When Rebecca Solnit and Bill McKibben asked Barry Lopez if he would join a group of writers in submitting a 350-word piece for 350.org’s effort to publicize that number — the maximum parts per million of carbon dioxide the atmosphere can maintain if we are to preserve the planet as we know it — he, like other writers busy with their own projects, initially declined. But then, compelled by the obligation of embracing this all-important limit in his creative work, he accepted. 

— The Editors

On a winter afternoon, along a trail in the Sierra Madre in the state of Mensajero, beneath an immense rampart of rising cumulonimbus cloud, a deeply imperfect man bent over to collect a small piece of black glass. He recognized its kind: obsidian, a thick sliver of it. When the molten interior of the Earth is thrown into the frigid sky and cools quickly, it becomes a stone like this. People say of its edges that no knife is sharper, and of its color that it is transparent but bottomless, like the sea’s, so it cannot be rendered on paper or canvas.

The man turned the spalled flake over in the palm of one hand with the fingers of the other. He tested the edge with his thumb and held it up to the sun. He knew of no volcanoes in these mountains, but the trail was many centuries old, and people had carried red coral, abalone shells, and turquoise up and down it for generations. Someone dropped this, he thought, in the time when his grandfather was alive, or in the year of his own birth, or a pilgrim might have dropped it, only days ago.

It glittered in his palm, like sunlight in ice, and he wondered, as the heaving clouds encroached on the sun and the shard of glass darkened, what his obligations were. Should he give it back to the trail or pocket it for the single daughter he was traveling to see? In another age he would not have hesitated to take it to the girl. Now he felt he must put it back, even if later someone else might take it. He believed he had come upon a time in his life when everything, even the things of God, needed protection. When he met his daughter, he would tell her he had found a black tear in the dust of the narrow path and understood he must leave it be. And she would ask whose tear it was, and he would have to use his imagination, in the way his people had once done.