By Valerie Volcovici

WASHINGTON (Reuters) - Mayors of 21 cities in Florida on Friday called on the moderators of next week's presidential debates in Miami to ask candidates how they would deal with rising sea levels caused by climate change, a concern of the state's coastal communities.

"It would be unconscionable for these issues of grave concern for the people of Florida to not be addressed in the upcoming debate you will be hosting in the state," the mayors wrote in an letter to CNN, The Washington Post, Univision and the other media outlets hosting the Democratic and Republican debates on March 9 and March 10 in Miami.

The bipartisan group of mayors, who represent coastal cities and towns from Miami to Tampa, have complained that the issue of climate change has not been raised in the dozens of televised debates held to date.

Of the four remaining Republican candidates, none say they believe that anything should be done to address climate change and all have called for federal environmental rules to be scaled back.

One of the letter's signatories, Pinecrest Mayor Cindy Lerner, traveled to New Hampshire in February to press Florida Republican candidate Senator Marco Rubio to discuss the issue.

Rubio has said that the climate has always been changing and that he will not support policies to combat climate change that will "destroy our economy."

Billionaire businessman Donald Trump has called climate change a conspiracy invented by China, while Texas Senator Ted Cruz called climate science "pseudoscientific theory." Ohio Governor John Kasich has said climate change is a problem but has not proposed a plan to address it.

Democratic hopefuls Hillary Clinton and Senator Bernie Sanders have made climate change a top-tier issue that needs to be addressed.

Southeast Florida is especially vulnerable to rising sea levels. Four counties in the region formed the Southeast Florida Regional Climate Change Compact six years ago to coordinate their response and pool funding to deal with its impact.

The issue has gotten little attention in the national presidential debates.

The mayors provided debate moderators three questions on climate change, all of which were directed to Rubio.

They warned that higher sea levels will pose a costly threat to Florida's economy. Florida coastal communities could experience about a 2-foot (60-cm) rise in sea level by 2060, the U.S. Geological Survey said.