Climate change is slow, gradual, almost imperceptible — until suddenly it’s not. One day, it seems like a normal summer. The next, the temperature soars to an unbearable 121 degrees. In Canada.

That record for the entire country was set last Tuesday in Lytton, British Columbia, as the Pacific Northwest suffered through its most punishing heat wave in recorded history. The same week, it was 108 in Seattle and 116 in Portland, Ore. — both all-time records.

In a region where many homes are not air-conditioned, it is not yet clear how many people died from the extreme heat; in British Columbia alone, officials said there were more than 700 “sudden and unexplained” deaths during the week, three times the normal average. At least 95 deaths in Oregon are suspected of being heat-related. Dozens of deaths are being investigated in Washington state as well.

The cause of all this was a massive, stubborn “heat dome” that parked itself on top of the region and refused to move. As you know, it is not possible to definitively attribute any specific weird, unprecedented weather event to climate change. But a pattern of increasingly frequent, weird, unprecedented weather events is precisely what climate scientists warned us about.

They also warned about other catastrophic phenomena. According to the Yale School of the Environment, sea level in the Miami area has risen by a foot since 1900 — with most of the increase taking place since the Champlain Towers complex in Surfside, Fla., was built 40 years ago. It is possible, perhaps likely, that some design or construction flaw led to the building’s shocking collapse. But greater-than-anticipated exposure to the corrosive effects of intrusive seawater certainly did not improve the building’s structural integrity, and could have diminished it.

The planet is warming because human activity has raised the concentration of carbon dioxide in the atmosphere by more than 47 percent since the dawn of the Industrial Revolution. Atmospheric carbon is now at a level not seen since 3 million years ago, an epoch when average sea level was 50 to 80 feet higher than today, according to the National Oceanic and Atmospheric Administration.

We desperately need to slash carbon emissions and stop making things worse. But we also need to reckon with the myriad implications of the damage we have already done.

That means reexamining all kinds of assumptions — however reasonable they once were — that went into the way human infrastructure has been designed and built. What seemed like normal environmental parameters may no longer apply.

Seattle, Portland and Vancouver, B.C., will still have plenty of cool, rainy days. But now we know that those cities — along with even unlikelier places, like the town of Verkhoyansk in arctic Siberia, which experienced a 100-degree day last year — are vulnerable to periods of extreme and unprecedented heat.

Does this mean that Alaskans and Norwegians should all run out and buy air conditioners? No, and in fact that would be counterproductive, since it would create new demand on the power grid, and much of that demand would be met by burning more fossil fuels. But it does mean that northern cities should develop robust plans for protecting vulnerable residents if an extreme heat wave — and, perhaps, accompanying wildfires — should strike.

It means that beachfront structures in vulnerable areas should be inspected, and their safety reevaluated, in light of the impact of rising seas. It means that projects to stem worsening flooding, such as a massive 20-foot sea wall that the Army Corps of Engineers proposes for Miami, should be given new urgency. It means that the price tag for such undertakings, however daunting it may seem, must be weighed against the cost in human life of doing nothing.

It means understanding that “100-year floods” or “500-year hurricanes” will no longer confine themselves to the intervals we have assigned them. Periods that once were considered extremely wet or dry will no longer be seen as extreme; the weather bell curve has widened and the temperature bell curve is shifting measurably to the warm side.

It means being smart enough to use the spending that will be necessary to adapt to climate change as an engine of economic growth and transformation. It means seeing adaptation not just as an environmental program but as a jobs program as well.

And it means being smart enough to limit the damage, not so much for ourselves as for our grandchildren.

The climate change and weather instability we are experiencing will be with us for centuries. But it will all get much worse unless humanity reduces carbon emissions by shifting to clean, renewable energy. One simple question for climate-change skeptics: What is it about 121 degrees in Canada that you don’t understand?

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