river runoff from recent rainfall, which gets backed up by the incoming tide.

Some cities are already experiencing ever higher king tides, also known as "sunny day" flooding. From 1970 to 2014, the annual number of sunny day floods in Charleston increased from two to 14.

In 2016, the city experienced a record of 50 tidal flood days, according to a study this year by the National Oceanic and Atmospheric Administration. Savannah, Ga., also set a record with 38 tidal flood days, as did Key West with 14 days.

Over the last century, sea level has risen about a foot in Charleston. That's one reason sunny day flooding has increased there and in other parts of the Carolinas, Virginia and Florida. With expected sea level rise in coming decades—caused by meltingof the ice sheets and expansion of warming ocean waters—Charleston could see 180 days of sunny-day flooding by 2045. That means that current king tides could become the normal high tides.

As sea levels and tides rise, coastal cities face hard choices about preventing inundation. These include building expensive sea walls, elevating streets and retrofitting sewage treatment plants and other infrastructure. Natural habitats are also at risk. In Morro Bay, which sits roughly halfway between Los Angeles and San Francisco, coastal managers fear loss of salt marshes and possibly eelgrasses, essential parts of the marine food web.

Citizen environmental monitoring has a long history, most notably with the Audubon Society's Christmas bird count. For king tides, the monitoring effort started in Australia in 2009, then spread quickly to North America