MIAMI — Experts appealed Thursday to President Barack Obama and Mitt Romney to address climate change when they debate in Florida, saying the coastal state is already hit by rising water levels.

More than 120 Floridians who either serve on official bodies on sea level or hold advanced degrees on the issue signed a letter to the two candidates ahead of their third and final debate October 22 in the coastal city of Boca Raton.

The letter asked Obama and Romney to explain policies they would take to reduce the risks of a future rise in sea levels and adapt to the impact, as well as how they would work with other nations on climate change.

The experts said that the sea level has risen eight inches (20 centimeters) in the 20th century and has resulted in flooding during high tide seasons, as well as salt water intrusion into drinking water wells.

"The porous limestone underlying much of Florida resembles Swiss cheese, making sea walls ineffective and the state particularly vulnerable to sea level rise by allowing subsurface water to penetrate far inland," the letter said.

"Because Florida is so densely populated, it is estimated 40 percent of the population and housing units at risk from sea level rise in the nation are here, in the state of Florida," it said.

Climate change has not figured prominently in the campaign, with surveys showing that the economy is voters' top concern, although the planet has charted a slew of record-hot temperatures in recent years.

Romney mocked Obama over climate change during the Republican convention in Tampa, Florida, saying that the president "promised to slow the rise of the oceans and heal the planet."

"My promise is to help you and your family," Romney said to applause and laughter from his supporters.

Obama hit back at the Democratic convention in North Carolina, saying bluntly that "climate change is not a hoax."

Democratic lawmakers tried early in Obama's term to approve restrictions on carbon emissions blamed for rising temperatures, but the proposal failed in the Senate where Republicans said it would impose too much of an economic burden and questioned the science behind climate change.