New York Mayor Michael Bloomberg on Tuesday unveiled a $19.5 billion plan to boost the city's defenses against climate change, seven months after superstorm Sandy devastated the US East Coast.

The blueprint features structures like a series of six-meter-high (20-foot) waterfront walls and dikes to prevent flooding of the kind that idled the city for days in the late October 2012 storm.

Sandy claimed 43 lives in New York alone -- the total US death toll was about 120 -- knocked out over half of Manhattan's power grid for several days, paralyzed transport and caused millions of dollars in damage in coastal areas.

Bloomberg announced the plan as he released a study of more than 400 pages on the risks that climate change entails for New York.

He warned that in 2050, a disaster similar to Sandy could have even more serious consequences. He said it is projected that a quarter of the city will be in areas prone to flooding.

"We have to look ahead to anticipate any and all future needs... Sandy cost our city $19 billion in damages and lost economic activity. We forecast that a storm like Sandy could cost around $90 billion by mid-century," the mayor said.

Bloomberg said he anticipates some projects on the waterfront will be controversial, especially those that block the view to the ocean.

He insisted however he was bent on safeguarding New York's 835 kilometers (520 miles) of coastline. Some 400,000 people already live in flood risk areas, where a total of 270,000 jobs are on the line.

"The alternative is to get flooded out -- or worse. We can't stop nature, and so if we're going to save lives, and protect the lives of communities, we're going to have to live with some new realities," Bloomberg said.

"This is a defining challenge of our future."

In 2009, scientists said that coastal waters around New York could rise as much as 12 centimeters (five inches) by 2020. Now, they say a 20-centimeter change is possible by that time, and up to 60 centimeters by 2050.

Eight percent of coastal areas could be regularly flooded, simply due to normal high tides, the study warned.

By 2050, temperatures could regularly exceed 32 degrees Celsius (90 degrees Fahrenheit) over nearly two months of every year -- 57 days, as compared with 18 days currently, experts warn.

The plan also calls for reinforcement or creation of dunes along vulnerable coastlines in Staten Island, Brooklyn and the Rockaways.

Construction will be carried out with the help of the Army Corps of Engineers and financing will come from a combination of city funding and federal relief money.

"There's no single solution to all of these challenges and we won't get all this work done at once. That would literally be impossible," Bloomberg said.

"But piece by piece over many years and even decades, we can build a city that's capable of preparing better."