I grew up in the part of the country where the summertime sun beats down on your head, rolling beads of sweat drip down your face and body, pasting your clothing onto your skin, and stultifying humidity steals your breath. Sweltering heat is a signature of the Mississippi Delta, the storied Southern land area, which encompasses my hometown of Memphis Tennessee. As a kid, I was oblivious to the official meteorological report, but I remember near 90-degree days as an everyday thing. Back then, I would repeatedly run in and out of the kitchen door — as kids do — inspiring a swift and loud rebuke by whichever adult happened to witness my action. “Close that door and stop letting all the air out! ” — they would shout, ‘air’ a shorthand for air conditioning.

I recall the first years of working in New England as a young professional. As a veteran of hot weather, I thought it was odd that so few people I knew had air conditioning, or even felt they needed it. Why, they would ask, when there are maybe a half dozen really hot summer days? Days, they would point out, that usually occurred in August when summer was just about over. Not anymore.

Extreme heat apparently knows no calendar. Greater Boston’s recent July wave of extreme heat prompted Boston Mayor Michelle Wu to declare a heat emergency. Here, temperatures reached 90 and above, and in California and parts of the Southwest topped 100 degrees. In Europe, it was hot enough to melt part of an airport runway in the U.K. where the temperature climbed to its highest rate on record. Heat-sparked wildfires are popping up around the world.

Climate change experts say this is the evidence of the long-predicted warming of the earth made worse by human activity. But even though I intellectually know global warming is linked to extreme heat, I am shocked by the current numbers of heat-related deaths. At least 1,500 people died of heat strokes and exhaustion during the last few weeks in Spain, Britain, Portugal and France. A number, experts say, that will probably climb. According to data from the Centers for Disease Control, there were 1,577 American heat-related deaths in 2021 — a 56 percent increase over the 1,012 deaths in 2018. This year’s deaths will include my friend Gwen, who died in May during a spike of extreme heat in Chicago. High temperatures so unseasonably early that the furnace in her apartment building was still on. Panicked residents say they complained to management and reached out to a local elected official who also reached out to the management company, but also got no response. Meanwhile, the extreme heat coupled with the heat from the building’s furnace quickly pushed inside temperatures up to 100-plus degrees. Gwen’s family is suing the building owners for alleged negligence, but that won’t bring her back.

Climate scientists and activists now have more grim evidence of the climate crisis. But I’m not sure the tragic consequences of this recent extreme heat will make people grasp how dire the situation is. Here the words of fierce young activist Greta Thunberg make it plain “I want you to act as if our house is on fire,” she once thundered. “Because it is.”