Mayor Michael R. Bloomberg’s $20 billion program to armor New York City against climate change over the next decade includes extensive details, right down to the size of the concrete bulkheads proposed for the coast of Queens.

Yet even though the mayor is being praised for his foresight, the projects themselves face numerous obstacles before they can become a part of the city’s landscape and shoreline, officials and experts said on Wednesday.

They will have to pass through an array of government agencies, in City Hall, Albany and Washington. Zoning panels and community groups will weigh in. Billions of dollars in financing must be secured.

And of course Mr. Bloomberg, who announced the plan on Tuesday, is leaving office at the end of the year, and there is no guarantee that his successors will embrace all of the components.