What might it be like to swim in an ocean that's 100 degrees?

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For a moment, as I followed the stories of this summer's devastating global heat wave, I found it hard to accept that our climate crisis has already become this catastrophic. The tragedies in Greece. The unrelenting, monthlong, historic high temperatures through wide corridors of the United States. The emerging forecasts that none of this is likely to be an aberration.

Then, a few weeks ago, the ocean temperature off Miami hit 95 degrees. A visceral alarm gripped my entire being. I kept repeating the number in stunned disbelief. It couldn't possibly hold, I told myself — and it didn't. By the end of the month, at least one reading had soared past 100 degrees.

Through all recorded time on planet Earth, humans have stood at the ocean's edge, gazing out at the horizon, in awe of the blue jewel we call home. Even from their vantage point a quarter million miles away, astronauts have expressed sheer wonder at the sight of our special pale blue dot, as Carl Sagan so eloquently put it.

Yet in recent weeks that blue dot has been suffering through a climate calamity most of us simply weren't prepared for. Yes, we've read about the accumulation of greenhouse gases that are warming our atmosphere. But I dare say, for many of us, the radical heating of our oceans is a frightening new juncture in human history that has gone largely unnoticed.

Millions of people dating back to ancient days have waded and bobbed and frolicked close to shore for exercise, peace and pleasure, and to connect with perhaps the grandest of all of Mother Nature's majestic features. But to step into the water off Miami late last month was akin to stepping into a hot Jacuzzi, the antithesis of refreshing and inspiring. Years from now, we may well remember the summer of 2023 as the beginning of an era when many of our oceans stopped serving as a glorious place of recreation.

My childhood was spent in the very waters off the Florida coast that recently registered temperatures in the triple digits. Growing up in Fort Lauderdale, the memories that loom largest are oceanic — spending all day splashing in the surf, laughing with my brother and sister, dunking one another, riding waves and playing endless underwater games, racing out to this or that buoy, flopping into bed at night exhausted and exhilarated by the magic caress of our irreplaceable backyard playground.

At age 9, after the Cuban Revolution, I searched the horizon to catch a glimpse of Cuba, this suddenly forbidden island. My mother pointed out across the ocean and said to me: "There. Havana is just across there. It's so close that you, you little swimmer, you could actually swim there." Later, after five attempts over 35 years, I finally did make that crossing. But I couldn't have made that swim last month. In such hot water, the body heat I'd generate from the duress of the effort — a continuous 52 hours and 54 minutes — would quickly lead to overheating and failure. And danger. Hyperthermia would conquer even the strongest of wills.

Of course, that would only be one small consequence of swimming in these heated waters.

Years ago, the Chambers of Commerce along Florida's shores were surely consumed with worries about the increase in jellyfish swarms that have come with warmer waters. Now they're no doubt huddled in meetings, contemplating the disaster that will ensue if these uncomfortable water temperatures drive tourists away for good.

And Florida is far from the only place where water temperatures are rising. Across the lower latitudes near the equator, this marine heat wave has been massive. From southern Mexico through the Caribbean and to the western Indian Ocean, 40% of the world's oceans have already fallen victim to the blunt force trauma of climate change. As of late June, it was warm enough to meet the criteria for a marine heat wave.

But somehow it's escaped our notice — overshadowed by the dire warnings of geophysicists who have described much of New York City going underwater and the scientists who chronicled the precipitous loss of habitat for animals that prowl the Arctic, as global warming shrinks the ice sheets they once used to hunt and fish. Every summer, it seems, we hear more about wildfires and the horrific toll they take: the loss of lives and homes. As fires ravaged the Australian bush, we witnessed in collective terror thousands of helpless animals running for their lives through burning forests.

It might be harder for us to relate to marine life, but the coral reefs, vital to the existence of many shallow water fishes, have been bleaching and dying practically overnight. Picture the dead fish floating on the ocean surface, the dead lobsters on the ocean floor: This is what the shores of Florida look like now. These creatures are fighting for their lives, much like the deer in the forests.

I've swum in every ocean except the Arctic, and I've often been asked to name my favorite one. But today I am not waxing poetic about their breathtaking beauty. I am grief-stricken at what we human beings have carelessly and greedily done to our home, our magnificent planet. Today I look across the vast expanse of any ocean and, beyond the majesty and mystery, I worry about ocean warming that will horrify us, that will diminish or even destroy our daily relationships with our blue jewel.

Diana Nyad is a writer, a public speaker and an endurance swimmer. This article originally appeared in the New York Times.