As the Northern Hemisphere marks the first day of summer, deadly heat waves are scorching cities across four continents, indicating that climate change can potentially make this summer the hottest in 2,000 years.

Record temperatures already reached in recent days are suspected to have caused hundreds if not thousands of deaths across Asia and Europe.

In Saudi Arabia, nearly 2 million Muslim pilgrims are finishing the hajj at the Grand Mosque in Mecca this week. But hundreds have died during the journey amid temperatures above 51 degrees Celsius (124 degrees Fahrenheit), according to reports from foreign authorities.

Egyptian medical and security sources told Reuters on Thursday that at least 530 Egyptians had died while participating in the haj – up from 307 reported as of yesterday. Another 40 remain missing.

Countries around the Mediterranean have also endured another week of blistering high temperatures that have contributed to forest fires from Portugal to Greece and along the northern coast of Africa in Algeria, according to the U.S. National Oceanic and Atmospheric Administration's Earth Observatory.

In Serbia, meteorologists forecast temperatures of around 40 degrees Celsius this week as winds from North Africa propelled a hot front across the Balkans. Health authorities declared a red weather alert and advised people not to venture outdoors.

Belgrade's emergency service said its doctors intervened 109 times overnight to treat people with heart and chronic health conditions.

In neighboring Montenegro, where health authorities also warned people to stay in the shade until late afternoon, tens of thousands of tourists sought refreshment on the beaches along its Adriatic coast.

Europe this year has been contending with a spate of dead and missing tourists amid dangerous heat. A 55-year-old American was found dead on the Greek island of Mathraki, police said on Monday – the third such tourist death in a week.

Parts of the U.S. Northeast and Midwest are also wilting under a heat dome, with more than 86 million people under a heat alert on Thursday, according to the National Weather Service.

A heat dome occurs when a strong, high-pressure system traps hot air over a region, preventing cool air from getting in and causing ground temperatures to remain high.

Under its heat emergency plan, New York City said it would open its cooling centers for the first time this year.

Meteorological authorities also issued an excessive heat warning for parts of Arizona, including Phoenix, on Thursday, with temperatures expected to reach 45.5 degrees Celsius.

Counting the dead

India's summer period lasts from March to May, when monsoons begin slowly sweeping across the country and breaking the heat.

But New Delhi on Wednesday registered its warmest night in at least 55 years, with India's Safdarjung Observatory reporting a temperature of 35.2 degrees Celsius at 1 a.m.

Temperatures normally drop at night, but scientists say climate change is causing nighttime temperatures to rise. In many parts of the world, nights are warming faster than days, according to a 2020 study by the University of Exeter.

New Delhi has clocked 38 consecutive days with maximum temperatures at or above 40 degrees Celsius since May 14, according to weather department data.

An official at the Indian Health Ministry said Wednesday there were more than 40,000 suspected heatstroke cases and at least 110 confirmed deaths between March 1 and June 18, when northwest and eastern India recorded twice the usual number of heat wave days in one of the country's longest such spells.

Gaining accurate death tolls from heat waves, however, is difficult. Most health authorities do not attribute deaths to heat, but rather the illnesses exacerbated by high temperatures, such as cardiovascular issues. Authorities therefore undercount heat-related deaths by a significant margin – typically overlooking thousands if not tens of thousands of deaths.

Record temperatures

The heat waves are occurring against a backdrop of 12 consecutive months that have ranked as the warmest on record in year-on-year comparisons, according to the European Union's climate change monitoring service.

The World Meteorological Organization says there is an 86% chance that one of the next five years will eclipse 2023 to become the warmest on record.

While overall global temperatures have risen by nearly 1.3 degrees Celsius above pre-industrial levels, climate change is fuelling more extreme temperature peaks – making heat waves more common, more intense and longer-lasting.

On average globally, a heat wave that would have occurred once in 10 years in the pre-industrial climate will now occur 2.8 times over 10 years, and it will be 1.2 degrees Celsius warmer, according to an international team of scientists with the World Weather Attribution (WWA) group.

Scientists say heat waves will continue to intensify if the world continues to unleash climate-warming emissions from the burning of fossil fuels.

If the world hits 2 degrees Celsius of global warming, heat waves would on average occur 5.6 times in 10 years and be 2.6 degrees Celsius hotter, according to the WWA.