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On a recent evening, about halfway between Norway and the North Pole, I stood alone on the wide-open top deck of a Russian ship called the Akademik Sergey Vavilov, hoping to spot a polar bear. Although my watch read 8 p.m., the Arctic sun blazed overhead like a Luxo lamp, and, from this high vantage, a sparkling, frozen ocean curled toward the horizon. Yard-thick chunks of ice, a hundred feet across or more, bobbed and scraped against the ship’s hull. The vessel plowed forward at an easy walking pace, splitting a few floes with gentle groans.

The Vavilov, a tough, Cold War-era oceanographic research ship, had been leased by an expedition cruise company to spend the summer touring the remote Arctic archipelago of Svalbard. It held an international complement of 95 adventurous passengers, plus Russian crew. At the moment, most of those guests were enjoying a barbecue on the lower back deck, four stories below my perch and out of sight behind a bunch of radar masts, but I’d bolted a burger and climbed up here as fast as I could. As an onboard naturalist, it was my job to spot wildlife—the main tourist attraction in Svalbard—and sea ice is prime territory for polar bears.

Polar bear spotting offers an exercise in extreme patience. You’re not looking for an animal-shaped profile so much as a distant, tiny, butter-yellow speck. I’d already been at it for eight hours since breakfast, peering through my spotting scope, with just an occasional kittiwake or harp seal to liven things up. But now, as I swept the horizon for the hundredth time, a form caught my attention. It was miles away, but as I watched, it moved perceptibly. I clicked on my VHF radio to call the ship’s bridge, one deck beneath my feet. “Got a PB at two o’clock,” I said. “And it looks like it’s on a kill.”

Things are heating up in our polar regions. More research, civilization, industry, tourism, exploration, inspiration, and concern are focused on the Arctic and Antarctica than ever before, and much of this attention is related to ����APP change. Our world is indisputably, inexorably warming—by a global average of about 1.4 degrees Fahrenheit since 1880—but the magnitude of that warming varies by region. The high Arctic and the Antarctic Peninsula have seen bigger temperature increases than anywhere else on earth, up to four or five degrees Fahrenheit in just the past 30 years.

Perhaps this is why, in discussions of ����APP change, the Arctic and Antarctica often get lumped together. But they are very different places—two extremes that are literally poles apart. The Arctic is an ocean surrounded by continents; Antarctica is a continent surrounded by oceans. The Arctic has thick sea ice that takes years to drift and melt; Antarctica has relatively little multi-year sea ice. Their ����APPs are both chilly, but otherwise dissimilar. Antarctica, on average, is colder, higher, drier, windier, and bleaker than the Arctic. It has no indigenous humans, and was first encountered by people less than 200 years ago. The Arctic, by contrast, has been settled for thousands of years, thanks to a (relatively) productive land environment. And the wildlife in the two regions couldn’t be more different: Antarctica’s hostile ����APP spurns flowering plants, insects, and land animals, all of which flourish in the far north.

Climate change is hard to comprehend because it is a large-scale phenomenon, out of sync with the tiny slices of space and time that any person occupies. In the past few years I have been fortunate to spend several seasons witnessing the changing at both ends of the earth, working alternately as a guide and researcher, entering icy worlds few get to visit, and staying there for months on end, year after year. And the more time I spend in these places, the more powerful and humbling they become. It’s the sweeping sense of scale that gets me. Here, where you can see the curvature of the earth—with no cell towers or office buildings to block the view—you begin to appreciate how small this world really is. Toward the poles, I feel I can almost grasp the globe.

Polar bears present a clear image of ����APP change in the Arctic, for good reasons (more on them in a minute). In the Southern Hemisphere, penguins are often portrayed as their natural counterpart. It’s easy to understand why: With warming temperatures, ice-loving polar bears and ice-loving penguins should both be in real trouble. But things are complicated in Antarctica, and in my experience, it’s not that simple. Populations of Antarctic penguins, for example, are stable on average. At least for now.

