A proposed $206 million overhaul of Miami Beach’s antiquated drainage system is just the first of many big-ticket bills South Florida faces.

Climate change may be the subject of debate in some places but in South Florida it’s become a costly reality.

In Miami Beach, where prolonged flooding in low-lying neighborhoods has become the norm after heavy storms, city leaders are weighing a $206 million overhaul of an antiquated drainage system increasingly compromised by rising sea level.

The plan calls for more pumps, wells to store storm runoff, higher sea walls and “back-flow’’ preventers for drain pipes flowing into Biscayne Bay. Those devices are intended to stop the system from producing the reverse effect it often does now. During seasonal high tides, the salty bay regularly puddles up from sewer grates in dozens of spots, such as near the local westside bar Purdy Lounge. Extreme high tides — like one in October 2010 — can push in enough sea water to make streets impassable, including blocks of the prime artery of Alton Road.

“It’s the first time, as far as I know, that any community in South Florida and actually in the entire state of Florida is taking into account sea level rise as they plan their storm water infrastructure,” said Fred Beckmann, the city’s public works director, during a public hearing on the plan earlier this month.

It won’t be the last time.

South Florida counties and cities, as well as the South Florida Water Management District which oversees flood control for the region, all are beginning to draw up projects for keeping the coastline dry as sea level creeps up. The potential costs could be staggering.

The district alone has identified three flood control gates along coastal Northeast Miami-Dade — critical to draining storm water from Pembroke Pines and Miramar in southwestern Broward — in fast need of retrofitting with massive pumps. Rising seas threaten to reduce the capacity of a system that now depends on gravity, the storm water flowing downhill into the Atlantic. Cost estimates run $50 million or more for each pump alone and buying land for them could double or triple the bill. Nine other gates could need similar work down the road.

Fort Lauderdale, where high tides also push salt water up storm drains in the ritzy Las Olas Isles section, is also planning to install back-flow preventers, said Jennifer Jurado, director of Broward’s environmental protection and growth management department. Hallandale Beach already had to install pumps on storm-water injection wells, at about $10 million each, to combat increasing back-pressure, she said.

“The overall issues are so much greater, I think we’re easily looking at hundreds of millions of dollars,’’ she said. That’s just for the next 20 to 30 years, to handle a moderate three to seven inch rise.

A study last year by the Florida Atlantic University Center for Environmental Studies found that the projected rise over the next 70 to 100 years would require one city alone, Pompano Beach, to spend from $500 million to $1 billion to overhaul drainage and water supply systems, as well as coastal roads and facilities.

“If 50 years from now we’re looking at a foot and a half or two feet and rising, our region is going to be confronted with some very serious problems,’’ said Barry Heimlich, an FAU researcher who co-authored the study. “It’s going to cost hundreds of billions of dollars.’’

Lawmakers in some states have blithely dismissed the threats of global warming, most notably those in North Carolina, where state lawmakers earlier this year passed a law ordering that only historic trends, not projections, be considered in coastal planning.

In South Florida, political leaders and planners aren’t in denial. In 2009, Miami-Dade, Broward, Palm Beach and Monroe counties formed a climate change “compact’’ to work together to confront a problem South Florida will see sooner than just about anywhere.

A string of studies by insurers, environmental groups and government and university researchers have singled out Miami-Dade County at the top of the list of at-risk cities, with tens of billions of dollars of property that could be damaged by heightened storm surge or flooding.

Earlier this year, a report from Climate Central, an independent research and journalism organization, suggested Miami-Dade and Broward counties alone have more people vulnerable to flooding than any state except Florida and Louisiana. Other studies suggest some of the lowest-lying Florida Keys may be the first to be inundated.

The compact’s draft projection of sea level in Southeast Florida — based on local trends and global forecasts — calls for a rise of three to seven inches by 2030 and nine to 24 inches by 2060. From there, many scientists predict the trend could accelerate.

Miami Beach and other low-lying barrier islands are particularly vulnerable to drainage problems but those are spreading to the mainland, said FAU’s Heimlich. The FAU study found a sea level rise of about six inches could cut flood-control capacity by more than half — with higher tides bottling up canals and structures that now drain with gravity as runoff flows downhill to the coast.

“This is a problem that is not far away,’’ said Heimlich. “It is already being experienced and will get worse in the next few years.’’

Broward and Miami-Dade counties both are doing more detailed analysis of how existing drainage systems might have to be retro-fitted or expanded.

Doug Yoder, deputy director of the Miami-Dade Water and Sewer Department, said sea level rise will also push more salty and brackish water into surface drainage and sewer systems, adding to the costs and volume of treating runoff. Worsening salt water intrusion, which can shrink and taint the underground Biscayne Aquifer, the county’s main source of drinking water, will also require more expensive treatment systems in the future.

Potentially, Yoder said, the county could have to move sewage treatment plants like the aging facility on Virginia Key inland and build them at higher elevations. Monroe County is already planning to do that with a new fire station in Key West, adding several feet to the ground-floor elevation.

While there are a wide range of potential costs, including raising roads, Yoder said solving drainage was critical. Without it, he said, “you wonder how long people will continue to live in a place that floods routinely.’’

James Murley, executive director of the South Florida Regional Planning Council, said Miami Beach is out front in accounting for sea level rise. On-going budget challenges could make it tough sell for some communities worried about spending too much to address impacts that might not come as soon as anticipated. Forecasts differ on the pace and impact.

The plan crafted by Miami Beach’s engineering consultant, CDM Smith, is intended to address sea level rise for just 20 years.

Environmentalists and other critics said that relatively short window, at least in terms of climate change impacts, seemed intended to minimize costs. But Mike Schmidt, a vice president with CDM Smith, said projects could be altered to account for faster or higher rises. More or larger pumps, for instance, could be added to force storm water out against the higher pressures of rising sea levels.

Much of Miami Beach’s drainage system dates back to the 1940s and there is limited data about how many outfalls were designed to remain above high tide or for how long. But an analysis performed by Coastal Systems International, another contractor assisting in the project, showed the ends of the drain pipes are spending more time submerged, with the mean high water elevation creeping up by about 1.68 inches over the last 14 years. The plan, which still must be approved by the Miami Beach Commission, is designed to handle another six inches by 2030

Beckmann, the public works director, said the city only needed to two pumps for stormwater when he started 11 years.

“Right now, we have 17 and we’ll probably call for another 14,’’ he said.

Schmidt said rainfall still accounts for 95 percent of the flooding in Miami Beach but in century or two, the city could be more like New Orleans, sitting below sea level with its safety dependent on sea walls and pumps. “Eventually, if the projections are true, you’re facing a position where the sea level rise would go above the land surface and then you’re raising critical infrastructure,’’ he said. “Your sea walls are going higher, you’re putting in locks and dams and you’re pumping almost everything.’’

For now, Miami Beach Mayor Matti Bower said her biggest concern was figuring out how to pay for the projects, saying she didn’t think it was fair for the city alone to be tackling the expense.

Normally, the city would issue a bond and raise stormwater rates to cover costs but because the drainage project is also designed to reduce environmental impacts to the bay, the city will explore options including seeking federal grants or money from other state or county agencies.

“I’m not even worried about 25 years from now because I’ll be 100 then,’’ Bower said, “but I do worry for the children and grandchildren.’’

By CURTIS MORGAN