The last time David Thoreson stared down the Northwest Passage from the bow of a sailboat, a claim few Iowa boys can make — heck, hardly anyone can make, the open expanse of Arctic Ocean stunned him into silence.

Where was the ice?

“We were utterly shocked,” he wrote in his journal in 2007. He’d gazed down the same channel 13 years earlier with Roger Swanson, a Minnesota farm boy, in an attempt to transit 6,000 miles of Arctic Ocean aboard the Cloud Nine, a ketch with Swanson’s hometown of Dunnell (Minn.) painted on its stern. (More on the phenomenon of the seafaring farmers in a bit.)

Cloud Nine then was thwarted by pack ice in the Barrow Strait and turned back. Now the boat sailed free. Icebergs were disintegrating. Brown earth emerged from the shoulders of snow-capped peaks.

“This is good for us, but not so good for the planet,” Thoreson wrote.

Today, Thoreson has merged his work as a professional photographer with accounts of his sailing voyages to Antarctica, through the Northwest Passage and an unusual circumnavigation of North and South America from the Arctic to the Antarctic oceans.

The result is a lush photo memoir, “Over the Horizon: Exploring the Edges of a Changing Planet.” An exhibit of his photos opens March 31 at Norway House in Minneapolis, and will be in place through May 7.

Today, Thoreson, 57, seeks to help people grasp the implications of climate change — not always easy when its evidence can be as subtle as tulips in March or as distant as glaciers in Antarctica. In many ways, his work harks back to how his boyhood fantasies in landlocked Iowa inspired a passion for the even more immense horizons of the sea.

“I think because I grew up so far from the ocean, I kind of became a daydreamer, thinking about a lot of things I didn’t really understand,” he said. The more he read and imagined scenes at sea, the more real they became. When he finally saw the ocean as a teenager, he understood it.

With his firsthand accounts and photographs of climate change’s effects, Thoreson aims to make what still seems theoretical to some people a reality that can help them make decisions about the future.

And to deniers who contend this simply is the most recent in a long history of climate cycles?

“First of all, I agree with them,” Thoreson said. “It is a long cycle. It is a natural cycle. We really do have two cycles going on — a natural climate change process with a long geological footprint, and a human-induced cycle that’s being exacerbated by 11 billion people on the planet burning fossil fuels as fast as we can get them out of the ground.

“The things that scientists are documenting now are so outside the terms of a normal cycle that it’s almost silly, this discussion about denial. So it’s important to tell my story, that this Iowa boy went off to sail the world’s oceans and witnessed this.”

Different sorts of waves

OK, about these seafaring farmers.

Thoreson grew up in Algona, Iowa, about 60 miles from Dunnell, where Swanson raised hogs. Cloud Nine’s crew became the first Americans to transit the Northwest Passage in a sailboat, without aid of an icebreaker.

And then there’s Mark Schrader, who also spent his younger days on a farm (in Nebraska) daydreaming about vast oceanic horizons. He would go on to become the first American to solo the world’s southern capes, off South America and Africa — twice — in circumnavigations in the 1980s.

Schrader was the captain of Ocean Watch, a 64-foot cutter-rigged sailboat that he sailed around the Americas, making 50 port stops in 13 countries to collect scientific data and build awareness of changes in the oceans and the threat to marine life.

Thoreson signed on as a crew member shortly after returning from his Northwest Passage trip with Swanson. (Swanson died five years later, in 2012, at the age of 81.)

“I felt an urgent call to action,” Thoreson said of being on Schrader’s expedition. The sailing was both enthralling and treacherous, as his photos show. Yet he knew that the truly hard work would begin once they were back on land: the work of telling the story.

“We all need these stories to stimulate our curiosity,” he said. “I think that’s what people like Will Steger and Ann Bancroft and Lonnie Dupre — all great Minnesota explorers — what they have done, and are doing.

“When Will Steger says the polar oceans are changing, I want to check it out, too,” whether that means actually going there, or learning more from dry land. Thoreson said it’s similar to how he once consumed the wilderness writings of Edward Abbey.

“I knew I may never get to Alaska, but I need to know it’s there. I need to know we have these vast wilderness areas that are protected, because it’s good for the spirit.”

Okoboji also tells its story

Thoreson grew up sailing on Lake Okoboji, near Spirit Lake, Iowa, where he lives today. His mother taught him on a little boat named Help, with the name written upside-down on the stern in case it capsized.

He comes from a lineage of sailors: His grandfather was a commodore of the Okoboji Yacht Club. His mother was its first female commodore. Eventually, Thoreson’s turn came, too.

A good Norwegian, he regards himself as sharing a lineage with Roald Amundsen, the first person to navigate the Northwest Passage, taking almost four years from 1903 to 1906, his crew iced in each winter before they could move forward. Now, with more open water for longer stretches of time, cruise ship companies are planning transits.

“To have been in the Arctic at a time when one era of exploration ended and another is beginning is humbling,” Thoreson said. “This has changed me completely into a person that I never really knew I would be, but it happened by getting out there and being curious.”

He’s adamant that “getting out there” can happen without leaving home. “Join a local water quality group, or the Sierra Club. Lessen your footprint.”

Thoreson had planned to return to the Northwest Passage this summer on a friend’s ship, but the trip has been postponed, which makes him think, looking out over the Iowa fields, that it may be time to find his own boat.