Ridge

She walked the marmot through the zones, because seeing the stack is the lesson.

"Start at the base — broadleaf forest, warm and green," Ridge said, gesturing up the slope. "Climb, and it shifts to cooler needleleaf forest — the trees change to conifers built for cold. Keep going and you hit the treeline — the altitude above which it's simply too cold and windy for trees to grow at all. Above that: alpine meadow, low tough plants hugging the ground. Higher still: bare rock, snow, a few lichens — practically tundra. Each band has its own climate and its own life, stacked one above the next, and you crossed all of them without going anywhere but up."

The subtext the older students heard was that the same underlying rule can produce the same pattern along completely different axes — that understanding why biomes stack by latitude lets you instantly predict how they'll stack by altitude, because it's one principle wearing two costumes.

Ridge connected it, carefully, to the warming world.

"Here's the sharp part," she said. "When the climate warms, every band on the mountain shifts upward — the conditions each species needs climb higher up the slope, and the species must climb to follow, just like the flatland biomes migrate poleward. But a mountain has a top. A creature living near the summit, when its climate band moves up, can climb for a while — and then it runs out of mountain. There's nowhere higher to go. Scientists call it the escalator to extinction: the highest-mountain species get pushed up and off the peak. That's the honest concern — not that things are moving, but that on a mountain, up eventually ends."

The marmot, who lived high up, went quiet. "So I can only climb so far."

"For now, you have room," Ridge said gently, "and knowing the pattern is exactly what lets people protect it — by keeping the warming slow enough that the bands don't climb faster than the life on them, and by protecting whole mountains so species have their full slope to move along. Understanding the escalator is the first step to keeping anyone from riding it off the top. Knowledge isn't the scary part. It's the useful part."