I slid onto the ice with the trepidation of a newly walking toddler, probing with my ski pole, lining my kids behind me like ducklings. You'd laugh if you were watching. But bear with me. It's hard to believe you really can walk on the sea, even when you're doing it.

It was the middle of March, nearly a year ago. My family of four had flown into Nome around the time a lot of people fly to Nome. Iditarod fans filled the streets. But I was a hundred yards offshore of downtown. I didn't fall in. Beneath my skis, the ocean was crystallized. Crystal touched crystal touched crystal, a sparkling line that reached to Unalakleet, where the first of the mushers had just reached Norton Sound.

We'd touched down in Nome with a tent we pitched in the glitter-cold air — on a friend's porch — drooping against the skis and poles and sled runners we'd leaned beside it. I was as curious as anyone to know which dog team would win, but I was mostly distracted, clutching crumpled paper lists. I was really there for the sea ice. On that day, March 15, 2015, the Arctic held 5.5 million square miles of it.

I crossed the last task off the list, cheered in the first few mushers, and stepped back onto the Bering Sea, ski tips pointed west. We planned to follow that ice until it was gone.

At its maximum, sea ice covers almost 6 million square miles of the Arctic. That's 10 times larger than Alaska, the largest state in the union. If you can't really picture that, you're in good company. Even in Alaska, fewer than 7 percent of us live where the sea turns solid.

It was as solid as anything, and we soon forgot about the "sea" in sea ice. We skied the ocean. Camped on the ocean. Drank the ocean.

I whaled at it with an ice ax until shards exploded across the snow and we melted them down and drank them, because even in the cold, salt melts the ice like it does on your front steps. Saltwater drains out, leaving fresh ice behind.

April 17, 2015: 5.4 million square miles of ice

The ice was solid but not flat. It was sculpted into monsters, castles, labryinths. We tugged our sleds through a toddler's playroom of shattered blocks. My 6-year-old's skis went skittering in opposite directions. He yelled "Crash!" then laughed, and then asked for a break. We climbed head-sized chunks of ice fused into a hill, and looked out.

The sky was dark ahead. Inky, ominous, foreboding. It hovered over an inexplicable line of blackness and we ran through every spooky adjective we could think of because something seemed so wrong.

It was a lack of albedo.

Albedo is Latin for "whiteness." It's a measurement climatologists use to express the amount of sunlight reflected from a surface. Ice with snow on top — which had been our home for a month — reflects more than 80 percent. That reflection burned the sky white, and burned our faces into raccoon-eye tans behind ski goggles. It burned and glared and it bounced into the sky and took all its warmth with it, leaving us huddled in our parkas.

We skied to the darkness. Found water. Waves bounced off the boundary between solid and liquid. The edge was several feet thick, but it undulated beneath us. Whales groaned in the distance. Bowheads, the locals told us later. The water looked black. I pushed up my ski goggles. Still black. Liquid ocean has an albedo of 6. It reflects just 6 percent of the light, and we could see it for miles because it turned the whole sky black.

This was a polynya — an inky slash through ice, a swath of open water that persists despite the cold. Wind blasted through the York Mountains, pummeled our tent, shoved the ice away as fast as it could form, and kept this water black and wet all winter. Open water keeps itself open too. Ice keeps itself icy. It's a feedback loop. White surfaces reflect heat, stay cold, and help freeze nearby areas. Dark surfaces warm, and melt their neighbors. On a much larger scale, that cascade is what can drop the Earth into an ice age, or pull it back out of one.

May 21, 2015: 4.8 million square miles of ice

I stood on top of the Seward Peninsula's last jumble-ice hill and played "guess the temperature" with the thermometer we carried. All four of us came in too low. After so much cold, it was hard to believe that 40 was possible, but there it was. The edge was wet and wobbly, and small floes streamed northward in the wind. You could push them out with a ski pole, and who knows how far they'd go? Was there any more ice out there? Our skis planed through a sheet of water atop the ice, reflecting marshmallow clouds and the kids' orange and yellow rain gear. We skied on a painting of the sky.

It was turquoise, not white. The albedo had dropped. Shallow pools reflected only 20 to 40 percent of the light, maybe only 15 percent in the ones so deep we didn't dare ski them. Sea urchins and clam shells scattered across the ice and when the sun hit them, each one melted its own deep hole.

The skiing was effortless. Slippery quick. Spring was coming faster and faster, and we were going faster and faster, too. Ten miles in a day sometimes, pretty good for a 6-year-old, and if the wind was going the right way, you could stand still and fly.

