As the Northern Hemisphere welcomes the first day of summer, cities across four continents are experiencing deadly heatwaves, marking a stark reminder of the ongoing impacts of climate change.

Recent record-breaking temperatures are suspected to have caused hundreds, if not thousands, of deaths across Asia and Europe, with the potential for this summer to surpass last year's as the warmest in 2,000 years.

In Saudi Arabia, nearly two million Muslim pilgrims have faced extreme heat while completing the haj at the Grand Mosque in Mecca.

Reports from foreign authorities indicate that hundreds have died due to temperatures soaring above 51 degrees Celsius (124 degrees Fahrenheit). Specifically, Egyptian medical and security sources reported that at least 530 Egyptians had perished during the pilgrimage, a significant increase from the 307 reported just a day earlier. Additionally, 40 pilgrims remain missing.

Countries around the Mediterranean have also been battling another week of blistering high temperatures, contributing to devastating forest fires from Portugal to Greece and along Algeria's northern coast. This data, provided by the U.S. National Oceanic and Atmospheric Administration's Earth Observatory, highlights the widespread nature of the crisis.

In Serbia, meteorologists forecast temperatures around 40 degrees Celsius (104 degrees Fahrenheit) this week, driven by hot winds from North Africa. Health authorities have declared a red weather alert and advised the public to stay indoors. Belgrade's emergency services reported 109 overnight interventions to treat individuals with heart and chronic health conditions.

Neighboring Montenegro issued similar warnings, urging people to stay in the shade until late afternoon. Meanwhile, tens of thousands of tourists sought relief on the beaches along the Adriatic coast. However, the heat has proven deadly for some tourists in Europe, including a 55-year-old American found dead on the Greek island of Mathraki, the third tourist to die in a week.

In the United States, a vast portion of the eastern region is suffering under a "heat dome," a phenomenon where a strong high-pressure system traps hot air, preventing cooler air from entering and causing sustained high ground temperatures. New York City has opened emergency cooling centers in libraries and senior centers, while surrounding suburban districts have sent students home early to avoid the heat. An excessive heat warning is also in effect for parts of Arizona, including Phoenix, where temperatures are expected to reach 45.5 degrees Celsius (114 degrees Fahrenheit).

In New Mexico, wildfires exacerbated by the extreme heat have claimed two lives, burned more than 23,000 acres, and destroyed 500 homes. While heavy rains could provide some relief, thunderstorms are complicating firefighting efforts by causing flash flooding.

Overall, nearly 100 million Americans were under extreme heat advisories, watches, and warnings as of Thursday, according to the National Integrated Heat Health Information System. While the brutal temperatures are expected to ease in New England by Friday, New York and the mid-Atlantic states will continue to face near-record heat into the weekend.