NEW YORK — Removable flood walls would be set up for much of lower Manhattan, a 15-to-20-foot levee would guard part of Staten Island and a system of gates and levees would protect a Brooklyn creek as part of a nearly $20 billion plan Mayor Michael Bloomberg proposed today to transform how New York City girds itself for storms and global warming.

Bloomberg’s proposals also include building dunes in Staten Island and the Rockaways, firming up the shoreline with bulkheads in neighborhoods and considering building a levee and a new “Seaport City” development at the South Street Seaport that would echo nearby Battery Park City.

The mayor also is suggesting giving $1.2 billion in grants to property owners to flood-proof their buildings and $50 million to nursing homes to improve theirs; making hospitals even in rarely flooded areas upgrade their pumps and electrical equipment; and expanding beaches and marshes, among other ideas.

“This plan is incredibly ambitious,” Bloomberg said in a speech at the Brooklyn Navy Yard, acknowledging that much of the work would extend beyond the end of his term this year. “This is urgent work, and it must begin now.

Fueled by the city’s blow from Superstorm Sandy last fall, the sweeping proposals represent a sizeable step up in scale and urgency for a mayor who has for years emphasized the threat climate change proposes to the nation’s biggest city, which has 520 miles of coastline.

It remains to be seen how the ideas will fare in a future mayoral administration, and what kind of support — financial and otherwise — they might ultimately get from the federal government and other entities that would be involved, not to mention from New Yorkers themselves.