With rising seas threatening to profoundly alter the landscape and life, South Florida has more at risk from climate change than any region in the country. Miami-Dade, Broward and Palm Beach alone have more than 5.5 million residents, more people than each of 30 entire states. Only 80,000 people live in Monroe, but the low-lying islands of the Keys rank among the nation’s most at-risk communities and will be the first measuring sticks of sea rise.

The threats go beyond surge from hurricanes or tropical storms.

With just an eight-inch sea level rise, drainage canals can lose 40 percent of capacity and salt intrusion will taint and squeeze underground drinking water aquifers. Several coastal cities in Broward are already spending tens of millions to move wells inland.

There are also increasing flooding risks from routine thunderstorms. The city of Miami Beach is already planning to spend more than $200 million to overhaul a drainage system that increasingly leaves streets flooded during high tides.

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. With a four-foot rise by 2100 — projected by Miami-Dade’s climate task force — the sea would cover much of the barrier islands and begin percolating up from the Everglades into low-lying western suburbs.

Jennifer Jurado, Broward’s director of natural resources management and planning, said the counties decided to collaborate to combine resources on a critical issue and, the counties hope, to increase clout with lawmakers in Tallahassee and Washington. A primary goal is to shape state and federal climate policies and steer more funding to South Florida communities most at risk.

“It’s been a strong political commitment from these four counties,’’ she said. Three more counties, Martin, St. Lucie and Indian River, this year are tapping into the work in an associated regional planning effort.

Still, there are major challenges to implementing plans that call for a wide-ranging overhaul of South Florida’s road, transit and sewer and water supply systems. The plan, for instance, doesn’t just call for elevating roads but for expanding mass transit to both protect transportation corridors and reduce greenhouse gases that scientists believe have worsened climate change.

Projects like raising seawalls, installing drainage pumps and moving well fields also will be costly to justify to counties and cities already struggling with slashed budgets.

Jurado said she doesn’t expect the region to do everything in “one fell swoop,” but rising tides — and future storms like Sandy — will increasingly force communities in coming decades to weigh the costs of adapting or potentially pulling back from vulnerable areas.