“Emperor and Adélie penguins are currently doing fine, except for some isolated instances,” says David Ainley, a California marine biologist who has been intensively studying penguins in Antarctica’s Ross Sea since the late 1960s. In 2002 Ainley published a book called The Adélie Penguin: Bellwether of Climate Change, which might seem to imply stormy skies for the birds. But neither Ainley’s book nor his current research shows continent-wide declines. Instead, in the past several decades, Antarctic penguin populations have generally held steady or even gone up. One 2013 paper, coauthored by Ainley, which described an Adélie Penguin colony that has grown rapidly since the 1990s, was even titled “Climate Change Winners.”

While most penguins seem to be thriving for the time being, some Antarctic penguins are not, and in the long term, ����APP change threatens them all.

Ainley’s Antarctic research focuses on frigid Ross Island, just off the Antarctic coast, almost due south of New Zealand; the island is home to Emperors and Adélies, the only two penguin species restricted to Antarctica. In 2009 I joined Ainley’s project for a hands-on field season at Cape Crozier, at Ross’s eastern tip, which hosts about 280,000 breeding pairs of Adélie Penguins (probably earth’s largest Adélie colony) as well as the world’s southernmost Emperor Penguin colony. A helicopter dropped me off with two other researchers at the beginning of the summer, and for the next two months, cut off from civilization, we slept on the ice in canvas tents, ate a lot of frozen cauliflower, and obsessed about penguins.

For me, staring at penguins for eight hours a day, seven days a week, was life changing. It sparked a personal love affair with Antarctica—or The Ice, as insiders call it. In the great penguin universe, Adélies are wind-up toys. Without natural land predators, the birds harbor little fear of humans, so it’s easy to appreciate their antics at close range. At first glance, the crisp black-and-white attire seems stiffly formal, nervous teenagers at prom. But these penguins have big personalities (in Happy Feet they were the charming little punks with Latin accents), and I soon grew used to gaggles of them at my heels, following me apparently out of sheer curiosity.

Cape Crozier’s landscape impressed me every bit as much as its charismatic avian residents. The Ross Sea remains nearly pristine, and will probably hold sea ice long after the ice has melted elsewhere. But even the Ross Sea is beginning to feel the effects of a shifting ����APP.

Large-scale changes in temperature and in the extent of sea ice will eventually touch Antarctica’s penguins. The apparent stability of penguin populations seems likely to be a near-term phenomenon. Already the birds’ ranges are shifting south. And while recent research by Ainley and other scientists has shown that penguins, including Adélies and Emperors, can move their nesting grounds to adapt to changing conditions, at some point even the most adaptable penguins won’t be able to go any farther toward the pole. In a recent paper Ainley and his coauthors warned that if global temperatures exceed 3.6 degrees Fahrenheit above preindustrial levels (predicted, in the same paper, to occur between 2025 and 2052), Emperor and Adélie penguin colonies north of 70 degrees south latitude could disappear—40 percent and 70 percent, respectively, of their current breeding populations. A tipping point is approaching.

In fact, it’s here. On the heavily studied northern tip of the Antarctic Peninsula, the continent’s “banana belt,” where midwinter temperatures have increased by as much as 9 degrees Fahrenheit since 1950 (more than anywhere else in the world), Adélie Penguin numbers are crashing. In early 2014 I witnessed several colonies there that are now half the size they were a decade ago. In the same area, Chinstrap Penguins are also disappearing, perhaps because of recovering whale populations and , and an adjacent Emperor Penguin colony also seems to be falling off. On the other hand, Gentoo Penguins, a more northerly species, are enthusiastically colonizing the peninsula. If trends continue, all these penguins will keep shifting southward. Nobody can say how far they can go before they reach the end of the world.

Antarctica’s penguins are relatively blessed to live in one of the last near-pristine places on the planet. Except on the northern Antarctic Peninsula, penguins on The Ice—tens of millions of them—are not currently in grave danger. But the meltdown happening now at the other end of the globe, in the high Arctic, suggests that a similar grim future awaits Antarctica—and the birds.

