overall stability of the climate should be the priority over desperately trying to stabilize the current conditions, which may or may not be optimum for human use.
It is my country bumpkin opinion that we should be concerning ourselves with how to make our climate wetter, not colder. I think city folks who live in more tropical locales lose sight of that fact, but those of us who live in the deserts are keenly aware of it.