Three hundred million years ago, the earth was holding its breath.
The forests were not like our forests. The trees were not like our trees. Lycopods sixty meters tall — tree-sized club mosses — rose from swamps with bark that looked like hammered tin, a diamond lattice of leaf scars. Horsetails the height of modern oaks. Dragonflies with two-foot wingspans riding the thick, oxygen-rich air. The atmosphere held forty percent more oxygen than it does now. Things burned easily and grew enormous.
The trees fell. They always do. What was different then: nothing could decompose them.
Lignin is the structural polymer that makes wood hard. The Carboniferous forests were full of it. But the fungi and bacteria capable of breaking lignin down — white rot fungi, the organisms we have now that allow dead trees to rot back into soil — hadn't evolved yet. The evolutionary machinery for digesting lignin simply did not exist.
So the lycopods fell, and sat. Were buried in sediment. Were buried deeper. The weight of the earth pressed on them for millions of years. What had been living wood — carbon that those ancient trees had pulled from the air through photosynthesis, carbon that was briefly a forest — compressed and concentrated and transformed, over unimaginable time, into seams of black rock.
Those seams are the compressed shadow of the Carboniferous forest. Each meter of coal represents enormous amounts of original organic material — trees and ferns and the organisms that lived among them — compacted by geology into something dense and dark and almost pure carbon.
What you're holding, when you hold coal, is the compressed ghost of a forest that no longer exists. Not just the species — the entire ecological regime is gone. Nothing grows like that anywhere on earth now. You are holding the last trace of a world.
When you burn coal, the carbon releases as CO₂.
Which is the same carbon. The exact atoms — many of them — that were in the Carboniferous atmosphere three hundred million years ago, that were captured by a lycopod and turned into wood, that fell and were buried and compressed over geological time, are now, as the coal burns, re-entering the air.
The Carboniferous forest is returning to the atmosphere.
Those enormous alien trees — those sixty-meter lycopods that no longer exist — are returning to the sky they once stood beneath. Not as trees. As gas. The carbon that was once the forest is becoming air again, and the air is warming, and the world those trees knew — hot, wet, oxygen-rich — is something like what we are slowly rebuilding.
The Carboniferous period ended, in part, because of itself. All that carbon sequestration — all those trees pulling CO₂ from the air and not decomposing — eventually lowered atmospheric carbon enough to cool the planet. A glaciation followed. The warm swamps contracted. The great forests died back. The period of coal formation wound down.
The earth breathed in. Held it. For three hundred million years. And then we began digging it up and burning it, and the earth is exhaling now, and the exhale is ancient.
There is a coal miner, going down into the earth, working in a seam. She is surrounded, on all sides, by the compressed shadows of trees that fell before there were eyes to see them fall. The ceiling of the mine is the bottom of an ancient swamp. The walls are the compacted residue of forests no human has ever seen, of organisms with no living relatives, of an atmosphere we would find poisonous.
She is inside deep time. She is inside the longest breath in the history of this planet.
And we are letting it out.