We have a problem and need to find the best way to fix it. Understanding the cause in order to find one possible fix is an obvious thing to do. So is looking for any other possible solutions. It is an unnecessary constraint that we need to solve the problem only by reversing its cause.

If the cause of the problem was a brighter sun but we could save the planet by internalizing the cost of our emissions, it seems like it would be wise to do that regardless of whether an earth umbrella might address the root cause more directly.