Deadly heatwaves are scorching cities on four continents as the Northern Hemisphere marks the first day of summer, a sign that climate change may again help to fuel record-breaking heat that could surpass last summer as the warmest in 2,000 years.

Record temperatures 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 reportedly died during the journey amid temperatures above 51 degrees Celsius.

Egyptian medical and security sources told Reuters on Thursday that at least 530 Egyptians had died while participating — up from 307 reported the day before.

More than 100 die from heat in India

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 35.2C at 1am.

Temperatures normally drop at night, but scientists say climate change is causing night-time temperatures to rise.

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

An official at the Indian health ministry said on Wednesday there were more than 40,000 suspected heat stroke cases and at least 110 confirmed deaths between March 1 and June 18.

Gaining accurate death tolls from heatwaves, however, is difficult.

Most health authorities do not attribute deaths to heat, but rather to 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.

The heatwave comes as a new report interviewing 2,178 people across India found one in three of the respondents had either already moved or considered moving because of weather-related disasters.

The report said 45 million people could be displaced in India by 2050 due to slow-onset impacts such as melting glaciers or rising seas if climate pledges and targets were not met.

Europe and US sweltering

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 US National Oceanic and Atmospheric Administration's Earth Observatory.

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.

A broad swath of the eastern US was also wilting for a fourth consecutive day under a heat dome, a phenomenon that 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.

In the nearby state of New Mexico, a pair of fast-moving wildfires abetted by the blistering heat have killed two people, burned more than 23,000 acres and destroyed 500 homes, according to authorities.

All told, nearly 100 million Americans were under extreme heat advisories, watches and warnings on Thursday, according to the federal government's National Integrated Heat Health Information System.

The brutal temperatures should begin easing in New England on Friday, the weather service said, but New York and the mid-Atlantic states will continue to endure near-record heat into the weekend.

More heat to come?

The heatwaves 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.