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With a baking hot summer just starting, millions of Americans have already begun flooding the nation’s beaches. They’ll face traffic jams, algae blooms, sunburn and a rising threat – painful jellyfish encounters.

Climate change, warming ocean temperatures, overfishing, invasive species and even fertilizer run-off mean there are more jellies than before in places where they didn't use to be, including box jellyfish, which have appeared in Florida and even New Jersey and in rare cases can be lethal.

While being stung by a jellyfish is seldom life-threatening, it is highly unpleasant, say those who have encountered them.

"It felt like someone slashed my skin with a knife," said Jana Paradiso of San Francisco, who was vacationing with friends in Positano, Italy a few years ago when she touched against what she thinks was a mauve stinger, a small jelly with long tentacles and a strong venom.

“It was a searing, sharp pain,” she said. By the time she got out of the surf, welts had started to appear across her arm. By the next day the pain was mostly gone, but not the memory.

“I didn’t go back in the water,” she said.

Why you're seeing more jellyfish

Marine scientists are recording large, regional shifts in jelly populations, many of which account for the feeling among beachgoers that there are more jellies out there than there used to be.

Jellyfish populations tend to wax and wane as conditions and their food supply does. In many species, this oscillation occurs in a 20-or-so-year cycle.

“It’s pretty clear that there are regional and local changes in jellyfish population,” said Laurence Madin of the Woods Hole Oceanographic Institution, who worked on a groundbreaking paper in 2013 that showed these oscillations.

But some overall trends are clear. One is that warmer ocean temperatures are allowing jellies to appear in new areas.

Moon jellies, for example, were never seen north of Cape Cod but now they’re often found up into Boston and the Gulf of Maine, Madin said. They are not deadly but cause unpleasant stings. In Norway, helmet jellyfish are showing in fjords.

“They want to remain in water temperatures that are suitable for them, so they’re moving north,” said Cathy Lucas, a professor of marine biology at the University of Southampton in the United Kingdom.

“If conditions are good, if they get a lot of food, you can have a jellyfish bloom,” said Jessica Schaub, an oceanographer at the Institute for the Oceans and Fisheries at the University of British Columbia in Canada who studies jellyfish.

Overfishing can also increase jelly populations because there are fewer things around to eat them.

“As their competitors die off, they just keep living,” said Schaub.

Fertilizer runoff from farms into places like the Gulf of Mexico causes huge areas of low-oxygen water known as dead zones. Fish can’t survive in these areas – but jellies can.

“Jellyfish require less oxygen and are more tolerant of these dead zones so they expand to fill in areas where finfish are no longer as successful,” said Maldin. “They tolerate a lot.”

Box jellyfish where they shouldn't be

One concerning shift has been in the range of box jellyfish, some species of which can be deadly.

"The box jellyfish that we have an abundance of in Hawaii has recently caused injuries in various beaches in Florida. The changing range of these jellies and increasing human population density, these things all work together in U.S. waters," said Angel Yanagihara, a research professor in the department of tropical medicine at the University of Hawaii who studies jellyfish venom.

Sometimes called "sea wasps," these jellyfish can be highly dangerous. They've generally been found in the tropics, including Hawaii, Puerto Rico and the Caribbean but now also exist in Florida and have been seen as far north as New Jersey.

In several places around the world, other non-native jelly species have appeared far outside their usual habitats.

The Atlantic coast comb jellyfish probably entered the Black Sea between Europe and Asia in a boat’s bilgewater. Then it began swamping the Black Sea's prized beaches.

Australian species have been seen in the Gulf of Mexico.

“They out-compete the native species there and there’s nothing to control their population,” said Schaub.

Immense jellyfish “blooms” forced the closing of beaches in Hawaii earlier this month and swarms of jellyfish blocked the intake valves at power plants in Sweden, Israel and the United Kingdom.

Finally, as more and more humans move to the coasts to be close to the sea, the amount of infrastructure built in the water, from piers to wharves to jetties, oil rigs and offshore wind turbines, all provides a perfect infrastructure for the juvenile form of the jellyfish, a polyp, to attach and grow, said Steven Haddock, a senior scientist at the Monterey Bay Aquarium Research Institute.

New research: No looming jelly-apocalypse

While there are more jellyfish around, there isn't a coming jellyfish apocalypse.

Two decades ago, some marine biologists began to fear climate change, pollution and ocean degradation could bring the collapse of many ocean species and pulsating masses of gelatinous jellies would take over the seas.

“There was a flurry of speculation and I think a lot of it was driven by the press, frankly,” the University of Hawaii's Yanagihara said.

An enormous study published in 2013 gathered all available data on global jellyfish populations and found that while jelly populations were increasing in some places, they were declining in others. Overall, the oceans weren’t going to turn into one great vat of jelly goo.

“The scientific community really tapped the brakes on that,” Yanagihara said.

Partly it was observational bias, said Jim Cutler, a senior scientist of the Mote Marine Laboratory and Aquarium’s Benthic Ecology Program in Sarasota, Florida. More people at the beach means more jellyfish encounters. “It’s like the phenomenon of shark sightings. A lot more people going to the beach means more shark sightings.”

In some areas, jellyfish populations are falling. In Florida, for example, they’re not as abundant as they were five years ago, he said.

“People think some of our predators, sharks and large groupers, might be learning that they can eat them,” Cutler said. “There can be a learning curve for predators to decide what they can and cannot eat.”

Now scientists are busy gathering data for a much larger jellyfish study with more than three times as much data as the 2013 study, some of it going back as long as a century. While analysis is still underway, one of the organizers says the findings overall are in line with the earlier research.

"There will be some regions where it’s increasing over time and some regions where it’s decreased over time,” said Lucas, one of the new study's leaders.

What to do if you're stung by a jellyfish

If you’re out swimming and see jellyfish, the first thing to do is get out of their way.

"You’re in the jellyfish’s habitat,” said Haddock. “So be smart and aware of the local wildlife. Use a wetsuit or rash guard, or stay out of the water if there are lots of stinging jellies around.”

If you see them on the beach, don’t touch them, said Schaub. “Jellyfish that wash up on the beach can still sting, the stinging cells remain active.”

If stung, don’t rub the area, which will just activate the venom packets, said Yanagihara.

Do not scrape the sting site or rub with sand. Pouring plain vinegar over the area can inactivate the venom packets so they don’t discharge, though it doesn’t neutralize the venom itself, she said.

To neutralize the venom, immerse the exposed area in warm water, such as a hot tub, for 30 to 40 minutes, she said. An over-the-counter preparation, StingNoMore, can also help.

Be cautious in the ocean

Two years ago, Lupe Aguilera of San Francisco decided to go for an ocean dip while on vacation in Puerto Vallarta, Mexico. She’d gotten about 20 feet offshore when she felt a burning sensation along her arm.

“When you swim in the ocean, you feel a little prick or tingle every so often. But this was different,” she said. “It felt like someone brushed me with a lit cigarette.”

“I started swimming back to shore and it just kept getting more intense. By the time I got to the point where I could stand up, I could see the lash marks, I had tentacles wrapped around my upper and lower arm.”

The hotel medic had her take a hot shower and then apply ice to the site. It took hours for the red marks to finally begin to fade away.

It hasn’t stopped her from swimming but she is more cautious.

“Now I ask the lifeguards, ‘Hey, what’s the jellyfish situation today?”