Within seconds of my radio call, people began to converge on the top deck of the Vavilov, anxious for a peek at the distant polar bear. One of the first to appear was Ian Stirling, perhaps the world’s best-known polar bear scientist, who, despite 40 years of research and more than 200 scientific publications, still gets excited every time he sees one. He squinted through my spotting scope for a few seconds.

“Sure looks like it’s tearing apart a seal,” he said.

Others lined up for a view of the bear, several miles away. Over the next couple of hours, our vessel eased closer, parting ice floes on a slow, direct approach. When we could make out bloodstains on the bear’s front legs and coils of intestine spilling from the seal carcass, Stirling began to interpret the scene.

“This looks like a healthy adult male bear,” he said. “It’s doing exactly what it should be doing right now: hunting seals on the sea ice. Polar bears around here get the majority of their annual calories during spring and early summer, when seals have their pups. For the rest of the year they eke out whatever sustenance they can find.”

In Svalbard the strategy is straightforward, but with one major flaw: If there is no sea ice, the bears get stuck on land and starve throughout the summer, because they can’t hunt very well from shore. Farther south we’d already seen several of these stranded animals—gaunt, mangy, desperate, dangerous—and even discovered the skin-and-bones carcass of one that had apparently collapsed in its tracks. Such encounters are increasingly common.

Arctic sea ice, unlike its counterpart in most of Antarctica, has been declining so spectacularly in recent years that some refer to it as a “death spiral.” Since 1980, by extent and thickness, Arctic sea ice volume has shrunk by up to 75 percent. On average, summer melt has arrived five days earlier each decade. As recently as 1988 four-year-old ice accounted for 26 percent of sea ice; by 2013 that had dropped to 7 percent. Various studies have forecast ice-free Arctic summers by about 2040—when one might paddle a kayak to the North Pole.

Of course, predicting the future is a notoriously unreliable business. Within the past couple of years scientists have criticized the Intergovernmental Panel on Climate Change for overestimating sea ice declines in the Arctic. But in fact the projections were too low: Arctic ice is melting even faster than the most extreme IPCC forecasts, and scientists recently reported seeing signs that both the West Antarctic and Greenland ice sheets had begun an irreversible collapse, a process that, when eventually complete, could add 40 feet to global sea levels.

In 2012 Stirling reviewed the available evidence of ����APP change on polar bears and concluded that the animals are in real trouble. Shrinking sea ice, so vital as a hunting platform, directly affects adult body condition and cub survival. Winter ice extent around Svalbard in 2014 was the fifth-lowest on record, and a new study found only 10 percent of females there had cubs in 2014, down from about half in the 1990s. By mid-century, Stirling says, the bears may disappear from all southern portions of their range. While a few northerly areas may become more favorable for bears in the short term, the rate of habitat change is downright scary. “Long-term viability,” Stirling says, with clinical understatement, “is uncertain.”

By 4 a.m. the vavilov had closed the gap and was parked in solid ice. A hundred yards off the bow, oblivious to us, the enormous male polar bear had nearly polished off its seal dinner. I took a quick head count: Of 95 guests on board, only four had gone to bed. Everyone else was still on deck, in brilliant sunshine, watching with rapt fascination.

Suddenly, the bear stopped eating. It blinked a few times, swung its massive head slowly from side to side, and took two hesitant steps backward. Then, almost comically, all four paws slid out, the belly flattened, and, by the time its nose touched the ice, the animal was fast asleep in a food coma. It stayed that way for most of the next two days, waking up occasionally to chew on pieces of seal, while our ship held position nearby. I could see the bear from my cabin’s porthole.

Stirling said it was unusual for a polar bear to linger so long on one meal. This particular animal, he assumed, must have been confident of defending its prize against intruders. It certainly didn’t look rushed. Finally, when only a few shards of seal bone and skin remained, the bear stirred, seeming to respond to some unseen Arctic pull. Just before noon, it wandered off, the buttery fur fading to a mere speck in the white landscape, and was gone.

Noah Strycker is associate editor at Birding magazine and author of, most recently, The Thing With Feathers: The Surprising Lives of Birds and What They Reveal About Being Human.