PAUL SMITHS, N.Y. — Jerry Jenkins, an ecologist with the Wildlife Conservation Society, narrates with encyclopedic precision the serene, timeless landscape of the Osgood River, as he moves past it with each dip of his canoe paddle.

That is sphagnum moss carpeting the banks of a bog that stretches across hundreds of acres, a signature feature of northern landscapes. Those are tamaracks and black spruces — cold-tolerant conifer trees found mostly in Canada — rising from the shores. A pair of gray jays alights on a branch: they, too, are at the southern end of their range.

Mr. Jenkins, who is the author of the book “Climate Change in the Adirondacks: The Path to Sustainability,” spends much of his time on the water and in the woods, documenting the ecosystem with a notebook and a camera. He thus brings an unusual perspective to the scene. Where a casual observer might behold diversity and continuity, he projects decades into the future and finds absence and loss.

“Nothing we see here is found at temperatures 10 degrees warmer, and very little makes it to five degrees warmer,” Mr. Jenkins said matter-of-factly on a mild fall day. “We will be in a climate that this community has never known in its history. One has to go back to world climate levels we haven’t seen in 15 million years.”