ATHENS — If you are Jeff Bezos, you probably don’t need to read this. That’s because if you’re a billionaire preparing to gallivant around Europe this summer, chances are you’ll be doing it at least partially aboard a yacht. For the rest of us, refreshing sea breezes are not a given as the Continent girds for what is shaping up to be the hottest summer on record. Tourists, Americans among them, are facing a silent killer in the form of heat stress. 

The image of a couple rescued from Joshua Tree National Park after running out of water seemingly has its counterpart in the mosaic of Greek islands, popular vacation spots whose natural environments can quickly turn hazardous beneath the summer sun’s harsh glare.

Summer hasn’t even officially started but the southern flank of Europe is already in Mother Nature’s crosshairs. On June 9, as Greece faced the first heat wave of the season, a popular British television host, Dr. Michael Mosley, succumbed to the elements on the Greek island of Symi. As of June 16, a retired American police officer visiting the Aegean island of Amorgos who set off on a hike Tuesday morning was still missing.

Also on Sunday, a 55-year-old American male who had been reported missing since June 11 at Mathraki was found dead on a beach on the remote island near Corfu. 

And on the island of Samos, a 74-year-old Dutch tourist who had gone missing after setting off on a hike on the island was found dead on Saturday and two French tourists were still missing Sunday on the small island of Sikinos, not far from the more popular Santorini.

Although the details of each situation are different, they add up to a cautionary tale for travelers this summer: without adequate protection from the heat a seemingly ordinary vacation outing can quickly turn into a struggle for survival.

Last year Europe faced a record number of “extreme heat stress” days, and this year is shaping up to be no different or even worse. As an official with the World World Meteorological Organization told the Agence France-Presse, “Heat is a silent killer, because symptoms are not so easily evident. And when these underlying conditions are present, the consequences can be very bad, and even catastrophic.”

Add to that, tragic, as in  the case of Dr. Mosley, who had an audience of millions through BBC programs like “Trust Me, I’m a Doctor.” According to his wife, he had set off from a beach area to head for the next town on a known but steep and rocky path, and at some point went in the wrong direction. The 67-year-old collapsed and died only 33 feet from his destination. Rescuers found his body after a four-day search.

The temperature at the time of his hike  was reportedly around 99 degrees. That may not sound like a lot to residents of the American Southwest, where daily summer temperatures higher than 115 degrees are often the norm. But the terrain of Mediterranean islands can be harsh and  shade is virtually nonexistent. Short distances that might look doable on a map can become daunting or impossible once heat stress sets in.

When that happens, the body’s natural cooling system can become overwhelmed. The trajectory from disorientation and dizziness to organ failure and death can be fast — particularly for the elderly. 

Or not so old. The American tourist who went missing on the island of Amorgos on Tuesday, Albert Calibet, is only 59. Mr. Calibert, a retired Los Angeles County police officer, was reportedly already familiar with the rocky island, having visited several times in the past. However, like Dr. Mosley, he set off on his hike alone — never a wise move.

He started in the village of Aegiali  with the intention of hiking to the island’s small port of Katapola. That is not an uncommon route on an island renowned for its hiking trails and sweeping sea views, but is still a four-hour journey on foot. Temperatures were above 100 degrees as Mr. Calibet got started on his hike.

An island shopkeeper told Greek television that Mr. Calibet had shown her the route he planned to take and had a bottle of water with him. But would one bottle have been enough?

The Amorgos deputy mayor for tourism, Popi Despotidi, told Greek media that “temperatures are very high, like all of Greece during the heatwave…we’re guessing he felt dizzy and collapsed somewhere.”

ABC reported that Mr. Calibet’s brother and the missing American’s girlfriend were on their way to Amorgos — a journey that typically involves a flight to Athens followed by a ferry trip of at least six hours — to help in search-and-rescue efforts.

Whether or not Dr. Mosley and Mr. Calibet suffered dehydration on their ill-fated hikes was not immediately clear. But the risk of dehydration, coupled with the risk of heatstroke, is what led Greek officials to shut down access to archaeological sites more than once last week between the hours of noon and 5pm. Temperatures in Athens soared  to 104 degrees on June 13, but by the weekend had cooled down.

However, the official start of summer is still a few days away. There are indications that some American travelers are already pivoting to cooler destinations like Brussels, Munich, and  Zurich.

The prospect of more heat waves in southern Europe, though, need not be a deterrent to travel in the affected areas. The old adage “When in Rome, do as the Romans do” applies now more than ever — in the hottest hours of the day on many Mediterranean islands, one sees very few locals out and about. They know from experience that peak heat of the day is best avoided.

Also, in hiking as in swimming, this old rule is as relevant as ever — never venture out alone. It could be hotter, as well as lonelier, than you think.