The government of a low-lying island nation is considering the construction of a seawall to protect its capital and economic hub from the rising seas brought on by climate change. The length and expense of the project depends on how high the wall needs to be — 3 feet? Four?
A wall that's too high would be needlessly expensive and intrusive. One that is too low would make the effort to protect the soon-to-be inundated city in vain, sapping resources that could have been put toward other preventative measures. Doing nothing could be disastrous.