Picture a beach along the same vast ocean you know today—the same powerful waves and shifting tides, reflecting the same beautiful sunsets, even the same green-blue water. Now imagine a crowd gathered at the shoreline, standing in a big circle, gawking at something that just washed up. Kids tug on their parents’ shirt sleeves, asking questions about the dead creature lying on the sand. Reporters arrive. The story is momentous even if the takeaway isn’t much fun. Everyone knows there used to be fish in the oceans—kind of like the ones that still live in some rivers and lakes, except they could be much bigger, sometimes meaner, more diverse, more colorful, more everything. But those mythical ocean fish all died. Except maybe this one. This one was alive in there, and now it’s dead too.

In 2017 , Payne and several colleagues looked into the source of the aforementioned extremely bad stuff that led to the Great Dying. They concluded that temperature-dependent hypoxia—loss of oxygen due to changes in temperature—caused about 70 percent of the losses. An oddly familiar culprit was fingered for this temperature change: "rapid and extreme climate warming." Payne and his pals weren't the first to draw comparisons between the events leading up to the Great Dying and the changes we're seeing today. A previous study had found that the Great Dying had resulted from rising carbon emissions—caused at that time by geothermal events—that occurred over the span of two to 20 millennia ; in other words, the blink of a geological eye.

We know that, about 250 million years ago, some extremely bad stuff happened, because almost everything on Earth that was alive at that time died very quickly, taking only a few million years to die off. This event is not to be confused with the meteorite impact that happened 65 million years ago—the one that supposedly wiped out the dinosaurs. That was nothing. A lot of those dinosaurs never went truly extinct; they're now known as "birds," and quite a few mammals made it, and evolved into humans, in pretty short order. This earlier event, the Permian–Triassic Extinction, is frequently called "the Great Dying" by paleontologists who like historical events to sound like Morrissey album titles. It made the Earth pretty quiet for a while—the oceans quietest of all.

To get into Payne's frame of mind, we have to look at two areas of history. First, there's pre-dinosaur times, where we can find a precedent for the kind of huge-scale extinction we're seeing now. Then, we have to look at the past few hundred years, to understand why our fishless future kind of looks like, uh, the present.

According to Stanford University paleobiologist Jonathan Payne, an expert in marine mass-extinction events, a scenario where all the ocean's fish, mammals, and other creatures—even tiny animals like krill—are all gone is far from science fiction. The type of die-off that would lead to a largely lifeless ocean has happened before, and we're well on our way to seeing it happen again.

"The relevant thing we know from these recent results is that the patterns of warming, and loss of oxygen from the ocean that can account for the extinction at the end of the Permian are the same features we're starting to see right now," explained Curtis Deutsch, a chemical oceanographer at the University of Washington and one of Jonathan Payne's colleagues on that 2017 study.

"Shifting baselines" have to do with everyone's gut-level perception of the natural world. The term refers to our tendency to perceive our own early experiences of ecology as the norm, in contrast to what we see later in life. To explain with a non-oceanic example, my own childhood memories of summers in California's Inland Empire include street gutters choked with thousands of California toads. Twenty years later, those toads are mostly gone—likely decimated by chytrid fungus infections. Their loss leaves me with the false impression that the natural order in Southern California has vanished in a very short time, when actually, the damage humanity has caused here is of much longer duration and much larger in scale than the loss of one species of toad (a species that arguably wasn't "supposed to be there" in the first place). Much more serious losses of biodiversity have been rolling out for centuries, but I don't miss animals like the Southern California kit fox, which went extinct over a century ago, because my own baseline never included them.

To be clear, the Great Dying wasn't 100 percent caused by warming either. But whatever the cause, 286 out of 329 marine invertebrate genera we know of died back then. All the trilobites and blastoids died, for instance. Every single one! But no one mourns the trilobites and blastoids, and that actually helps illustrate why we fail to grasp that we're annihilating life in the oceans. There's actually a sociological term for this phenomenon: it's called a shifting baseline.

Holistic accounting of the numbers of various species in the oceans have only begun recently, so it's hard to pin down exact numbers, but according to a 2015 report by the World Wildlife Fund , the oceans lost 49 percent of all vertebrates in just the time between 1970 and 2012. So rather, we should try and imagine the perspectives of people who saw the oceans when they were teeming with life, and Deutsch suggests reading accounts from the Age of Exploration. If they could time travel, Deutsch said, the Spanish explorers who first visited the New World would look at our ocean today, and say, Wow, that's dead.

Aesthetically, it won't be very different, according to Payne. A point I came across again and again in my research is that crystal-clear blue waters are often relatively lifeless. It's rare to look at the ocean and see strong indications of life—even plant life. "It's not carpeted in green, there aren't cells everywhere photosynthesizing," Payne said. "The color you see is mostly just the physics of light absorption and water." So in most places, you wouldn't actually see anything at all by looking at the ocean, just as a flight over the Great Plains doesn't tell you anything about the decline of the American buffalo.

Similarly, according to Deutsch, we won't collectively care about the death of all the fish, because when it finally happens, our baselines will have shifted so much that the lack of fish will seem normal.

"They would describe coming in on their ships through the Gulf of the Caribbean and not even being able to get to shore because the backs of the sea turtles were just so thick they couldn't get their boats in," Deutsch said. Indeed, when Columbus arrived, there were so many turtles, they thunked against the hull of his ship all night, keeping his crew awake. Today, spotting a sea turtle is a momentous event, because the number of sea turtles in the Caribbean is down to about 3 to 7 percent of what it was before Europeans arrived.

I have seen precisely one wild sea turtle in my entire life, and that was because I was searching for one.

I was off the northeast coast of Queensland, Australia, at the time, snorkeling in the Great Barrier Reef in the hopes that it might at least partially correct my own shifting baseline vis-a-vis ocean biodiversity. Even if you've never had the extreme privilege of visiting a coral reef, you've undoubtedly seen one, as magnificently CG-rendered in Finding Nemo, or majestically photographed for the BBC's Blue Planet TV series, which means you know the broad strokes of what a coral reef is—a place so teeming with life that it's one of the rare places for which the word "teeming" seems appropriate.