Hurricane season is upon us. The activity of individual seasons goes up and down, but trends are becoming clear: rising sea level and increasing storm intensity and rainfall are elevating the hazard.
Rising sea levels is one of the most robust consequences of a warming climate. Warmer water expands and takes up more volume. Melting land-based ice also adds more water. The consequences for Florida are dire.
Globally, sea level has risen 8 to 9 inches since 1880, an increase greater than any equal-length period in thousands of years. And the increase is accelerating.
Coastal communities feel the effects of sea-level rise during flooding.  More frequent nuisance flooding — flooding that can occur at high tide even in fair weather — is obvious in many parts of Florida. In Naples in 1950, sea level 14 inches above high tide occurred about once a year on average. It now occurs about five times a year.
It’s going to get worse. Seawater has a lot of thermal inertia. It takes ocean temperatures decades to respond to the greenhouse-gas-induced heat from the atmosphere.  Even if additional heating stopped tomorrow (unlikely!), the ocean would continue rising for decades.
How much higher global sea level will be by 2050 is uncertain. It depends on the amount of greenhouse gases humans continue to add to the atmosphere and how rapidly the ice sheets in Greenland and Antarctica break apart. Middle range estimates are 9 to 17 inches above 2016 levels globally, and high estimates are up to two feet. In South Florida, we could see 12 to 30 additional inches by 2050.
If Hurricane Irma had taken a slightly different path up Florida’s west coast with maximum winds pointing into Tampa Bay, the outcome would have been vastly different.
Florida’s shallow Gulf Coast waters and the shape of Tampa Bay make Tampa the most surge-vulnerable city in the United States. Three other Florida cities are in the top 10.
A category 4 hurricane would produce a 1-in-100-year surge in the Tampa area (26 percent odds over a 30-year mortgage). That would drive a 10-foot surge and cause $175 billion in damage. Add 30 inches of sea level to that and most of Tampa is under water. From a risk-planning perspective, we have to prepare for these disaster scenarios.
It’s not just sea-level rise that’s alarming, but also changes in storm intensity and rainfall. Ocean temperature affects the “potential intensity” of a hurricane, the speed limit on its strength.
While most hurricanes don’t reach their potential, some do. Warming oceans are raising the speed limit, and it’s likely that a greater percentage of hurricanes will reach the highest intensity categories. A few will exceed the most intense levels seen historically.
Meanwhile, intense rain events are becoming more frequent. Warmer air holds more water vapor. When the conditions are right, as they are in a hurricane, rainfall is more intense. The energy that drove Hurricane Harvey’s astonishing rain on Houston last year came directly from the extremely warm waters in the Gulf of Mexico. Extreme rainfall is becoming more common as the climate warms.
We have the power to prevent the worst-case climate scenarios that would make swaths of coastal Florida uninhabitable by the 22st century.
A huge shift away from fossil fuels would reduce carbon emissions and ease the warming curve. It’s encouraging that renewable electricity generation rose globally by 6.3 percent in 2017, and now provides about 21 percent.
But we have our work cut out for us. Global carbon emissions rose in 2017 after several years of flat emissions. Warming is continuing and sea levels are rising because of the heat that’s already in the atmosphere. That means that the need to adapt with increased community resiliency and insurance incentives is crucial.
Let the start of a new hurricane season underscore the need to respond to the increasing hazard.
Dr. Timothy Hall is a Senior Scientist at NASA’s Goddard Institute for Space Studies in New York City.Â
“The Invading Sea” is a collaboration of four South Florida media organizations — the South Florida Sun Sentinel, Miami Herald, Palm Beach Post and WLRN Public Media.