You could stand still and float, too. A skip from the shore, we paused on muddy globs that scraped at our skis and brought the sleds we towed to a halt. A thin crack separated us from land. I ate my granola bar, snapped together a paddle, clipped my kids' life vests, and didn't look up until we were ready to get in the boats.

The floe we were standing on was drifting out to sea.

We didn't camp on the ice anymore. Instead, I sneezed from the dust and mold on the just-thawed tundra, and our tent steamed and stank from the pile of sodden ski boots by the stove. The sun didn't set enough to notice, and every night, it sounded like the ducks were about to storm the tent.

May 28, 2015: 4.6 million square miles of ice

I heard it begin, in a long low rush like the flapping of wings. Sure, it was birds. I turned over, went back to sleep, as the last bridge between ice and land dripped away. The ice was unmoored now. By morning, it surrounded us. It crushed against the boulders that ringed the shore, piling 10 feet high where there'd been nothing. Just before bed, I'd moved our skis inland, out of some quiver of caution I couldn't quite articulate. The ice would have buried them.

I wouldn't have missed them. The ice that remained cracked into floating, see-through islands the size of city blocks, cluttering the open water. My husband ventured out on one. It rolled under his skis like a waterbed. I stood on a narrow rim of solid ice, and we both held our breaths, watching the ripples fade out across the ice. I clutched at the safety of my raft. My kids were already inside it.

"We are NOT crossing that."

We strapped our skis to the rafts and dodged the islands as they blew to shore, where every rocky point became the prow of a bulldozer, encased in a rubble of slushy white bricks.

The ice was losing 19,000 square miles a day now. It would shrink faster and faster, abandoning its southern fringe as it pulled toward a September minimum, when the ice is always at its lowest level. It would cling, even then, to the northern edge of Greenland, but by the end of May, it was leaving the Chukchi Sea behind.

Kittiwakes and murres poured off the cliffs — a waterfall of black and white that screamed and grunted and groaned. The murres' wings beat heavy and fast, legs splayed out and barely holding their fat bodies aloft as they skimmed over us. My daughter stretched out her arms. She thought she might catch one. The sea was littered with them.

Rainbows led us to camp, where my daughter ran barefoot singing with a fistful of blooming tundra. I couldn't help but root for spring. The end of the ice was raucous and dramatic while spring crashed and squawked, gleaming blue and pink, smelling of salt, mud and flowers. It happens every year. Spring is normal.

But this wasn't normal. We traveled 360 miles from Nome to Deering, and when we arrived, they said they'd never seen it break up so early. We followed the ice from March through May, and in no other spring could we have seen so little. We followed the ice, and every day it was about two standard deviations smaller than normal — that much smaller than the 1981-2010 average. What's that? Imagine you woke up and walked outside tomorrow, and every man you saw was just 5 feet 2 inches tall. That's two standard deviations below the norm. The ice has been about that small in every year this decade. A decade or two from now, imagine that those 5-foot-2-inch guys look tall. That's how normal things are on the sea ice.

We followed the sea ice because I knew it was important and because it's harder to understand what you've never seen. I saw ice waver and crack. I saw it crumble and wobble and pile and drip. The water swallowed the heat of the sun and that loss of albedo fed back on itself, nibbling at those white edges and puddling in turquoise pools until there was more and more darkness to swallow that heat. Once the spring thaw began, it never froze again — not even a skim on the puddles. It's the feedback that makes the transformation so fast.

I didn't know that some of the murres that littered that ocean would probably litter my beach back home in Kachemak Bay this winter — dead. That's been blamed on warm water. The difference between the cold Arctic and the warmer south is what keeps the jet stream from getting stuck. With a warmer north, the jet stream kinks. Warm air can flow north. Cold air can flow south. Weather patterns can stall out, bringing persistent heat, cold, flood or drought.

Before long, the Iditarod racers will get used to starting in Fairbanks. Rain will keep pounding on my roof, maybe every day of February like it did in nearly every day of January. Not all years, but more years. The albedo feedback loop makes the Arctic more sensitive to warming than anywhere else on the planet. It also gives the Arctic an outsized influence on how much the rest of the planet warms.

Darkness begets darkness, warmth begets warmth.

Erin McKittrick is an award-winning writer, adventurer and scientist based in Seldovia. Author of "A Long Trek Home: 4,000 Miles by Boot, Raft and Ski," "Small Feet, Big Land: Adventure, Home and Family on the Edge of Alaska" and "Tracing the Heart of Alaska" (due out in spring 2017). Her children's picture book "My Coyote Nose and Ptarmigan Toes: An Almost True Alaskan Adventure" comes out next month. You can find her at GroundTruthTrekking.org.