Why Heat Warning for 28 states in US?

The National Weather Service has warned that extreme heat will affect more than 100 million people in the US this week, with temperatures reaching triple digits in some states and temperature records being broken in many areas across the country.

  "Above-normal temperatures will continue to prevail across much of the U.S. through the end of the week, with a significant portion of the population still subject to heat advisories and warnings," the agency said.

  Heat advisories and warnings have been put in place for 28 states, with central and southern states facing the brunt of scorching heat. Parts of Oklahoma reached 115F (46C) this week, while the Dallas area reached 109F (42C).

  EMS in Tulsa, Oklahoma reported an increase in heat-related emergency calls this year. The city's Office of Emergency Medical Services said it has received nearly 250 calls this year, a number that is usually reached later in the summer.

  It's very concerning and alarming for lives living out there. Especially because the number of patients being transported suggests that some of these calls are heat strokes that can be fatal.

  In the northeast, where temperatures are settling in at over 32C, city officials have warned residents to limit outdoor activities during the hottest times of the day.

  Philadelphia declared a "heat health emergency" for Thursday and set up a "heat line" number for residents experiencing intense heat. Boston opened community centers and swimming pools as places for residents to cool off.

  "It's clear that a changing climate is a risk to our health," Boston Mayor Michelle Wu said Wednesday. "I urge everyone to stay cool and safe and check on your neighbors this week."

  Extreme heat warnings were in place for Phoenix, America's hottest city, on Thursday and Friday. Temperatures are forecast to reach 113 F (45 C) Thursday afternoon and 115 F (46 C) Friday afternoon. The authority only issue heat advisories when temperatures are above average for the season, and in Phoenix in July that means temperatures over 112F. So far this year, the city has broken or tied four daily record highs and nine nighttime lows.

  The effect of heat is cumulative, and the body only begins to recover when temperatures drop below 80F. Climate scientists have warned that heatwaves - which have spread across Europe and Asia this summer - will become more intense and longer if the climate emergency is not addressed. A study published in May showed that human influence on the climate has made a particular heat wave in South Asia 30 times more likely.

  Joe Biden said on Wednesday in Somerset, Mass., about the climate crisis that global warming is an "emergency," but stopped short of declaring a national emergency, as activists had hoped the president would do. Such a declaration would allow Biden to block oil imports or order the military to work on renewable energy production. The White House said a formal emergency declaration was "still on the table."

  "It is literally, not figuratively, a clear and present danger," Biden said. "The health of our citizens and communities is literally at stake."