On a predictably gorgeous South Florida afternoon, Coral Gables Mayor Jim Cason sat in his office overlooking the white-linen restaurants of this affluent seaside community and wondered when climate change would bring it all to an end. He figured it would involve a boat.

When Cason first started worrying about sea-level rise, he asked his staff to count not just how much coastline the city had (47 miles) or value of the property along that coast ($3.5 billion). He also told them to find out how many boats dock inland from the bridges that span the city’s canals (302). What matters, he guessed, will be the first time a mast fails to clear the bottom of one of those bridges because the water level had risen too far.

“These boats are going to be the canary in the mine,” said Cason, who became mayor in 2011 after retiring from the U.S. foreign service. “When the boats can’t go out, the property values go down.”

If property values start to fall, Cason said, banks could stop writing 30-year mortgages for coastal homes, shrinking the pool of able buyers and sending prices lower still. Those properties make up a quarter of the city’s tax base; if that revenue fell, the city would struggle to provide the services that make it such a desirable place to live, causing more sales and another drop in revenue.

And all of that could happen before the rising sea consumes a single home.