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It’s easy to take ice cubes for granted when all you do is pop them out of a freezer tray or catch them tumbling from a dispenser. But before the advent of mechanical refrigeration, “harvesting” ice involved as much work as farming a crop.

Richardson Nature Center in Bloomington lets visitors experience this labor-intensive, cold-climate tradition each January. Recently, visitors sawed through the crust of a frozen pond and floated giant blocks through a channel using tools from a century ago. After clamping a block with an ice tong attached to a rope, a half-dozen kids gave it a tug, popping the ice chunk out of the water and sending it skittering after them.

After all that effort, the 100-pound block was still far too big to drop in a glass. So the kids filled metal ice cube cutters with hot water and let them melt into the block. Finally, after chipping the cubes out with a handheld pick, they had their finished product.

Harvesting cold sure warmed the body up.

Although the Three Rivers Park District has been hosting its annual ice harvest for more than two decades, frequently drawing several hundred people, it may not go on forever.

In recent years, the nature center has paired the ice demonstration with displays and workshops on climate change that show how the area’s average winter temperatures are rising. Minnesota is among the fastest-warming states in the country, and if that trend continues, the ice harvesting tradition could melt out of existence.

The driving force behind Richardson’s ice harvest is Tim Graf, a retired telecommunications engineer whose grandparents ran an ice business in Worthington, Minn., up until the 1940s, when electric refrigeration made the industry obsolete.

Graf is a font of ice harvesting knowledge, which he shared with visitors to the nature center while assisting with the demonstration.

Graf began volunteering with the park district in the late 1990s after he’d stopped by one of its winter events and found that a promised ice harvesting demonstration was little more than a small hole in the ice and a hand saw.

“So I found a park person and said, ‘I have this entire garage full of real ice harvesting tools, maybe we can expand this?’ ” Graf recalled.

For a decade, Graf’s grandfather and his crew had used many of those tools to cut roughly 40,000 tons of ice — about the area of 40 football fields — from Worthington’s Okabena Lake each January. (The commercial ice industry started in New England in the early 1800s.)

The Okabena harvest took two to three weeks and went 24 hours a day, with the night shift working by kerosene lantern, Graf explained.

After plowing away the snow, the “ice field” was laid out in a grid, with blocks cut partway through using horse-drawn plows, or, starting in the 1920s, Model T motors mounted on skids with big circular blades.

The hand saws came in next. Then a quick blow with a long-handled chisel called a spud bar cleaved off a 250- to 300-pound block. A human-powered assembly line floated the blocks to shore, where a conveyor system ferried the blocks to an insulated icehouse in which they were stored for the year.

Ice harvesting was exhausting, bone-chilling work. Not to mention dangerous. Working alongside the open water, it was not uncommon for workers to fall in.

“You ever do the polar plunge?” Graf likes to ask program attendees. “Back in the ice harvesting days, that would be called a really bad day at work.”

One of the most memorable stories from Graf’s grandparents’ ice business involved a worker who had made an inadvertent polar plunge. He thought he’d run home quick to change his clothes — without anticipating the impact of below-zero temps on wet pants. “All of a sudden you’re not walkin’ anymore,” Graf said, describing how the man froze in his tracks. “His buddies picked him up like a store mannequin and carried him home and just stood him up in the bathtub until the pants thawed enough so they could get them off.”

On thin ice

Over the years, as Graf has helped host Richardson’s annual demonstration for schoolkids and the public, he’s seen awareness of ice harvesting decline.

“When we first started, you’d have teachers who had a little familiarity, but nowadays even the teachers are like, ‘Oh, wow!’ he said. “People who actually harvested ice are 80 and 90 years old now.”

Young people accustomed to the luxury of lingering with the refrigerator door flung wide open, Graf said, are shocked there was once a “five-second rule” for getting milk out of the icebox.

Graf often starts his presentation for schoolkids by referencing the opening scene of “Frozen,” where a crew of singing men cuts and slings ice blocks in choreographed rhythm.

“It was the best thing to happen to ice harvest historians and teachers,” Graf said of the Disney blockbuster. “Before that you really had to do a lot of work to get kids in tune to what you were trying to talk about.”

For the past several years, Richardson has paired the ice harvest with information sessions related to climate change, including energy efficiency and composting.

It’s a natural link, Graf said, as weather conditions during the annual harvest have grown warmer, on average, over the past two decades.

The first time Graf led the event, the snow was so deep they couldn’t cut ice. During the early years, temperatures were frequently in the single digits and the pond ice would be 16 to 20 inches thick, the same as Graf’s grandparents’ harvests.

But during the past decade, several harvests have taken place on days when the temperatures were in the 30s and 40s. The ice was often only a foot thick, sometimes less. One year, there was a thunderstorm.

Very few people “put up ice” today — those who do live in off-grid homes or run fishing resorts Up North and use it to pack their clients’ catch. But Richardson’s ice harvest serves as a reminder: Even if the industry has long since evaporated, the loss of its raw material could have serious repercussions.