Red tide is killing Florida’s southwest coast. Fish, manatees, sea turtles — some of them endangered — and nine dolphins have washed up dead on the beaches, and all of them are confirmed or suspected to have been poisoned by the algal bloom. The body of a young whale shark was found on a beach in late July, and biologists believe that it was the first known whale shark to have been killed by red tide.

Now the toxic algae — Karenia brevis — is working up the coast from Sanibel Island to Tampa Bay. Respiratory irritation in humans has been reported as far north as Manatee County, just south of Tampa Bay, where high concentrations of the algae were measured last week. The water off Pinellas County — Clearwater, Largo, St. Petersburg — had elevated concentrations of red tide beyond a normal “background” state for the first time this month.

Florida Gov. Rick Scott declared a state of emergency for seven counties on the southwest coast Tuesday.

Algal blooms are common in Florida and along the rest of the Gulf Coast, but they don’t always get this bad. This one began in 2017 and, over the past few months, has slowly ballooned into a nightmare scenario for residents and business owners — not to mention the thousands of animals that have died.

There are several ways human activity can exacerbate a bloom, but the main culprit is allowing nitrogen-rich material such as fertilizer to run off into natural water sources. The same fertilizer that helps sugar cane, tomatoes and corn grow in the Sunshine State feeds algae when it reaches the ocean.

Humans are also playing a role by driving up global temperatures via greenhouse gas emissions. In a letter published by the journal Environmental Science & Technology, researchers at the University of Florida and the University of North Carolina said that “climate change will severely affect our ability to control blooms, and in some cases could make it near impossible.”

As air and ocean temperatures increase, the environment becomes more hospitable to toxic algal blooms in several ways, according to scientists and the Environmental Protection Agency.

Warmer water

In freshwater such as the Great Lakes, a different kind of “algae” — cyanobacteria — flourishes at warmer temperatures. Combined with fertilizer runoff, red tides due to cyanobacteria have spiraled out of control in recent years, particularly in western Lake Erie. In freshwater cases, the harmful algal bloom doesn’t just threaten wildlife, it also threatens the water that people drink and bathe in. In 2014, Toledo’s water supply was so poisoned with cyanobacteria toxins that the entire city had to drink bottled water for three days, according to the Cleveland Plain Dealer.


Temperature trend in the eastern Gulf of Mexico since 1975. (Data from NOAA/Graphic by Climate Central)

Global ocean temperature has risen about 2 degrees since 1900. Maximum annual temperature in the eastern Gulf of Mexico has also climbed about 2 degrees since 1977, according to buoy data collected by the National Oceanic and Atmospheric Administration.

K. brevis — the algae affecting Florida — has an interesting response to rising temperature. It thrives in water temperatures up to about 83 degrees, but if it gets much warmer than that, the algae doesn’t grow as quickly. However, researchers have found that K. brevis can tolerate higher temperatures and grow faster given more carbon dioxide. Atmospheric carbon dioxide surpassed a concentration of 400 parts per million in 2015 and will continue to rise as society burns more fossil fuel.

Extreme rain events wash more fertilizer into the ocean

For the eastern United States, 2018 has been the year of floods. Week after week, torrential downpours on saturated ground have pushed rivers and streams beyond their banks.

What doesn’t get caught in reservoirs or absorbed into the ground eventually reaches the ocean, carrying all of the minerals, pollution and nutrients it picked up along the way. If torrential rain happens to flood regions with heavy agricultural production, the runoff tends to be high in fertilizer — chemical or manure. In either case, it’s liquid gold for coastal algae.

More shallow water along the coast

Average sea level across the globe has risen 7 to 8 inches since 1900, according to the 2017 National Climate Assessment, a significant portion of which has occurred since 1993. Relative to the ocean level in 2000, scientists project about another 0.5-foot rise by 2030, up to 1.2 feet by 2050 and as much as 4.3 feet by 2100.

Even the low end of the projection calls for one foot by the end of the century, which is more than enough to push water into areas that it hasn’t inhabited in at least a couple of millennia. As places such as Tampa, Miami and Charleston, S.C., lose shoreline, the ocean gains more shallow, warm water along the coast, and a larger area of highly favorable breeding ground for algae.

More CO2 = more plant growth

Like land plants, algae breathes in carbon dioxide and exhales oxygen. The more carbon dioxide in the atmosphere, the more the plant can grow and multiply. Rapid growth is possible with higher levels of CO2, “especially toxic blue-green algae that can float to the surface of the water,” according to the EPA.

Droughts lead to saltier freshwater

One of the effects of climate change is more precipitation extremes, in both directions. In some parts of the world, droughts will become more intense and prolonged. Without rain to replenish lakes and ponds, the water becomes more saline — or salty — as it evaporates. If it gets salty enough, algae that typically only grows in the oceans will invade freshwater systems, too.

Editor’s note: A previous version of this story placed algae firmly in the plant kingdom. Turns out, that is the subject of debate. Cyanobacteria are considered bacteria and thus not part of the plant kingdom. What can be said about all algae and cyanobacteria, though, is that it relies on photosynthesis for energy.