Different people give different answers.
Some argue that since we cannot be sure how climate will react to changes in the atmosphere and the biosphere, we cannot know the outcome of global warming. Others say that because we do not know how the water supply will be affected at the regional level, there's no point in spending time and resources on what is only a possibility. It would make sense to follow both of these strategies. While this time lag makes it all the more important to reduce greenhouse gas emissions immediately, it also means that we shall have to adapt to some change in climate. Adaptive measures include conservation and efficiency strategies that are beneficial even without climate change.