For 400 years the sea has been kind to Boston. Maritime trade fed the city’s early economic rise, and countless cod laid down their lives to feed its inhabitants. With the exception of boatloads of occupying redcoats, the Atlantic hasn’t given the city much to complain about. When the area has flooded — as it did this spring — it hasn’t been because of the mighty Atlantic, but the placid Charles.

But when Boston’s planning visionaries think about the future, increasingly, it’s the sea they’re worried about. Huge swaths of the city are on landfill, just a few feet above sea level, and as ocean levels rise in the coming decades — as most earth scientists project they will — Boston faces the prospect of an ocean that is higher and more dangerous than the one it has long known. So-called 500-year floods — freak meteorological events of extreme destructive power, now expected only twice a millennium — will become 100-year floods, and 100-year floods will become 20-year floods.

The specter of rising seas has long been invoked by environmentalists as a kind of warning, to spur action on climate change. But in recent years, a growing chorus of planners and scientists have begun talking about higher sea levels not as some cautionary scenario, but as a fact of life — one that everyone who lives near the ocean, whatever his views on the climate, needs to prepare for.

What does Boston need to do? Architects, city officials, insurers, and engineers have begun to lay the groundwork

for a new version of the city, one prepared to keep pace with rising tides. In the past few years, the conversation has gained momentum, with meetings in City Hall, academic conferences on the topic, and a widely discussed article in the current issue of the journal ArchitectureBoston.

The ideas run the gamut from basic infrastructure fixes — raising the entrances to the city’s subway and highway tunnels, or moving electrical equipment out of downtown basements and onto the roofs — to zoning changes that discourage construction in high-risk areas. And a pair of architects is proposing a megalithic building project that would completely reshape Boston Harbor, using massive sea gates that could swing shut to seal the city off from the most devastating storm surges.

“This isn’t just an environmental issue,” says James W. Hunt III, the city’s chief of environmental and energy services. He ticks off the agencies currently involved in the effort: the Boston Redevelopment Authority, the Mayor’s Office of Homeland Security, the office of Environmental and Energy Services, the transportation department, the public health commission.