Opinion | My 500-Mile Journey Across Alaskas Thawing Arctic

Flames were leaping out of the forest beneath the float plane taking us deep into the remote interior of northern Alaska.Our destination was the glacial Walker Lake, which stretches 14 miles through Gates of the Arctic National Park.Nearly 100 miles from the nearest dirt road, Walker Lake is within an expanse of uninhabited tundra, scraggly boreal forests and the seemingly endless peaks of the Brooks Range in a wilderness bigger than Belgium.

Once we arrived, we saw wide bear trails bulldozed through alder thickets and plentiful signs of moose and wolves.We had come here to begin a 500-mile journey that would take us in pack rafts down the Noatak River, believed to be the longest undeveloped river system left in the United States, and on foot, slogging the beaches of the Chukchi Sea coastline.Our goal was to get a close-up look at how warming temperatures are affecting this rugged but fragile Arctic landscape.

Worldwide, roughly twice the amount of the heat-trapping carbon now in the atmosphere has been locked away in the planet’s higher latitudes in frozen ground known as permafrost.Now that ground is thawing and releasing its greenhouse gases.The fire we flew over was our first visible sign of the changes underway.While wildfires are part of the landscape’s natural regenerative cycle, they have until recently been infrequent above the Arctic Circle.

But now the rising heat of the lengthening summers has dried out the tundra and the invasive shrubs that have recently moved north with the warmth.This is a tinderbox for lightning strikes.

The fires expose and defrost the frozen soil, allowing greenhouse gases to escape into the atmosphere.I have slept more than 1,000 nights on frozen terrain while exploring the Far North.My first Arctic venture was in 1983, with a fellow National Park Service ranger in a tandem kayak on the Noatak River in Gates of the Arctic.

We awoke one morning, startled by the sounds of a big animal running through low willows.It jumped...